Background of the Invention
[0001] The invention relates generally to sound reproduction. More specifically, the invention
relates to multiple channel sound reproduction systems having improved listener perceived
characteristics.
[0002] Multiple channel sound reproduction systems which include a surround-sound channel
(often referred to in the past as an "ambience" or "special-effects" channel) in addition
to left and right (and optimally, center) sound channels are now relatively common
in motion picture theaters and are becoming more and more common in the homes of consumers.
A driving force behind the proliferation of such systems in consumers' homes is the
widespread availability of surround-sound home video software, mainly surround-sound
motion pictures (movies) made for theatrical release and subsequently transferred
to home video formats (e.g., videocassettes and videodiscs).
[0003] Although home video software formats have two-channel stereophonic soundtracks,
those two channels carry, by means of amplitude and phase matrix encoding, four channels
of sound information--left, center, right, and surround, usually identical to the
two-channel stereophonic motion-picture soundtracks from which the home video soundtracks
are derived. As is also done in the motion picture theater, the left, center, right,
and surround channels are decoded and recovered by consumers with a matrix decoder,
usually referred to as a "surround-sound" decoder. In the home environment, the decoder
is usually incorporated in or is an accessory to a videocassette player, videodisc
player, or television set/video monitor. Although nearly universal in motion picture
theater environments, the center channel playback is often omitted in home systems.
A phantom-image center channel is then fed to left and right loudspeakers to make
up for the lack of a center channel speaker.
[0004] Motion picture theaters equipped for surround sound typically have at least three
sets of loudspeakers, located appropriately for reproduction of the left, center,
and right channels, at the front of the theater auditorium, behind the screen. The
surround channel is usually applied to a multiplicity of speakers located other than
at the front of the theater auditorium.
[0005] It is the recommended and common practice in the industry to align the sound system
of large auditoriums, particularly a motion picture theater's loudspeaker-room response,
to a standardized frequency response curve or "house curve." The current standardized
house curve for movie theaters is a recommendation of the International Standards
Organization designated as curve X of ISO 2969-1977(E). The use of a standardized
response curve is significant because in the final steps of creating motion picture
soundtracks, the soundtracks are almost always monitored in large (theater-sized)
auditoriums ("mixing" and "dubbing" theaters) whose loudspeaker-room responses have
been aligned to the standardized response curve. This is done, of course, with the
expectation that such motion picture films will be played in large (theater-sized)
auditoriums that have been aligned to the same standardized response curve. Consequently,
motion picture soundtracks inherently carry a built-in equalization that takes into
account or compensates for playback in large (theater-sized) auditoriums whose loudspeaker-room
responses are aligned to the standardized curve.
[0006] The current standardized curve, curve X of ISO 2969, is a curve having a significant
high-frequency rolloff. The curve is the result of subjective listening tests conducted
in large (theater-sized) auditoriums. A basic rationale for such a curve is given
by Robert B. Schulein in his article "In Situ Measurement and Equalization of Sound
Reproduction Systems,"
J. Audio Eng. Soc., April 1975, Vol. 23, No. 3, pp. 178-186. Schulein explains that the requirement
for high-frequency rolloff is apparently due to the free fold (i.e., direct) to diffuse
(i.e., reflected or reverberant) sound field diffraction effects of the human head
and ears. A distant loudspeaker in a large listening room is perceived by listeners
as having greater high frequency output than a closer loudspeaker, if aligned to measure
the same response. This appears to be a result of the substantial diffuse field to
free field ratio generated by the distant loudspeaker; a loudspeaker close to a listener
generates such a small diffuse to direct sound ratio as to be insignificant.
[0007] More recently the rationale has been carried further by Gunther Theile ("On the Standardization
of the Frequency Response of High-Quality Studio Headphones,"
J. Audio Eng. Soc., December 1986, Vol. 34, No. 12, pp. 956-969) who hypothesized that perceptions of
loudness and tone color (timbre) are not completely determined by sound pressure and
spectrum in the auditory canal. Theile relates this hypothesis to the "source location
effect" or "sound level loudness divergence" ("SLD") which occurs whenever auditory
events with differing locations are compared: a nearer loudspeaker requires more sound
level (sound pressure) at the ear drums to cause the same perceived sound loudness
as a more distant loudspeaker and the effect is frequency dependent.
[0008] It has also been recognized that the sound pressure level in a free direct field
exceeds that in a diffuse field for equal loudness. A standard equalization, currently
embodied in ISO 454-1975 (E) of the International Standards Organization, is intended
to compensate for the differences in perceived loudness and, by extension, timbre
due to frequency response changes between such sound fields.
[0009] Perceived sound loudness and timbre thus depends not only on the location at which
sound fields are generated with respect to the listener but also on the relative diffuse
(reflected or reverberant) field component to free (direct) field component ratio
of the sound field at the listener.
[0010] One major difference between the home listening environment and the motion picture
theater listening environment is in the relative sizes of the listening rooms--the
typical home listening room, of course, being much smaller. While there is no established
standard curve to which home sound systems are aligned, the high-frequency rolloff
house curve applicable to large auditoriums is not applicable to the considerably
smaller home listening room because of the above-mentioned effects.
[0011] Unlike home video software media having soundtracks transferred from motion picture
film soundtracks, recorded consumer software sound media (e.g., vinyl phonograph records,
cassette tapes, compact discs, etc.) have a built-in equalization that compensates
for typical home listening room environments. This is because during their preparation
such recordings are monitored in relatively small (home listening room sized) monitoring
studios using loudspeakers which are the same or similar to those typically used in
homes. Relative to large auditorium theater environments, the response of a typical
modern home listening room-loudspeaker system or a small studio listening room-loudspeaker
system can be characterized as substantially "flat," particularly in the high-frequency
region in which rolloff is applied in the large auditorium house curve. A consequence
of these differences is that motion pictures transferred to home video software media
have too much high-frequency sound when reproduced by a home system. Consequently,
the musical portions of motion picture soundtracks played on home systems tend to
sound "bright." In addition, other undesirable results occur--"Foley" sound effects,
such as the rustling of clothing, etc., which tend to have substantial high-frequency
content, are over-emphasized. Also, the increased high-frequency sensitivity of home
systems often reveals details in the makeup of the soundtrack that are not intended
to be heard by listeners; for example, changes in soundtrack noise level as dialog
tracks are cut in and out. These same problems, of course, occur when a motion picture
soundtrack is played back in any small listening environment having consumer-type
loudspeakers, such as small monitoring studios.
