[0001] This invention relates to a loudspeaker line examination system for examining whether
there are any problems such as line breakage or short-circuiting in loudspeaker lines
in a public address system built in a building or the like.
[0002] An example of prior examination systems of the above-described type is disclosed
in Patent Literature 1. In a public address system according to Patent Literature
1, a plurality of loudspeakers are connected to a loudspeaker line in parallel with
each other, and a power amplifier is connected to the loudspeaker line. An audio signal
and a test signal are combined in a stage preceding a power amplifier, and the power
amplifier amplifies the resultant composite signal and applies it to the loudspeaker
line. The test signal is a signal at a constant voltage. A detecting circuit is disposed
in the output of the power amplifier, which includes a filter deriving test signal
current flowing to respective loudspeakers through the loudspeaker line. Since the
voltage of the test signal is constant, an output signal of the filter represents
a composite impedance of the loudspeaker line and the respective loudspeakers. In
the loudspeaker line examination system disclosed in Patent Literature 1, the value
of the output signal of the filter is compared with a threshold value for use in detecting
line breakage and a threshold value for use in detecting short-circuiting, to judge
whether line breakage or short-circuiting has occurred. The examination system uses,
as a reference value, the value of the filter output signal developed when the loudspeaker
line operates properly, and uses a value resulting from adding a first predetermined
value to the reference value as the line breakage detection threshold value, and a
value resulting from subtracting a second predetermined value from the reference value
as the short-circuiting detection threshold value.
[0004] As described, the background art examination system determines the two threshold
values, using the value of the output signal of the filter developed when the loudspeaker
line is in the proper operating state, and, therefore, in order to detect line breakage
and short-circuiting with high accuracy, these threshold values must be set accurately.
However, sometimes there is a large difference between the output signal value measured
by the described examination system for determination of the threshold values, and
the output signal value measured thereafter in the normal operating state in which
there is no loudspeaker line breakage or short-circuiting occurred. For example, there
may be a difference between the output signal value measured when only the test signal
is supplied to the loudspeaker line, and the output signal value measured in the normal
use of the public address system in which the audio signal at various frequencies
and the test signal are supplied to the loudspeaker. The difference in value is significant
when the level of the audio signal is larger than the level of the test signal. Accordingly,
the examination system of the background art may make an erroneous judgment as if
there were line breakage or short-circuiting, while the loudspeaker line is in the
proper operating conditions, which erroneous judgment is caused by accurate setting
of the first and second values.
[0005] An object of the present invention, therefore, is to provide an examination system
which can make accurate detection of line breakage or impedance decrease in a loudspeaker
line, with such erroneous judgment minimized as much as possible.
[0006] According to the present invention there is provided a loud speaker line examination
system as defined in claim 1.
[0007] The test signal may be an analog signal, or an analog signal resulting from converting
a digital signal by a digital-to-analog converter. The test signal is combined with
the audio signal in a combiner, and the resultant composite is supplied to the amplifier.
Thus, the amplified audio and test signals are supplied to the respective loudspeakers
through the loudspeaker line. Impedance determining means derives the test signal
component contained in the output signal of the amplifier and determines the impedance
viewed from the output of the amplifier toward the respective loudspeakers, based
on the derived test signal component. For example, the impedance determining means
can determine one or more impedances by deriving a voltage and current of the test
signal contained in the amplifier output. Alternatively, the impedance determining
means may perform frequency analysis of the amplifier output as described later, to
derive a frequency component of the test signal, to thereby determine one or more
impedances corresponding to the frequency of the test signal. Judging means compares
the impedance determined by the impedance determining means with a predetermined threshold
value and judges the presence of at least one of line breakage and impedance decrease
in the loudspeaker line. For example, when the line breakage detection threshold value
is used, it is judged that line breakage of the loudspeaker line or inadequate connection
of some loudspeaker has occurred if the measured impedance is larger than the line
breakage threshold value. When the impedance decrease detection threshold value is
used, it is judged that impedance decrease has occurred in the loudspeaker line if
the determined impedance is lower than the impedance decrease detection threshold
value. It can be arranged that both the line breakage detection threshold value and
the impedance decrease detection threshold value may be prepared so as to enable judgment
of both. When the ratio of the resultant composite signal to the test signal is larger
than a predetermined value, and the resultant composite signal is increasing, threshold
revising means revises the threshold value in the direction to lower the degree of
accuracy of judgment made in the judging means. The threshold value is set based on
the impedance measured, while only the test signal is being supplied from the amplifier
to the loudspeaker line.
