[0001] This invention relates to a method of processing a plural channel audio signal including
left and right channels, the information in the channels representing a three dimensional
sound-field for generation by respective left and right loudspeakers arranged at a
given distance from the preferred position of a listener in use.
[0002] The processing of audio signals to reproduce a three dimensional sound-field on replay
to a listener having two ears has been a goal for inventors for many years. One approach
has been to use many sound reproduction channels to surround the listener with a multiplicity
of sound sources such as loudspeakers. Another approach has been to use a dummy head
having microphones positioned in the auditory canals of artificial ears to make sound
recordings for headphone listening. An especially promising approach to the binaural
synthesis of such a sound-field has been described in EP-B-0689756, which describes
the synthesis of a sound-field using a pair of loudspeakers and only two signal channels,
the sound-field nevertheless having directional information allowing a listener to
perceive sound sources appearing to lie anywhere on a sphere surrounding the head
of a listener placed at the centre of the sphere.
[0003] The goal of researchers developing and studying the synthesis of 3D sound-fields
from conventional two speaker systems has been to provide for complete and effective
transaural crosstalk cancellation.
[0004] The fundamental Head Response Transfer Function (HRTF) characteristics which are
required to implement a transaural crosstalk cancellation scheme are the left- and
right-ear transfer functions associated with the azimuth angle at which the loudspeakers
are situated (Figure 1). For most applications, this is commonly accepted to be ±30°.
The near-ear function is sometimes referred to as the "same" side function (or "
S" function), and the far-ear function as the "alternate" (or "
A") function. These
A and
S characteristics form the basis of all transaural crosstalk cancellation schemes (Figure
2). Transaural crosstalk cancellation is described in more detail in WO 95/15069.
The
A and
S functions are combined to form filter blocks of the form:
(where
C = (-
A/S) ), and:
These terms are often compounded together and simplify to form:
It is not possible to obtain reliable measurements of HRTF data (A and S) at low
frequencies for several reasons, including the following.
1. Poor LF response of measurement actuator (loudspeaker).
In practise, it is known to make measurements from an artificial head in order to
derive a library of HRTF data. It is common practise to make these measurements at
distances of 1 metre or thereabouts, for several reasons. Firstly, the sound source
used for such measurements is, ideally, a point source, and usually a loudspeaker
is used. However, there is a physical limit on the minimum size of loudspeaker diaphragms.
Typically, a diameter of several inches is as small as is practical whilst retaining
the power capability and low-distortion properties which are needed. Hence, in order
to have the effects of these loudspeaker signals representative of a point source,
the loudspeaker must be spaced at a distance of around 1 metre from the artificial
head. (As it is often required to create sound effects for PC games and the like which
possess apparent distances of several metres or greater, and so, because there is
little difference between HRTFs measured at 1 mere and those measured at much greater
distances, the 1 metre measurement is used.) However, loudspeakers of this size and
configuration possess very poor LF performance, and their LF response begins to fail
at frequencies of around 200 Hz and below.
2. Poor LF response of measurement sensor (microphone in artificial head).
3. DC offsets in instrumentation.
It is not uncommon to find spurious DC level offsets of 5 - 10 mV in digital tape
recorders and other instruments used in HRTF measurements. (A DC offset corresponds
directly to a gain error at 0 Hz.)
4. Wind pressure artefacts.
In an anechoic measurement chamber, external wind pressure can cause significant pressure
fluctuations within the chamber, giving rise to substantially large data offsets.
Consequently, it is convenient to filter off the LF components of the HRTF signals
prior to recording them, thus making the mid and high frequency information reliable
and reproducible, but at the expense of loss of LF data.
5. Standing waves.
Even in an anechoic chamber, residual reflected energy can combine to cause standing
waves. and these are most apparent at long wavelengths, hence procedures used for
(4), above are doubly useful.