[0012] There is yet another difference between the home sound systems and motion picture
theater sound systems that detracts from creating a theater-like experience in the
home. It has been the practice at least in certain high-quality theater sound systems
to employ loudspeakers that provide a substantially directional sound field for the
left, center, and right channels and to employ loudspeakers that provide a substantially
non-directional sound field for the surround channel. Such an arrangement enhances
the perception of sound localization as a result of the directional front loudspeakers
while at the same time enhancing the perception of ambience and envelopement as a
result of the non-directional surround loudspeakers.
[0013] In contrast, home systems typically employ main channel (left and right channel)
loudspeakers designed to generate a compromise sound field that is neither extremely
directional nor extremely non-directional. Surround channel loudspeakers in the home
are usually down-sized versions of the main channel loudspeakers and generate similar
sound fields. In the home environment, little or no attention has been given to the
proper selection of directional characteristics for the main channel and surround
channel speakers.
[0014] Also, with respect even to the above-mentioned high-quality theater sound systems,
no compensation has been employed for the differences in listener perceived timbre
between the main channels and the surround channel resulting from the generation of
predominantly direct sound fields by the main channel speakers and the predominantly
diffuse sound field produced by the surround channel speakers.
[0015] In addition, with respect to home systems and to the above-mentioned high quality
theater sound systems, a single (monophonic) surround-sound channel is applied to
multiple loudspeakers (usually two, in the case of the home, located to the left and
right rear of home listening room and usually more than two, in the case of a motion-picture
theater, located on the side and rear walls). Particularly in the home environment,
the result is that the surround-sound channel sounds to a listener seated on the center
line as though it were in the middle of the head.
Summary of the Invention
[0016] Aspects of the present invention are directed primarily to surround-sound reproduction
systems in relatively small listening rooms, particularly those in homes. With respect
to such, the invention solves the problem of spectral imbalance (e.g., alteration
in timbre), particularly excessive high-frequency energy, when playing pre-recorded
sound material that is equalized for playback in a large (theater-sized) auditorium
whose room-loudspeaker system is aligned to a frequency response curve having a significant
high-frequency rolloff. In a preferred embodiment, re-equalization according to
a correction curve is provided in the playback system in order to restore to a "flat"
response the perceived spectral balance of recordings transferred from motion picture
soundtracks having an inherent high-frequency boost because of their intended playback
in large (theater-sized) auditoriums aligned to the standard house curve. Such re-equalization
restores the spectral distribution (timbre) intended by the creators of the pre-recorded
sound material.
[0017] With respect to small (home-sized) listening rooms, a further aspect of the invention
is to generate generally directional sound fields in response to the left and right
sound channels and in response to the center sound channel, if used, and to generate
a generally non-directional sound field in response to the surround-sound channel.
[0018] A directional sound field is one in which the free (direct) component of the sound
field is predominant over the diffuse component at listening positions within the
listening room. A non-directional sound field is one in which the diffuse component
of the sound field is predominant over the free (direct) component at listening positions
within the listening room. Directionality of a sound field depends at least on the
Q of the loudspeaker or loudspeakers producing the sound field ("Q" is a measure of
the directional properties of a loudspeaker), the number of loudspeakers, the size
and characteristics of the listening room, the manner in which the loudspeaker (or
loudspeakers) is (or are) acoustically coupled to (e.g., positioned with respect to)
the listening room, and the listening position within the room. For example, multiple
high Q (directional) loudspeakers can be distributed so as to produce a non-directional
sound field within a room. Also, the directionality of multiple loudspeakers reproducing
the same channel of sound can be affected by their physical relationship to one another
and differences in amplitude and phase of the signal applied to them.
[0019] This aspect of the invention is not concerned
per se with specific loudspeakers nor with their acoustic coupling to small listening rooms,
but rather it is concerned, in part, with the generation of direct and diffuse sound
fields for the main (left, right, and, optionally, center) channels and for the surround
channel, respectively, in a small (home-sized) room surround-sound system using whatever
combinations of available loudspeakers and techniques as may be required to generate
such sound fields. This aspect of the invention recognizes that excellent stereophonic
imaging and detail combined with sonic envelopement of the listeners can be achieved
not only in large (theater-sized) auditoriums but also in the small (home-sized) listening
room by generating generally direct sound fields for the main channels and a generally
diffuse sound field for the surround channel. In this way, the home listening experience
can more closely re-create the quality theater sound experience.
[0020] According to a further aspect of the invention, the listening impression created
by the generation of direct sound fields for the main channels and a diffuse sound
field for the surround channel can be improved even further, for all sizes of listening
rooms, by the addition of equalization to compensate for the differences in perceived
timbre between direct and diffuse sound fields. In other words, the full benefit of
the use of direct and diffuse sound fields for the main and surround channels, respectively,
is not achieved unless appropriate equalization is provided in the surround channel
in order to compensate for its reproduction of a diffuse sound field. Although the
standard curve ISO 454 is intended to compensate for the frequency dependent differences
in perceived loudness between direct and diffuse fields, it has not heretofore been
appreciated that such a correction is relevant to surround-sound systems.
[0021] According to yet a further aspect of the invention, the listener's impression of
the surround-sound channel can be improved, for all sizes of listening rooms, by
decreasing the interaural cross-correlation of the surround-sound channel sound field
at listening positions within the room. Preferably, this is accomplished by a technique
such as slight pitch shifting between multiple surround loudspeakers, which does not
cause undesirable side effect. While this aspect of the invention may be employed
without the aforementioned generation of generally direct sound fields for the main
channels and a generally diffuse sound field for the surround channel, the combination
of these aspects of the invention provides an even more psychoacoustically pleasing
listening experience. Preferably, the combination further includes the aspect of the
invention providing for surround channel equalization to compensate for the listener
perceived difference in timbre between a direct sound field and a diffuse sound field.
Brief Description of the Drawings
[0022]
Figure 1 is a block diagram of a surround-sound reproduction system embodying aspects
of the invention.
Figure 2 is a block diagram of a surround-sound reproduction system embodying aspects
of the invention.
Figure 3 is a loudspeaker-room response curve used by theaters, curve X of the International
Standard ISO 2969-1977(E), extrapolated to 20 kHz.
Figure 4 is a correction curve, according to one aspect of this invention, to compensate
for the large room equalization inherent in motion picture soundtracks when played
back in small listening rooms.
Figure 5 is a schematic circuit diagram showing the preferred embodiment of a filter/equalizer
for implementing the correction curve of Figure 4.
Figure 6 is a diagram in the frequency domain showing the locations of the poles and
zeros on the s-plane of the filter/equalizer of Figure 5.