[0008] The ratio of the composite signal to the test signal being large means that the proportion
of the audio signal in the composite signal is large, and that the impedances of the
loudspeaker line and the loudspeakers are under the influence of the audio signal.
Therefore, erroneous judgment may result if the current threshold value is used, which
is the reason why the threshold value is revised.
[0009] When the tendency of the composite signal to increase changes to the tendency to
decrease, the threshold revising means may raise the degree of judgment accuracy which
was lowered when the composite signal was increasing. In this case, the rate of change
in the direction to lower the judgment accuracy is larger, and the rate of change
in the direction to raise the judgment accuracy is smaller.
[0010] When the impedance of the loudspeaker line and the loudspeakers changes due to, for
example, increase of the level of the audio signal, some time period may be necessary
for the impedance to return to the original level from the level to which they changed,
even when the level of the audio signal decreases. In order to cope with such situation,
a smaller rate of change in the direction to raise the degree of judgment accuracy
is employed.
[0011] A loudspeaker line examination system according to an example which does not fall
in the scope of protection is also installed in a public address system of the same
arrangement as the one described with reference to the previous embodiment. The examination
system includes a test signal source, too, but the test signal contains both frequencies
near the lowest and highest frequencies of the human audio frequency band. The test
signal is combined with the audio signal in a combiner, and the resultant composite
signal is supplied to the amplifier. The impedance determining means derives the two
frequency components of the test signal contained in the output signal of the amplifier,
and determines, based on the derived frequency components, the impedance viewed from
the output of the amplifier toward the loudspeakers. This impedance determining means
is similar to the impedance determining means described with respect to the previous
embodiment. Judging means compares the determined impedance with a predetermined threshold
value and judges that the loudspeaker line and the loudspeakers have been open-circuited
and/or that the impedance has decreased.
[0012] Since the test signal contains different frequency components, and the impedances
are determined based on these two frequency components, with these impedances being
compared with the threshold value, it is possible to judge, with a higher degree of
accuracy, at least one of open-circuiting of the loudspeaker line and the loudspeakers
and impedance decrease.
[0013] In the above-described embodiment and example, the impedance determining means may
determine the impedance in a time period during which the audio signal source stops
operating. In such case, threshold setting means sets the threshold value based on
the determined impedance. Second judging means judges whether the determined impedance
is within a predetermined allowable range.
[0014] The impedances of the loudspeaker line and loudspeakers change with time. Accordingly,
if the threshold value set on the basis of the impedances of the loudspeaker line
and loudspeakers determined at a certain time is continuously used, a difference may
be arisen between the actual impedances of the loudspeaker line and loudspeakers and
the impedance of the loudspeaker line and loudspeakers determined for use in determining
the threshold value. Therefore the impedance of the loudspeaker line and loudspeakers
is determined during a time period during which no audio signal is supplied, and the
threshold value is set on the basis of the thus determined impedance, in order to
avoid erroneous judgment. Furthermore, by using the second judging means to judge
whether the thus determined impedance Z is within the allowable range or not, it is
possible to know when the loudspeakers should be replaced.
[0015] In the above-described embodiment and example, the impedance determining means may
include current detecting mean for detecting current flowing through the loudspeaker
line, and voltage detecting means for detecting a voltage applied to the loudspeaker
line. In such case, frequency component detecting means detects the frequency components
of the test signal contained in the detected current and in the detected voltage.
The detection of the frequency components may be done by the cross-spectrum analysis
of the detected current and voltage, for example. Operating means computes the impedance
from the detected test signal frequency components.
[0016] Since the impedance of the loudspeaker line and loudspeakers is measured based on
the test signal components as described above, the impedance of the loudspeaker line
and loudspeakers can be measured without being affected by the audio signal.