6. Impulse measurement method
HRTFs are measured by means of impulse responses, and this measurement does not provide
LF data, because there is insufficient energy in the transient impulse below around
200 Hz. Even when a "stretch" pulse method is used, this is still the case.
7. Time domain windowing
When measuring HRTFs, it is essential to "window" the measured impulses in the time
domain to a period of several milliseconds in order to eliminate incorporating reflected
waves into the measurement (even in an anechoic chamber), and this cuts off the spectrum
of the resultant data, again, below around 200 Hz.
[0005] As a consequence, HRTFs measured by the prior art methods do not contain LF information,
although, of course, the LF response is present in reality. The results of a typical
HRTF measurement are shown in Figure 3, depicting the
A and
S functions at 30° azimuth, measured from a commercial artificial head. The uncertainty
in the non-valid data, below several hundred Hz, is apparent. Accordingly, the missing
LF properties must be replaced in order to create valid HRTFs, and this is conveniently
done by extrapolating the amplitude data at the lowest valid frequency (200 Hz) back
to 0 Hz (or in practise, back to the lowest practical frequency, say 10 Hz). However,
although the LF amplitude data do not contain a great deal of "detail" (unlike the
HF characteristics), and therefore it might be supposed that back-extrapolation might
be simple, it is not entirely straightforward. This is because the HRTF curves are
not flat at the lowest valid frequency, but still curving, and the near- and far-ear
characteristics exhibit slightly differently shaped curves. Consequently, one must
make an intelligent estimate of the y-axis intercept, and extrapolate both curves
accordingly, as is shown in Figure 4. Any LF errors can create significant quality
problems, as low-frequency artefacts are very noticeable in high quality audio applications,
often termed "phase errors". For this reason any LF errors in the processing must
be avoided), and so in practice both near- and far-ear characteristics of the HRTF
are extrapolated to the same value at low frequencies.
[0006] Prior art transaural crosstalk cancellation methods have always used
A and
S functions which tend to the same value at low frequencies (see for example, Atal
and Schroeder, US 3,236,949). Using such functions, the anticipated crosstalk signal
at the far ear is equal to the primary signal at the near ear at low frequencies,
hence the ratio of crosstalk signal to primary signal is always 1:1 at low frequencies.
[0007] In WO 95/15069, it is disclosed that partial transaural crosstalk cancellation using
such transfer functions can broaden the region of space over which a listener can
hear full 3D sound-field effects.
[0008] According to a first aspect of the invention there is provided a method as specified
in claims 1 - 4. According to a second aspect of the invention there is provided apparatus
as specified in claim 5 and 6. According to a third aspect of the invention there
is provided an audio signal as specified in claim 7.
[0009] Embodiments of the invention will now be described, by way of example only, with
reference to the accompanying diagrammatic drawings, in which:-
Figure 1 shows a plan view of a listener, loudspeakers, and transfer functions,
Figure 2 shows a prior art transaural crosstalk cancellation scheme,
Figure 3 shows typical experimentally measured A and S functions,
Figure 4 shows prior art modified A and S functions with forced convergence below
200 Hz,
Figure 5 shows a listener with reference sphere and co-ordinate system,
Figure 6 shows a plan view of the space around the listener in the horizontal plane,
Figure 7 shows how near ear distances are calculated in the horizontal plane,
Figure 8 shows how far ear distances are calculated in the horizontal plane,
Figure 9 shows A and S functions according to the present invention, and
Figure 10 shows the transaural crosstalk cancellation factor (X) as a function of
speaker angle and distance in the horizontal plane.
[0010] The inventor of the present invention has discovered that the amount of transaural
crosstalk which actually occurs, relative to the primary signal, is dependent upon
the distance of the loudspeakers from the listener (and this distance dependency is
also a function of azimuthal position). In the present description and claims the
term "transaural crosstalk" is defined to be the intensity ratio of the far ear signal
with respect to the near ear signal. As these two functions have a different frequency
dependence, this ratio will in general be a function of frequency. However, in the
prior art the ratio approaches unity at low frequencies because A and S are forced
to the same value below about 200 Hz. That is, the transaural crosstalk signal (far
ear signal) is equal in magnitude to the primary signal (near ear signal) for such
low frequencies. Hence it can be said that in all the prior art schemes the transaural
crosstalk signal is substantially equal to (100% of) the primary signal at low frequencies,
regardless of loudspeaker distance and/or angle. Consequently, all the prior art methods
of transaural crosstalk cancellation have not been optimal for the arrangements/distances
of loudspeakers used in practice.