Figure 7 is a schematic circuit diagram showing the preferred embodiment for implementing
the surround channel direct/diffuse sound field equalizer according to another aspect
of the invention.
Figure 8 is a block diagram showing an arrangement for deriving, by means of pitch
shifting, two sound outputs from the surround-sound channel capable of providing,
according to another aspect of the invention, sound fields having low-interaural cross-correlation
at listening positions.
Detailed Description of the Invention
[0023] Figures 1 and 2 show, respectively, block diagrams of two surround sound reproduction
systems embodying aspects of the invention. Figures 1 and 2 are generally equivalent,
although, for reasons explained below, the arrangement of Figure 2 is preferred. Throughout
the specification and drawings, like elements generally are assigned the same reference
numerals; similar elements are generally assigned the same reference numerals but
are distinguished by prime (′) marks.
[0024] In both Figures 1 and 2, left (L), center (C), right (R), and surround (S) channels,
matrix encoded, according to well-known techniques, as left total (LT) and right total
(RT) signals, are applied to decoding and equalization means 2 and 2′, respectively.
Both decoding and equalization means 2 and 2′ include a matrix decoder that is intended
to derive the L, C, R, and S channels from the applied LT and RT signals. Such matrix
decoders, often referred to as "surround sound" decoders are well-known. Several variations
of surround sound decoders are known both for professional motion picture theater
use and for consumer home use. For example, the simplest decoders include only a passive
matrix, whereas more complex decoders also include a delay line and/or active circuitry
in order to enhance channel separation. In addition, many decoders include a noise
reduction expander because most matrix encoded motion picture soundtracks employ noise
reduction encoding in the surround channel. It is intended that the matrix decoder
4 include all such variations.
[0025] In the embodiment of Figure 1, re-equalizer means 6 are placed in the respective
LT and RT signal input lines to the matrix decoder 4, whereas in the embodiment of
Figure 2, the re-equalizer means 6 are located in the L, C, and R output lines from
the matrix decoder 4. The function of the re-equalizer means 6 are explained below.
In both the Figure 1 and Figure 2 embodiments, an optional direct/diffuse equalizer
means 8 is located in the S output line from the matrix decoder 4. The function of
the direct/diffuse equalizer means 8 is also explained below.
[0026] In both embodiments, the L, C, R, and S outputs from the decoding and equalization
means 2 feed a respective loudspeaker or respective loudspeakers 10, 12, 14, and 16.
In home listening environments the center channel loudspeaker 12 is frequently omitted
(some matrix decoders intended for home use omit entirely a center channel output).
Suitable amplification is provided as necessary, but is not shown for simplicity.
[0027] The arrangements of both Figures 1 and 2 thus provide for the coupling of at least
the left, right, and surround (and, optionally, the center) sound channels encoded
in the LT and RT signals to a respective loudspeaker or loudspeakers. The loudspeakers
are intended to be located in operating positions with respect to a listening room
in order to generate sound fields responsive to at least the left, right, and surround
(and, optionally, the center) channels within the listening room.
[0028] Because of the requirement to accurately preserve relative signal phase of the LT
and RT input signals for proper operation of the matrix decoder 4, which responds
to amplitude and phase relationships in the LT and RT input signals, the placement
of the re-equalizing means 6 (a type of filter, as explained below) before the decoder
4, as in the embodiment of Figure 1, is less desirable than the alternative location
after the decoder 4 shown in the embodiment of Figure 2. In addition, the re-equalizing
means 6, if placed before decoder 4, may affect proper operation of the noise reduction
expander, if one is employed, in the matrix decoder 4. The arrangement of Figure 2
is thus preferred over that of Figure 1. The preferred embodiment of re-equalizer
means 6 described below assumes that they are located after the matrix decoder 4 in
the manner of the embodiment of Figure 2. If the re-equalizer means 6 are located
before the matrix decoder 4 in the manner of Figure 1 it may be necessary to modify
their response characteristics in order to minimize effects on noise reduction decoding
that may be included in the matrix decoder 4 and, also, it may be necessary to carefully
match the characteristics of the two re-equalizer means 6 (of the Figure 1 embodiment)
in order to minimize any relative shift in phase and amplitude in the LT and RT signals
as they are processed by the re-equalizer means 6.
[0029] Figure 3 shows curve X of the International Standard ISO 2969-1977(E) with the response
extrapolated to 20 kHz, beyond the official 12.5 kHz upper frequency limit of the
standard. It is common practice in many theaters, particularly dubbing theaters and
other theaters equipped with high quality surround sound systems, to align their response
to an extended X characteristic. The extended X curve is a
de facto industry standard. The X characteristic begins to roll off at 2 kHz and is down 7
dB at 10 kHz. The extended curve is down about 9 dB at 16 kHz, the highest frequency
employed in current alignment procedures for dubbing theaters. In public motion picture
theaters, which are larger than dubbing theaters, the X curve is extended only to
12.5 kHz because the high frequency attenuation of sound in the air becomes a factor
above about 12.5 kHz in such large auditoriums. The X curve, and particularly its
extension, are believed by some in the industry to be too rolled off at very high
frequencies. In contrast to the X curve and the extended X curve, a good quality modern
home consumer sound system, although not aligned to a specific standard, tends not
to exhibit such a high-frequency room-loudspeaker response roll off. Relative to the
X curve and extended X curve, modern home consumer systems may be characterized as
relatively flat at high frequencies.
[0030] As explained above, in the creation of a motion picture soundtrack, the soundtrack
is usually monitored in a theater that has been aligned to the extended X response
curve, with the expectation that such motion picture films will be played in theaters
that have been aligned to that standardized response curve. Thus, motion picture soundtracks
inherently carry a built-in equalization that takes into account or compensates for
playback in theater-sized auditoriums whose loudspeaker-room response is aligned to
the standardized curve. However, for the reasons discussed above, this built-in equalization
is not appropriate for playback in home listening environments: the soundtracks of
motion pictures transferred to home video software media have too much high frequency
sound energy when reproduced by a home system. Correct timbre is not preserved and
details in the soundtrack can be heard that are not intended to be heard.
[0031] According to one aspect of this invention, a correction curve is provided to compensate
for the large room equalization inherent in motion picture soundtracks when played
back in small listening rooms. The correction curve was empirically derived using
a specialized commercially-available acoustic testing manikin. The correction curve
is a difference curve derived from measurements of steady-state one-third octave sound
level spectra taken in representive extended X curve aligned large auditoriums in
comparison to a good quality modern home consumer loudspeaker-room sound system. The
correction curve is shown in Figure 4 as a cross-hatched band centered about a solid
line central response characteristic. The correction band takes into account an allowable
tolerance in the correction of about ±1 dB up to about 10 kHz and about ±2 dB from
about 10 kHz to 20 kHz, where the ear is less sensitive to variation in response.