[0011] The invention provides a means for creating optimal transaural crosstalk cancellation
particularly, though not exclusively, for users of Personal Computer (PC) - based
multimedia systems, in which the loudspeakers are relatively dose to the listener
and might be at a variety of differing angles and distances, depending on the individual
user's set-up configuration and preferences. The amount of transaural crosstalk which
occurs is also influenced by the angle of the loudspeakers. (Note that this is not
to be confused with the use of the appropriate azimuth angle
A and
S functions, which is well known: i.e. use 30° A and S functions for speakers at 30°;15°
A and S functions for speakers at 15°, and so on).
[0012] This realisation enables the precise calculation of the relative transaural crosstalk
intensity which occurs for any given loudspeaker distance and angle, which result
can in turn be used to control the amount of transaural crosstalk cancellation which
is implemented. WO 95/15069 described a method which provided improved transaural
crosstalk cancellation by reducing the amount of cancellation to around 95% for hi-fi
distanced loudspeakers. This was based on subjective testing at a speaker distance
of 2.5 metres, in that this value provided the best audio results in critical listening
tests, but it was not clear at the time exactly why this should be so. The present
discovery explains this earlier result, and in fact shows that the theoretical optimum
value (see Table 1) for the above testing was 94%, thus confirming the subjective
assessment that "about 95% cancellation" was best.
[0013] It is standard procedure (as described in WO/15069) to use
A and
S functions which are artificially "forced" to converge at several hundred Hz and be
virtually identical at frequencies below this value. As a result of this, as noted
before, the anticipated transaural crosstalk signal will be equal to the primary signal
at low frequencies, resulting in "100%" transaural crosstalk cancellation. Such 100%
transaural crosstalk cancellation would be appropriate for loudspeakers which are
infinitely distant. It is also a reasonably good approximation for speakers which
are several metres away from the listener, such as in a conventional hi-fi arrangement.
However, at distances closer than several metres, a 100% cancellation signal is significantly
excessive and therefore the transaural crosstalk is "overcancelled": it not cancelled
properly. Hence the overall effectiveness is reduced. This has not previously been
recognised.
[0014] The present invention is a transaural crosstalk cancellation means based on "standard",
1 metre
A and
S functions. The method employs an algorithm which controls the intensity of the transaural
crosstalk cancellation signal relative to the near-ear intensity, using a crosstalk
cancellation factor which is a function of loudspeaker proximity and spatial position.
The invention is based on the observation that when a sound source moves relatively
closely towards the head (say, from a distance of several metres), then the individual
far- and near-ear properties of the HRTF do not change a great deal in terms of their
spectral properties, but their amplitudes, and the amplitude difference between them,
do change substantially, caused by a distance ratio effect.
[0015] For practical reasons, it is useful to consider the typical range of loudspeaker
position angles and distances representative of present multimedia loudspeaker configurations.
Such loudspeaker azimuthal angles lie in the range ±10° (for notebook PCs) to ±30°
(for desktop PCs), and the distances (loudspeaker to ear) range from about 0.2 metres
to 1 metre respectively. These ranges will be used here for illustrative purposes,
but of course the invention is not restricted to these parameters.
[0016] As a general illustration of the effects of using a relatively close loudspeaker,
first consider the approximate relative intensities at the far- and near-ear. When
a lateral sound source moves towards the head from, say, 1 metre distance, the distance
ratio (far-ear to sound source vs. near-ear to sound source) becomes greater. For
example, at 45° azimuth in the horizontal plane, at a distance of 1 metre from the
centre of the head, the near ear is about 0.95 metre distance and the far-ear around
1.06 metre. So the distance ratio is (0.95 / 1.06) = 0.90. When the sound source moves
to a distance of 0.5 metre, then the ratio becomes (0.45 / 0.57) = 0.79, and when
the distance is only 20 cm, then the ratio is approximately (0.16 / 0.27) = 0.59.
The intensity of a sound source diminishes with distance as the energy of the propagating
wave is spread over an increasing area. The wavefront is similar to an expanding bubble,
and hence the energy density is related to the surface area of the propagating wavefront,
which is related by a square law to the distance travelled (the radius of the bubble).
This is described in the Appendix. Hence the intensity ratios of left and right channels
are related to the ratio of the squares of the distances. Hence, the intensity ratios
for the above examples at distances of 1 m, 0.5 m and 0.2 m are approximately 0.80,
0.62 and 0.35 respectively. In dB units, these ratios are -0.97 dB, -2.08 dB and -4.56
dB respectively.
[0017] It is important to note, however, that the far ear to near ear intensity ratio differences
are position dependent. For example, if the aforementioned situation were repeated
for a frontal sound source (azimuth 0°) approaching the head, then there would be
no difference between the left and right channel intensities, by symmetry. In this
instance, the intensity level of both channels simply would increase according to
the 1/R
2 law.
[0018] Accordingly, it is desirable to derive an expression which defines the relative intensity
ratio at the far- and near-ears, caused by a local sound source, as a function of
both the distance and angular position of the source relative to the listener. As
a frame of reference, Figure 5 shows a diagram of the near space around the listener,
together with the reference planes and axes which will be referred to during the following
descriptions, in which P-P' represents the front-back axis in the horizontal plane,
intercepting the centre of the listener's head, and with Q-Q' representing the corresponding
lateral axis from left to right.
[0019] The near-ear distance can be determined, for example, by the following calculation.
Figure 6 shows a plan view of the listener's head, together with the near area surrounding
it. For the present purpose, we are interested in the front-right quadrant in order
to derive an expression for the source to near-ear distance. The situation is trivial
to resolve, as shown in Figure 7, if the "true" source-to-ear paths for the close
frontal positions (such as path "A") are assumed to be similar to the direct distance
(indicated by "B"). This simplifies the situation, as is shown on the left diagram
of Figure 7, indicating a sound source S in the front-right quadrant, at an azimuth
angle of θ degrees with respect to the listener. Also shown is the distance, d, of
the sound source from the head centre, and the distance, p, of the sound source from
the near-ear. The angle subtended by S-head_centre-Q' is (90° - θ). The near-ear distance
can be derived using the cosine rule, from triangle S-head_centre-near_ear:
If we assume the head radius, r, is 7.5 cm, then p is given by:
[0020] The far-ear distance can be determined, for example, by the following calculation.
Figure 8 shows a plan view of the listener's head, together with the near-field area
surrounding it. Once again, we are particularly interested in the front-right quadrant.
However, the path between the sound source and the far-ear comprises two serial elements,
as is shown dearly in the right hand detail of Figure 8. First, there is a direct
path from the source, S, tangentially to the head, labelled q, and second, there is
a circumferential path around the head, C, from the tangent point to the far-ear.
As before, the distance from the sound source to the centre of the head is d, and
the head radius is r. The angle subtended by the tangent point and the head centre
at the source is angle R.
The tangential path, q, can be calculated simply from the triangle:
...and also the angle R:
Considering the triangle S-T-head_centre, the angle P-head_centre-T is (90 - θ -
R), and so the angle T-head_centre-Q (the angle subtended by the arc itself) must
be (θ + R). The circumferential path can be calculated from this angle, and is:
Hence, by substituting (7) into (8), and combining with (6), an expression for the
total distance (in cm) from sound source to far-ear for a 7.5 cm radius head can be
calculated:
[0021] Now that working expressions for the distances to each ear from the sound source
have been established, it is possible to derive an expression which defines the distance-dependent
(and azimuth position-dependent) amount of crosstalk, relative to 100% (corresponding
to equal transaural crosstalk signal and primary signal at low frequencies, as suitable
for a distant source). As the source moves closer, the relative intensity between
the ears decreases, and so there is relatively less crosstalk. This "crosstalk factor"
(call it X) characterises the amount of transaural crosstalk relative to an infinitely
distant source, where the near-ear and far-ear signals are virtually equal in amplitude
at very low frequency (they tend to the same value at 0 Hz). Thus it is convenient
to describe the crosstalk factor, which is the ratio of (far-ear/near-ear) intensities,
as a fraction or percentage of this limiting, 100% value. This, in turn, would define
how much attenuation should be applied to the crossfeed path in a transaural crosstalk
cancellation system ("C" in Figure 2) based on conventional "infinitely distant"
A and
S functions.