In practice, the tolerance for the initial flat portion of the characteristic, below
about 2 kHz, may be tighter. The form of the correction curve band is generally that
of a low-pass filter with a shelving response: the correction is relatively flat up
to about 4 to 5 kHz, exhibits a roll off, and again begins to flatten out above about
10 kHz. About 3 to 5 dB roll off is provided at 10 kHz. The extended X curve response
is also shown in Figure 4 for reference. As mentioned above, the X curve, and particularly
its extension are believed by some in the industry to be too rolled off at very high
frequencies. It will be appreciated that the optimum correction curve would change
in the event that a modified X curve standard is adopted and put into practice.
[0032] A filter/equalizer circuit can be implemented by means of an active filter, such
as shown in Figure 5, to provide a transfer characteristic closely approximating the
solid central line of the correction curve band of Figure 4. The correct frequency
response for the filter/equalizer is obtained by the combination of a simple real
pole and a "dip" equalizer section. The real pole is realized by a single RC filter
section with a -3dB frequency of 15 kHz. The dip equalizer is a second order filter
with a nearly flat response. The transfer function of the section is:

The complex pole pair and the complex zero pair have the same radian frequency but
their angles are slightly different giving the desired dip in the frequency response
with minimum phase shift. The same dip could be achieved with the zeros in the right
half plane, but the phase shift would be closer to that of an allpass filter--180
degrees at the resonant frequency. The parameters of the dip section in the filter/equalizer
are:
fo = 12.31
kHz
Q = 0.81
γ=0.733
where
fo = 2πω
o . Another way of interpreting these parameters is that the Q of the poles is 0.81
and the Q of the zeros is

. The dip section can be realized by a single operational amplifier filter stage
and six components as shown in Figure 5. The filter stage in effect subtracts a bandpass
filtered signal from unity giving the required transfer function and frequency response
shape. The circuit topology, one of a class of single operational amplifier biquadratic
circuits, is known for use as an allpass filter (
Passive and Active Network Analysis and Synthesis by Aram Budak, Houghton Mifflin Company, Boston, 1974, page 451).
[0033] The rectangular coordinates of the poles and zeros of the overall filter equalizer
are as follows (units are
radians/sec in those locations on the
s-plane):
Real Pole:
α
rp = -9.4248
x10⁴
Complex Poles:
α
p±
jβ
p = -4.7046
x10⁴±
j5.9962
x10⁴
Complex Zeros:
α
z ±
jβ
z = -3.4485
x10⁴±
j6.7967
x10⁴
Figure 6 shows the location of the poles and zeros on the s-plane.
[0034] When implemented with the preferred component values listed below, the resulting
characteristic response of the filter/equalizer circuit of Figure 5 is:
Frequency, Hz |
Response, dB |
20 |
0 |
100 |
0 |
500 |
0 |
1,000 |
0 |
2,000 |
-0.2 |
3,150 |
-0.4 |
4,000 |
-0.7 |
5,000 |
-1.1 |
6,300 |
-1.8 |
8,000 |
-2.8 |
10,000 |
-4.2 |
12,500 |
-5.2 |
16,000 |
-5.4 |
20,000 |
-5.7 |
As mentioned above, there is an allowable tolerance of about ±1 dB up to about 10
kHz and about ±2 dB from about 10 kHz to 20 kHz. The preferred component values of
the circuit shown in Figure 5 are as follows:

[0035] The filter/equalizer circuit of Figure 5 is one practical embodiment of the re-equalizer
means 6 of Figure 2. Many other filter/equalizer circuit configurations are possible
within the teachings of the invention.
[0036] Referring again to the embodiments of Figures 1 and 2, the loudspeaker or loudspeakers
10, 12 (if used), and 14 are preferable directional loudspeakers that generate, when
in their operating positions in the listening room, left, center (if used), and right
channel sound fields in which the free (direct) sound field component is predominant
over the diffuse sound field component of each sound field at listening positions
within the listening room. The loudspeaker or loudspeakers 16 is (or are) preferably
non-directional so as to generate, when in its or their operating positions in the
listening room, a surround channel sound field in which the diffuse sound field component
is predominant over the free (direct) sound field component at listening positions
within the listening room. A non-directional sound field for reproducing the surround
channel can be achieved in various ways. Preferably, one or more dipole type loudspeakers
each having a generally figure-eight radiation pattern are oriented with one of their
respective nulls generally toward the listeners. Other types of loudspeakers having
a null in their radiation patterns can also be used. Another possibility is to use
a multiplicity of speakers having low directivity arranged around the listeners so
as to create an overall sound field that is diffuse. Thus, depending on their placement
in the listening room and their orientation with respect to the listening positions,
even directional loudspeakers are capable of producing a predominantly diffuse sound
field.
[0037] In order to obtain the full sonic benefits of directional and non-directional speakers
as just set forth, it is preferred that the arrangements of the Figure 1 and Figure
2 embodiments use the optional direct/diffuse equalizer 8. Such an equalizer compensates
for the differences in listener perceived timbre between direct and diffuse sound
fields. The use of a direct/diffuse equalizer with the directional and non-directional
speakers as just set forth is applicable to both large (theater-sized) auditoriums
and to small (home) listening rooms. As applied to large (theater-sized) auditoriums,
the arrangements of Figures 1 and 2 would, of course, not require the re-equalizer
means 6.
[0038] The preferred embodiment of the direct/diffuse equalizer 8 is an active filter/equalizer
circuit that substantially implements (within 0.3 dB) the inverse of the curve defined
by the difference data set forth in ISO 454-1975(E). In practice, such a close tolerance
is not required. The difference data in that standard is a table of the amount by
which the sound pressure level in a free field exceeds that in a diffuse field for
equal loudness. The data is as follows:
Frequency, Hz |
Difference, dB |
50 |
0 |
63 |
0 |
80 |
0 |
100 |
0 |
125 |
0 |
160 |
0 |
200 |
0.3 |
250 |
0.6 |
315 |
0.9 |
400 |
1.2 |
500 |
1.6 |
630 |
2.3 |
800 |
2.8 |
1,000 |
3.0 |
1,250 |
2.0 |
1,600 |
0 |
2,000 |
- 1.4 |
2,500 |
- 2.0 |
3,150 |
- 1.9 |
4,000 |
- 1.0 |
5,000 |
0.5 |
6,300 |
3.0 |
8,000 |
4.0 |
10,000 |
4.3 |
There is a suggestion in the above-cited article by Theile that ISO 454 does not
properly take into account the SLD effect, discussed above. Accordingly, the compensation
provided by the standard may be somewhat in error. It is intended that the either
ISO 454 or a corrected version thereof should provide the basis for the practical
implementation of the equalizer 8.