[0022] Alternatively, the crosstalk cancellation factor, X, could be converted into dB units
of sound intensity, X(dB) and used to define the LF asymptote difference of an
A and
S function pair, as shown in Figure 9, which could then be used in a conventional crosstalk
cancellation scheme (for example Figure 2, corresponding to Atal and Schroeder, US
3,236,949) to the same effect. Thus the
A function LF asymptote would be set so as to lie X(dB) below the
S asymptote (because the far (A) ear is always more distant).
[0023] The crosstalk factor X is the far-ear LF intensity (I
F) expressed as a fraction of the near-ear LF intensity (I
N). The intensities are related to the distances from the source to far-ear (D
F) and near-ear (D
N) by the square law relationship (see Appendix), as follows.
From equation (5), the near-ear distance is:
And from equation (9), the far-ear distance is:
Hence the crosstalk factor X (i.e. the LF intensity ratio), as a function of the
distance from the source to the head centre, d, and source azimuth angle, θ, is as
shown below in equation 13.
This can be expressed in dB in the usual manner, thus:
[0024] It is worthwhile computing the X factor as a function of distance from the listener's
head at various azimuth angles. This has been done in the range 10 degrees to 30 degrees,
and is both tabulated in Table 1 below and depicted graphically in Figure 10, where
the X factor has been expressed as a fraction, according to equation 13.
TABLE 1
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Distance-Dependent Transaural Crosstalk Cancellation |
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(Head radius assumed to be 7.5 cm) |
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X Factor as a Ratio of Intensities (Far/Near) |
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d (cm) |
<< - Speaker angle (deg) - >> |
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10 |
12 |
14 |
16 |
18 |
20 |
22 |
24 |
26 |
28 |
30 |
20 |
0.782 |
0.745 |
0.709 |
0.675 |
0.643 |
0.612 |
0.582 |
0.554 |
0.527 |
0.501 |
0.477 |
25 |
0.818 |
0.786 |
0.755 |
0.725 |
0.697 |
0.669 |
0.642 |
0.617 |
0.592 |
0.569 |
0.546 |
30 |
0.844 |
0.816 |
0.789 |
0.762 |
0.737 |
0.712 |
0.689 |
0.666 |
0.643 |
0.622 |
0.601 |
35 |
0.864 |
0.839 |
0.815 |
0.791 |
0.768 |
0.746 |
0.725 |
0.704 |
0.684 |
0.664 |
0.645 |
40 |
0.879 |
0.857 |
0.835 |
0.814 |
0.793 |
0.773 |
0.754 |
0.735 |
0.716 |
0.698 |
0.681 |
45 |
0.892 |
0.871 |
0.852 |
0.832 |
0.813 |
0.795 |
0.777 |
0.760 |
0.743 |
0.726 |
0.710 |
50 |
0.902 |
0.883 |
0.865 |
0.847 |
0.830 |
0.813 |
0.797 |
0.781 |
0.765 |
0.750 |
0.735 |
55 |
0.910 |
0.893 |
0.876 |
0.860 |
0.844 |
0.828 |
0.813 |
0.798 |
0.784 |
0.769 |
0.755 |
60 |
0.917 |
0.901 |
0.886 |
0.871 |
0.856 |
0.841 |
0.827 |
0.813 |
0.799 |
0.786 |
0.773 |
65 |
0.923 |
0.908 |
0.894 |
0.880 |
0.866 |
0.852 |
0.839 |
0.826 |
0.813 |
0.801 |
0.789 |
70 |
0.928 |
0.915 |
0.901 |
0.888 |
0.875 |
0.862 |
0.850 |
0.837 |
0.825 |
0.814 |
0.802 |
75 |
0.933 |
0.920 |
0.907 |
0.895 |