[0039] Figure 7 shows a schematic diagram of a practical embodiment of the direct/diffuse
equalizer 8 that implements the inverse of the curve defined by ISO 454-1975(E).
It will be noted that the standard provides data up to 10 kHz. This is more than adequate
because the frequency response of the surround channel in the standard matrix surround
sound system is limited to about 7 kHz. Equalizer 8 employs four sections having a
total of five operational amplifiers. Except for the second section, which is a simple
RC single pole low pass filter (25 kHz) and buffer (op amp 40), the sections are basically
the same circuit topology identified above as known for use as an allpass filter.
The first section, including op amp 38, functions as a dip equalizer with a -5.6 dB
gain at 1 kHz. The third section, including op amps 42 and 44, uses op amp 42 to provide
a phase inversion, causing the section to function as a boost equalizer having a gain
of 9 dB at 2.5 kHz. The last section is a further dip equalizer having a -6 dB gain
at 8 kHz. The preferred circuit values are as follows:
component |
value |
48 |
6K98 |
50 |
6K19 |
52 |
22N |
54 |
22N |
56 |
6K98 |
58 |
6K81 |
60 |
2.4K |
62 |
2700 pF |
64 |
6K81 |
66 |
30K1 |
68 |
4K99 |
70 |
10N |
72 |
10N |
74 |
6K81 |
76 |
10K |
78 |
10K2 |
80 |
7K5 |
82 |
2N7 |
84 |
2N7 |
86 |
7K5 |
[0040] The equalizer circuit of Figure 7 is one practical embodiment of the equalizer means
8 of Figures 1 and 2. Many other filter/equalizer circuit configurations are possible
within the teachings of the invention.
[0041] In a modification of the embodiments of Figures 1 and 2, the monophonic surround-sound
channel advantageously may be split, by appropriate de-correlating means, into two
channels which, when applied to first and second surround loudspeakers or groups of
loudspeakers, provide two surround channel sound fields having low-interaural cross-correlation
with respect to each other at listening positions within the listening room. Preferably,
each of the two de-correlated surround channel sound fields is generated by a single
loudspeaker. The use of more than a single loudspeaker to generate each field may
make it more difficult to match the timbre of the diffuse surround channel sound field
to that of the direct left, center, and right channel sound fields. This may be a
result of a comb filter effect produced when more than two loudspeakers are used to
generate each of the de-correlated surround channel sound fields.
[0042] It has previously been established that human perception favors dissimilar sound
present at the two ears insofar as the reverberant energy in a listening room is concerned.
In order to provide such a dissimilarity when using matrix audio surround-sound technology,
added circuitry is needed beyond simple encoding and decoding, since only a monaural
surround track is encoded. In principal this circuitry may employ various known techniques
for synthesizing stereo from a monaural source, such as comb filtering. However, many
of these techniques produce undesirable audible side effects. For example, comb filters
suffer from audible "phasiness," which can readily be distinguished by careful listeners.
[0043] Preferably, the decorrelation circuitry used in the practical embodiment of this
aspect of the invention employs small amounts of frequency or pitch shifting, which
is known to be relatively unobtrusive to critical listeners. Pitch shifting, for example,
is currently used, besides as an effect, to allow the increase of gain before feedback
in public address systems, where it is not easily noticed, the amount of such shifts
being small, in the order of a few Hertz. A 5 Hz shift is employed in a modulation-demodulation
circuit for this purpose described in "A Frequency Shifter for Improving Acoustic
Feedback Stability," by A.J. Prestigiacomo and D.J. MacLean, reprinted in
Sound Reinforcement, An Anthology, Audio Engineering Society, 1978, pp. B-6 - B-9.
[0044] Frequency or pitch shifting may be accomplished by any of the well-known techniques
for doing so. In addition to the method described in the Prestigiacomo and MacLean
article, as noted in the
Handbook for Sound Engineers, the New Audio Cyclopedia, Howard W. Sams & Co. First Edition, 1987, page 626, delay can form the basis for
frequency shift: the signal is applied to the memory of the delay at one rate (the
original frequency) and read out at a different rate (the shifted frequency).
[0045] The surround channel signal is applied to two paths. At least one path is processed
by a pitch shifter. Preferably, the frequency or pitch shift is fixed and is small,
sufficient to psychoacoustically de-correlate the sound fields without audibly degrading
the sound: in the order of a few Hertz. Although more complex arrangements are possible,
they may not be necessary. For example, pitch shifting could be provided in both paths
and the pitch could be shifted in a complementary fashion, with one polarity of shift
driving the surround channel signal in one path up in frequency, and the other driving
the signal in the other path downward in frequency. Other possibilities include varying
the pitch shift by varying the clocking of a delay line. The shift could be varied
in accordance with the envelope of the surround channel audio signal (e.g., under
control of a circuit following the surround channel audio signal having a syllabic
time constant--such circuits are well known for use with audio compressors and expanders).
[0046] Although either analog or digital delay processing may be employed, the lower cost
of digital delay lines suggests digital processing, particularly the use of adaptive
delta modulation (ADM) for which relatively inexpensive decoders are available. Conventional
pulse code modulation (PCM) also may be used. Although waveform discontinuities ("splices")
occur at the signal block sample junctions as the output signal from the delay line
is reconstructed whether ADM or PCM is used, such splices tend to be inaudible in
the case of ADM because the errors are single bit errors. In the case of PCM, special
signal processing is likely required to reduce the audibility of the splices. According
to the above cited
Handbook for Sound Engineers, several signal-processing techniques have successfully reduced the audibility of
such "splices."
[0047] Referring to Figure 8, the surround output from matrix decoder 4 (optionally, via
direct/diffuse equalizer 8) of Figures 1 or 2 provides the input to the decorrelator
which is applied to an anti-aliasing low-pass filter 102 in the signal processing
path and to an envelope generator 122 in the control signal path. The filtered input
signal is then applied to an analog-to-digital converter (preferably, ADM) 104, the
digital output of which is applied to two paths that generate, respectively, the left
surround and right surround outputs. The assignment of the "left" and "right" paths
is purely arbitrary and the designations may be reversed. The paths are the same and
include a clocked delay line 106 (114), a digital-to-analog converter 108 (116) and
an anti-imaging low-pass filter 110 (118).
[0048] The control signal for controlling the pitch shift by means of altering the clocking
of the delay lines 106 and 114 is fixed or variable, according to the position of
switch 124, which selects the input to a very low frequency voltage controlled oscillator
(VCO) 128 either from the envelope generator 122, which follows the syllabic rate
of the surround channel audio signal, or from a fixed source, shown as a variable
resistor 126. VCO 128 operates at a very low frequency, less than 5 Hz. The output
of the low frequency VCO 128 is applied directly to a high frequency VCO 130 which
clocks delay line 106 in the left surround path and is also inverted by inverter 132
for application to a second high frequency VCO 134 which clocks delay line 114 in
the right surround path. When there is no output from the low frequency VCO 128, the
two high frequency VCOs are set to the same frequency (in the megahertz range, the
exact frequency depending on the clock rate required for the delay lines, which in
turn depends on the digital sampling rate selected). The low frequency oscillator
128 modulates the high frequency oscillators, producing complementary pitch shifts.
[0049] Alternatively, the decorrelator of Figure 8 may be simplified so that the surround
output from the matrix decoder is applied without processing in a first path to either
the left surround loudspeaker(s) 112 or right surround loudspeaker(s) 120. The other
path is applied to the other of the loudspeaker(s) via frequency or pitch shift processing,
preferably fixed, including anti-aliasing low-pass filter 102, analog-to-digital converter
104, delay 106, digital-to-analog converter 108, anti-imaging low-pass filter 110.
Delay 106 is controlled as shown in Figure 8, preferably with switch 124 selecting
the fixed input from potentiometer 126. The amount of frequency shifting required
in this variation in which the pitch is shifted only in one channel is about twice
that provided to each of the paths in the embodiment of Figure 8.
[0050] The output of the paths is applied (through suitable amplification), respectively,
to one (preferably) or a group of left surround loudspeakers 112 and to one (preferably)
or a group of right surround loudspeakers 120. The loudspeakers should be arranged
so that they generate first and second sound fields generally to the left (side and/or
rear) and right (side and/or rear) of listening positions within the listening room.
The aforementioned techniques regarding the generation of a predominantly diffuse
sound field are preferably applied to the decorrelated surround channel.
1. A surround-sound system for reproducing pre-recorded multiple sound channels,
including left, right, and surround-sound channels, in a relatively small listening
room, such as in a home, wherein said left and right sound channels are equalized
for playback in a large auditorium whose room-loudspeaker system is aligned to a response
curve having a high-frequency roll off, comprising
loudspeaker means for generating, when located in its or their operating positions
with respect to the listening room, in response to first and second input signals,
first and second sound fields at listening positions within the listening room,
means for coupling said left and right sound channels, as said first and second input
signals, to said loudspeaker means, said means for coupling said left and right sound
channels to said loudspeaker means including means for re-equalizing said left and
right sound channels to compensate for said large auditorium equalization,
additional loudspeaker means for generating, when located in its or their operating
positions with respect to the listening room, in response to a third input signal,
a third sound field at listening positions within the listening room, and
means for coupling said surround-sound channel, as said third input signal, to said
additional loudspeaker means.
2. The system of claim 1 wherein said surround-sound system is also for reproducing
a center sound channel, said loudspeaker means generating, when located in its or
their operating positions with respect to the listening room, in response to a fourth
input signal a fourth sound field at listening positions within the listening room,
said means for coupling also coupling said center sound channel, as said fourth input
signal, to said loudspeaker means, said means for coupling said center sound channel
to said loudspeaker means including means for re-equalizing said center sound channel
to compensate for said large auditorium equalization.
3. The system of claim 1 wherein said additional loudspeaker means includes first
and second additional loudspeakers or groups of loudspeakers and wherein said means
for coupling said surround-sound channel further includes means for deriving two sound
channels from said surround-sound channel, which, when reproduced by said first and
second additional loudspeakers or groups of loudspeakers located in their operating
positions with respect to the listening room, generate first and second surround-sound
fields having low-interaural cross-correlation with respect to each other at listening
positions within the listening room and said means for coupling couples said two sound
channels to said first and second surround-sound channel loudspeakers or groups of
loudspeakers.
4. The system of claim 3 wherein said means for deriving two sound channels from said
surround-sound channel includes means for shifting the pitch of said two sound channels
with respect to each other.
5. The system of claims 1, 2, or 3 wherein said means for re-equalizing comprises
a circuit having a transfer characteristic of a low-pass filter with a shelving response
such that its characteristic response is relatively flat up to about 4 to 5 kHz, rolls
off between about 4 to 5 kHz and about 10 kHz, and is relatively flat above about
10 kHz.
6. The system of claim 5 wherein said characteristic response, subject to a tolerance
of about ±1 dB up to about 10 kHz and about ±2 dB from about 10 kHz to 20 kHz, is:
Hz |
dB |
20 |
0 |
100 |
0 |
500 |
0 |
1K |
0 |
2K |
-0.2 |
3K15 |
-0.4 |
4K |
-0.7 |
5K |
-1.1 |
6K3 |
-1.8 |
8K |
-2.8 |
10K |
-4.2 |
12K5 |
-5.2 |
16K |
-5.4 |
20K |
-5.7 |
7. A surround-sound system for reproducing pre-recorded multiple sound channels,
including left, right, and surround-sound channels, in a relatively small listening
room, such as in a home, wherein said left and right sound channels are equalized
for playback in a large auditorium whose room-loudspeaker system is aligned to a response
curve having a high-frequency roll off, comprising
loudspeaker means for generating, when located in its or their operating positions
with respect to the listening room, in response to first and second input signals,
first and second sound fields in which the direct sound field component of each sound
field is predominant over the diffuse sound field component at listening positions
within the listening room,
means for coupling said left and right sound channels, as said first and second input
signals, to said loudspeaker means, said means for coupling said left and right sound
channels to said loudspeaker means including means for re-equalizing said left and
right sound channels to compensate for said large auditorium equalization,
additional loudspeaker means for generating, when located in its or their operating
positions with respect to the listening room, in response to a third input signal,
a third sound field in which the diffuse sound field component is predominant over
the direct sound field component at listening positions within the listening room,
and
means for coupling said surround-sound channel, as said third input signal, to said
additional loudspeaker means.