0.883 |
0.871 |
0.859 |
0.847 |
0.836 |
0.825 |
0.814 |
80 |
0.937 |
0.925 |
0.913 |
0.901 |
0.890 |
0.878 |
0.867 |
0.856 |
0.845 |
0.835 |
0.824 |
85 |
0.940 |
0.929 |
0.918 |
0.907 |
0.896 |
0.885 |
0.874 |
0.864 |
0.854 |
0.844 |
0.834 |
90 |
0.944 |
0.933 |
0.922 |
0.912 |
0.901 |
0.891 |
0.881 |
0.871 |
0.861 |
0.852 |
0.842 |
95 |
0.947 |
0.936 |
0.926 |
0.916 |
0.906 |
0.896 |
0.887 |
0.877 |
0.868 |
0.859 |
0.850 |
100 |
0.949 |
0.939 |
0.930 |
0.920 |
0.911 |
0.901 |
0.892 |
0.883 |
0.874 |
0.866 |
0.857 |
250 |
0.979 |
0.975 |
0.971 |
0.967 |
0.963 |
0.959 |
0.955 |
0.952 |
0.948 |
0.944 |
0.940 |
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X Factor in dB |
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d (cm) |
<< - Speaker angle (deg) - >> |
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10 |
12 |
14 |
16 |
18 |
20 |
22 |
24 |
26 |
28 |
30 |
20 |
-1.067 |
-1.279 |
-1.491 |
-1.704 |
-1.919 |
-2.134 |
-2.349 |
-2.566 |
-2.783 |
-3.001 |
-3.219 |
25 |
-0.872 |
-1.046 |
-1.220 |
-1.395 |
-1.570 |
-1.746 |
-1.922 |
-2.098 |
-2.274 |
-2.449 |
-2.625 |
30 |
-0.736 |
-0.883 |
-1.030 |
-1.178 |
-1.325 |
-1.473 |
-1.621 |
-1.768 |
-1.915 |
-2.062 |
-2.208 |
35 |
-0.635 |
-0.763 |
-0.890 |
-1.017 |
-1.144 |
-1272 |
-1.399 |
-1.525 |
-1.651 |
-1.777 |
-1.902 |
40 |
-0.559 |
-0.671 |
-0.783 |
-0.894 |
-1.006 |
-1.118 |
-1.229 |
-1.340 |
-1.450 |
-1.560 |
-1.670 |
45 |
-0.499 |
-0.598 |
-0.698 |
-0.798 |
-0.897 |
-0.996 |
-1.095 |
-1.194 |
-1.292 |
-1.390 |
-1,487 |
50 |
-0.450 |
-0.540 |
-0.630 |
-0.719 |
-0.809 |
-0.898 |
-0.987 |
-1.076 |
-1.164 |
-1.252 |
-1.339 |
55 |
-0.410 |
-0.492 |
-0.573 |
-0.655 |
-0.737 |
-0.818 |
-0.899 |
-0.979 |
-1.060 |
-1.139 |
-1.218 |
60 |
-0.376 |
-0.451 |
-0.526 |
-0.601 |
-0.676 |
-0.750 |
-0.825 |
-0.898 |
-0.972 |
-1.045 |
-1.117 |
65 |
-0.348 |
-0.417 |
-0.486 |
-0.555 |
-0.624 |
-0.693 |
-0.762 |
-0.830 |
-0.897 |
-0.965 |
-1.032 |
70 |
-0.323 |
-0.387 |
-0.452 |
-0.516 |
-0.580 |
-0.644 |
-0.708 |
-0.771 |
-0.834 |
-0.896 |
-0.958 |
75 |
-0.302 |
-0.362 |
-0.422 |
-0.482 |
-0.542 |
-0.601 |
-0.661 |
-0.720 |
-0.778 |
-0.836 |
-0.894 |
80 |
-0.283 |
-0.339 |
-0.396 |
-0.452 |
-0.508 |
-0.564 |
-0.619 |
-0.675 |
-0.730 |
-0.784 |
-0.838 |
85 |
-0.266 |
-0.320 |
-0.373 |
-0.426 |
-0.478 |
-0.531 |
-0.583 |
-0.635 |
-0.687 |
-0.738 |
-0.789 |
90 |
-0.252 |
-0.302 |
-0.352 |
-0.402 |
-0.452 |
-0.501 |
-0.551 |
-0.600 |
-0.649 |
-0.697 |
-0.745 |
95 |
-0.239 |
-0.286 |
-0.334 |
-0.381 |
-0.428 |
-0.475 |
-0.522 |
-0.568 |
-0.614 |
-0.660 |
-0.706 |
100 |
-0.227 |
-0.272 |
-0.317 |
-0.362 |
-0.407 |
-0.451 |
-0.496 |
-0.540 |
-0.584 |
-0.627 |
-0.670 |
250 |
-0.091 |
-0.109 |
-0.127 |
-0.145 |
-0.163 |
-0.180 |
-0.198 |
-0.216 |
-0.233 |
-0.250 |
-0.267 |
[0025] From Table 1, the optimal X values for transaural crosstalk cancellation schemes
applicable to, say, (a) a hi-fi system, (b) a desktop PC, and (c) a laptop PC can
be ascertained, as tabulated below in Table 2.
TABLE 2
|
Speaker Distance (m) |
Speaker Angle |
X Factor |
X (dB) |