8. The system of claim 7 wherein said surround-sound system is also for reproducing
a center sound channel, said loudspeaker means generating, when located in its or
their operating positions with respect to the listening room, in response to a fourth
input signal a fourth sound field in which the direct sound field component of the
sound field is predominant over the diffuse sound field component at listening positions
within the listening room, said means for coupling also coupling said center sound
channel, as said fourth input signal, to said loudspeaker means, said means for coupling
said center sound channel to said loudspeaker means including means for re-equalizing
said center sound channel to compensate for said large auditorium equalization.
9. The system of claim 7 wherein said additional loudspeaker means includes first
and second additional loudspeakers or groups of loudspeakers and wherein said means
for coupling said surround-sound channel further includes means for deriving two sound
channels from said surround-sound channel, which, when reproduced by said first and
second additional loudspeakers or groups of loudspeakers located in their operating
positions with respect to the listening room, generate first and second surround-sound
fields having low-interaural cross-correlation with respect to each other at listening
positions within the listening room and said means for coupling couples said two sound
channels to said first and second surround-sound channel loudspeakers or groups of
loudspeakers.
10. The system of claim 9 wherein said means for deriving two sound channels from
said surround-sound channel includes means for shifting the pitch of said two sound
channels with respect to each other.
11. The system of claims 7, 8, 9 or 10 wherein said means for re-equalizing comprises
a circuit having a transfer characteristic of a low-pass filter with a shelving response
such that its characteristic response is relatively flat up to about 4 to 5 kHz, rolls
off between about 4 to 5 kHz and about 10 kHz, and is relatively flat above about
10 kHz.
12. The system of claim 11 wherein said characteristic response, subject to a tolerance
of about ±1 dB up to about 10 kHz and about ±2 dB from about 10 kHz to 20 kHz, is:
Hz |
dB |
20 |
0 |
100 |
0 |
500 |
0 |
1K |
0 |
2K |
-0.2 |
3K15 |
-0.4 |
4K |
-0.7 |
5K |
-1.1 |
6K3 |
-1.8 |
8K |
-2.8 |
10K |
-4.2 |
12K5 |
-5.2 |
16K |
-5.4 |
20K |
-5.7 |
13. The system of claim 11 wherein said means for coupling said surround channel to
said additional loudspeaker means including means for equalizing the surround channel
to compensate for the listener perceived difference in timbre between a direct sound
field and a diffuse sound field.
14. The system of claim 13 wherein said means for equalizing the surround channel
comprises a circuit having a transfer characteristic substantially implementing the
inverse of the response curve defining the amount by which the sound pressure level
in a direct sound field exceeds that in a diffuse sound field for equal loudness.
15. The system of claim 14 wherein the said response curve is defined by the international
standard of ISO 454-1975(E).
16. The system of claims 7, 8, 9, or 10 wherein said means for coupling said surround
channel to said additional loudspeaker means including means for equalizing the surround
channel to compensate for the listener perceived difference in timbre between a direct
sound field and a diffuse sound field.
17. The system of claim 16 wherein said means for equalizing the surround channel
comprises a circuit having a transfer characteristic substantially implementing the
inverse of the response curve defining the amount by which the sound pressure level
in a direct sound field exceeds that in a diffuse sound field for equal loudness.
8. The system of claim 17 wherein the said response curve is defined by the international
standard of ISO 54-1975(E).
19. A surround-sound system for reproducing pre-recorded multiple sound channels,
including left, right, and surround-sound channels, in a listening room, comprising
loudspeaker means for generating, when located in its or their operating positions
with respect to the listening room, in response to first and second input signals,
first and second sound fields in which the direct sound field component of each sound
field is predominant over the diffuse sound field component at listening positions
within the listening room,
means for coupling said left and right sound channels, as said first and second input
signals, to said loudspeaker means,
additional loudspeaker means for generating, when located in its or their operating
positions with respect to the listening room, in response to a third input signal,
a third sound field in which the diffuse sound field component is predominant over
the direct sound field component at listening positions within the listening room,
and
means for coupling said surround-sound channel, as said third input signal, to said
additional loudspeaker means, said means for coupling said surround channel to said
additional loudspeaker means including means for equalizing the surround channel to
compensate for the listener perceived difference in timbre between a direct sound
field and a diffuse sound field.
20. The system of claim 19 wherein said means for equalizing the surround channel
comprises a circuit having a transfer characteristic substantially implementing the
inverse of the response curve defining the amount by which the sound pressure level
in a direct sound field exceeds that in a diffuse sound field for equal loudness.
21. The system of claim 20 wherein the said response curve is defined by the international
standard of ISO 454-1975(E).
22. The system of claims 19, 20, or 21 wherein the system is for reproducing said
pre-recorded multiple sound channels in a relatively small listening room, such as
in a home, and wherein said left and right sound channels are equalized for playback
in a large auditorium whose room-loudspeaker system is aligned to a response curve
having a high-frequency roll off, said means for coupling said left and right sound
channels to said loudspeaker means including means for re-equalizing said left and
right sound channels to compensate for said large auditorium equalization.
23. The system of claim 22 wherein said means for re-equalizing comprises a circuit
having a transfer characteristic of a low-pass filter with a shelving response such
that its characteristic response is relatively flat up to about 4 to 5 kHz, rolls
off between about 4 to 5 kHz and about 10 kHz, and is relatively flat above about
10 kHz.
24. The system of claim 23 wherein said characteristic response, subject to a tolerance
of about ±1 dB up to about 10 kHz and about ±2 dB from about 10 kHz to 20 kHz, is:
Hz |
dB |
20 |
0 |
100 |
0 |
500 |
0 |
1K |
0 |
2K |
-0.2 |
3K15 |
-0.4 |
4K |
-0.7 |
5K |
-1.1 |
6K3 |
-1.8 |
8K |
-2.8 |
10K |
-4.2 |
12K5 |
-5.2 |
16K |
-5.4 |
20K |
-5.7 |
25. The system of claim 22 wherein said additional loudspeaker means includes first
and second additional loudspeakers or groups of loudspeakers and wherein said means
for coupling said surround-sound channel further includes means for deriving two sound
channels from said surround-sound channel, which, when reproduced by said first and
second additional loudspeakers or groups of loudspeakers located in their operating
positions with respect to the listening room, generate first and second surround-sound
fields having low-interaural cross-correlation with respect to each other at listening
positions within the listening room and said means for coupling couples said two sound
channels to said first and second surround-sound channel loudspeakers or groups of
loudspeakers.
26. The system of claim 25 wherein said means for deriving two sound channels from
said surround-sound channel includes means for shifting the pitch of said two sound
channels with respect to each other.
27. The system of claims 19, 20, or 21 wherein said additional loudspeaker means includes
first and second additional loudspeakers or groups of loudspeakers and wherein said
means for coupling said surround-sound channel further includes means for deriving
two sound channels from said surround-sound channel, which, when reproduced by said
first and second additional loudspeakers or groups of loudspeakers located in their
operating positions with respect to the listening room, generate first and second
surround-sound fields having low-interaural cross-correlation with respect to each
other at listening positions within the listening room and said means for coupling
couples said two sound channels to said first and second surround-sound channel loudspeakers
or groups of loudspeakers.
28. The system of claim 27 wherein said means for deriving two sound channels from
said surround-sound channel includes means for shifting the pitch of said two sound
channels with respect to each other.
29. The system of claim 19 wherein said surround-sound system is also for reproducing
a center sound channel, said loudspeaker means generating, when located in its or
their operating positions with respect to the listening room, in response to a fourth
input signal a fourth sound field in which the direct sound field component of the
sound field is predominant over the diffuse sound field component at listening positions
within the listening room, and said means for coupling also coupling said center sound
channel, as said fourth input signal, to said loudspeaker means.
30. The system of claim 29 wherein the system is for reproducing said pre-recorded
multiple sound channels in a relatively small listening room, such as in a home, and
wherein said left, center, and right sound channels are equalized for playback in
a large auditorium whose room-loudspeaker system is aligned to a response curve having
a high-frequency roll off, said means for coupling said left, center, and right sound
channels to said loudspeaker means including means for re-equalizing said left, center,
and right sound channels to compensate for said large auditorium equalization.
31. A method for reproducing pre-recorded multiple sound channels, including left,
right, and surround-sound channels, in a relatively small listening room, such as
in a home, wherein said left and right sound channels are equalized for playback in
a large auditorium whose room-loudspeaker system is aligned to a response curve having
a high-frequency roll off, comprising
re-equalizing said left and right sound channels to compensate for said large auditorium
equalization,
generating, in response to the re-equalized left and right sound channels, first and
second sound fields at listening positions within the listening room, and
generating, in response to said surround-sound channel, a third sound field at listening
positions within the listening room.
32. A method for reproducing pre-recorded multiple sound channels, including left,
right, and surround-sound channels, in a relatively small listening room, such as
in a home, wherein said left and right sound channels are equalized for playback in
a large auditorium whose room-loudspeaker system is aligned to a response curve having
a high-frequency roll off, comprising
re-equalizing said left and right sound channels to compensate for said large auditorium
equalization,
generating, in response to the re-equalized left and right sound channels, first and
second sound fields at listening positions within the listening room, and
generating, in response to said surround-sound channel, third and fourth sound fields
having low-interaural cross-correlation with respect to each other at listening positions
within the listening room.
33. A method for reproducing pre-recorded multiple sound channels, including left,
right, and surround-sound channels, in a relatively small listening room, such as
in a home, wherein said left and right sound channels are equalized for playback in
a large auditorium whose room-loudspeaker system is aligned to a response curve having
a high-frequency roll off, comprising
re-equalizing said left and right sound channels to compensate for said large auditorium
equalization,
generating, in response to the re-equalized left and right sound channels, first and
second sound fields in which the direct sound field component of each sound field
is predominant over the diffuse sound field component at listening positions within
the listening room, and
generating, in response to said surround-sound channel, a third sound field in which
the diffuse sound field component is predominant over the direct sound field component
at listening positions within the listening room.
34. The method of claim 33, the method further comprising equalizing the surround
channel to compensate for the listener perceived difference in timbre between a direct
sound field and a diffuse sound field.
35. A method for reproducing pre-recorded multiple sound channels, including left,
right, and surround-sound channels, in a relatively small listening room, such as
in a home, wherein said left and right sound channels are equalized for playback in
a large auditorium whose room-loudspeaker system is aligned to a response curve having
a high-frequency roll off, comprising
re-equalizing said left and right sound channels to compensate for said large auditorium
equalization,
generating, in response to the re-equalized left and right sound channels, first and
second sound fields in which the direct sound field component of each sound field
is predominant over the diffuse sound field component at listening positions within
the listening room, and
generating, in response to said surround-sound channel, third and fourth sound fields
in which the diffuse sound field component of each sound field is predominant over
the direct sound field component at listening positions within the listening room
and in which the sound fields have low-interaural cross-correlation with respect
to each other at listening positions within the listening room.
36. The method of claim 35, the method further comprising equalizing the surround
channel to compensate for the listener perceived difference in timbre between a direct
sound field and a diffuse sound field.
37. A method for reproducing pre-recorded multiple sound channels, including left,
right, and surround-sound channels, in a listening room, comprising
generating, in response to said left and right sound channels, first and second sound
fields in which the direct sound field component of each sound field is predominant
over the diffuse sound field component at listening positions within the listening
room,
equalizing the surround channel to compensate for the listener perceived difference
in timbre between a direct sound field and a diffuse sound field, and
generating, in response to the equalized surround-sound channel, a third sound field
in which the diffuse sound field component is predominant over the direct sound field
component at listening positions within the listening room.
38. The method of claim 37 wherein the method is for reproducing said pre-recorded
multiple sound channels in a relatively small listening room, such as in a home, and
wherein said left and right sound channels are equalized for playback in a large auditorium
whose room-loudspeaker system is aligned to a response curve having a high-frequency
roll off, the method further comprising re-equalizing said left and right sound channels
to compensate for said large auditorium equalization.
39. A method for reproducing pre-recorded multiple sound channels, including left,
right, and surround-sound channels, in a listening room, comprising
generating, in response to said left and right sound channels, first and second sound
fields in which the direct sound field component of each sound field is predominant
over the diffuse sound field component at listening positions within the listening
room,
equalizing the surround channel to compensate for the listener perceived difference
in timbre between a direct sound field and a diffuse sound field,
generating, in response to the equalized surround-sound channel, third and fourth
sound fields in which the diffuse sound field component of each sound field is predominant
over the direct sound field component at listening positions within the listening
room and in which the sound fields have low-interaural cross-correlation with respect
to each other at listening positions within the listening room.
40. The method of claim 39 wherein the method is for reproducing said pre-recorded
multiple sound channels in a relatively small listening room, such as in a home, and
wherein said left and right sound channels are equalized for playback in a large auditorium
whose room-loudspeaker system is aligned to a response curve having a high-frequency
roll off, the method further comprising re-equalizing said left and right sound channels
to compensate for said large auditorium equalization.