hi-fi System |
2.5 |
30° |
0.940 |
-0.267 |
Desktop PC |
0.6 |
30° |
0.773 |
-1.117 |
Laptop PC |
0.3 |
15° |
0.789 |
-1.030 |
[0026] The implementation of the invention is straightforward: the transaural crosstalk
cancellation factor X is incorporated into the filter design procedure, thus allowing
a range of different transaural crosstalk cancellation filters to be created from
standard low frequency convergent A and S functions, but with differing values of
X, for a range of speaker configurations, such that the end user can select the most
appropriate one for their particular speaker configuration. For example, after inspection
of the data shown in Table 1 it would be reasonable to create a range of filters for
X values in the range, say, 0.5 to 1.0 in 0.05 increments. These 11 filters would
cover most situations.
[0027] This is very convenient, because Microsoft's new Windows98 (trademark) operating
system includes the provision to select about a dozen different loudspeaker set-ups.
The present invention would fit into this system easily, allowing the user to specify
(a) the separation between speakers, and (b) the distance from head to speaker centre-line,
for example, and then the software could select the optimal transaural crosstalk filtering
arrangements.
[0028] In principle, as an alternative to the above method, it is possible to make
A and
S measurements at differing distances, say 1 metre, 0.9 metre, 0.8 metre and so on,
and create different crosstalk filters for these differing distances and for different
loudspeaker configurations. This would "build-in" the correct amount of transaural
crosstalk cancellation. However, the same problems would exist in attempting to work
out what exactly the low-frequency characteristics of
A and
S were. Also, as already noted above, such close measurements are compromised by the
loudspeaker diaphragm dimensions which depart from point-source properties at these
distances, and so it is not possible to make accurate measurements doser than about
0.8 metre.
[0029] A further disadvantage of this alternative approach is that it would require many
measurements at different distances and angles, and would result in quantised-distance
effects: an optimum value could not be calculated and easily be provided for all loudspeaker
configurations. The present invention allows both distance and angle parameters to
be used to calculate a single crosstalk cancellation factor, from which an associated
filter is selected, based on accurate, 1 metre measurement.
[0030] The above description has been related to loudspeakers which lie in the horizontal
plane of the listener: this has been for illustrative purposes only, and the invention
is not limited horizontal-plane loudspeaker configurations. The principles described
above are equally applicable to loudspeakers which do not lie on the horizontal plane,
and the equations may be re-formatted accordingly.
[0031] Finally, the content of the accompanying abstract is hereby incorporated into this
description by reference.