[0017] In order that the invention may be well understood, the will now be described some
embodiments thereof given by way of example, reference being made to the accompanying
drawings, in which:
FIGURE 1 is a block diagram of a public address system with a loudspeaker examination
system according to a first embodiment of the invention.
FIGURE 2 is a flow chart of frequency analyzing processing performed by a DSP of the
examination system of FIGURE 1.
FIGURE 3 is a flow chart of root-mean-square value measuring processing performed
by the DSP of the examination system of FIGURE 1.
FIGURE 4 is a flow chart of short-circuiting, line breakage and impedance increase
judging processing performed by the DSP of the examination system of FIGURE 1.
FIGURE 5 is a flow chart of threshold revising processing performed by the DSP of
the examination system of FIGURE 1.
FIGURE 6 is a detailed flow chart of Zopen and Zinc revising processing in the threshold
revising processing of FIGURE 5.
FIGURE 7 is an illustration of change a degree of measurement accuracy Ra as revised
according to the processing of FIGURE 6.
FIGURE 8 is a detailed flow chart of Z1open and Z1inc computation in the Zopen and
Zinc revising processing of FIGURE 6.
FIGURE 9 is a flow chart of aging judging processing performed by the DSP of the examination
system of FIGURE 1.
Best Mode for Carrying out the Invention
[0018] An examination system according to one embodiment of the present invention is embodied
in a public address system like the one shown in FIGURE 1. The public address system
is a system for announcing in various places in, for example, a large-scale store.
The public address system includes a signal source 2 providing an audio signal. The
signal source 2 may be, for example, a sound source for providing background music
over the store, or a microphone through which information about the store and emergency
announcement is given. The audio signal from the signal source 2 is applied through
a notch filter 3 to an amplifier, e.g. a power amplifier 4, where the audio signal
is amplified, and applied to a plurality of loudspeakers 8 through a loudspeaker line
6 connected to the output of the power amplifier 4. The notch filter 3 is used to
attenuate those frequency components of the audio signal which are the same as frequency
components of a later-described test signal for the purpose of avoiding interference
with the test signal. Accordingly, a circuit arrangement may be employed in which
the audio signal is inputted to the amplifier through the notch filter 3 only when
the test signal is being outputted, and is inputted to the amplifier without passing
through the notch filter 3 while the test signal is not being outputted. In place
of the notch filter 3, a low-pass filter and/or a high-pass filter may be used. The
loudspeakers are disposed at various locations in the store. In FIGURE 1, although
only one loudspeaker line 6 is shown, the loudspeaker line 6 is actually composed
of a pair of lines. The loudspeakers 8 are actually connected between the pair of
loudspeaker lines 6 in parallel with each other.
[0019] The examination system includes a DSP (digital signal processor) 10 functioning as
a signal source of the test signal at an inaudible frequency. The DSP 10 provides
a digital test signal as the test signal. The digital test signal is converted to
an analog test signal in a D/A (digital-to-analog) converter 12. The analog test signal
and the audio signal from the signal source 2 are combined in a combiner 13. The resultant
composite signal from the combiner 13 is applied to the power amplifier 4. The analog
test signal is a signal containing two frequency components at, for example, 40 Hz
and 20 KHz, and has a constant voltage value. Generally, the human audio frequency
band is from 20 Hz to 20 KHz. The loudspeakers 8 are so designed as to give optimum
sound in this human audio frequency band. The test signal is used for the purpose
of measuring a composite impedance of the loudspeaker line and loudspeakers 8 connected
in parallel to the loudspeaker line. Accordingly, although the frequency of the test
signal desirably is within the audio frequency band, it is not desirable for the test
signal components in the resultant signal, which results from combining the test signal
with the audio signal, to be delivered as noise to human ears. Then, it is desirable
to use, as the frequency of the test signal, either one or both of a frequency near
the lowest frequency or a frequency near the highest frequency within the audio frequency
band which is or are hard for human ears to sense. The loudspeakers 8 are supplied
with the audio signal and the test signal as amplified in the power amplifier 4. The
test signal is continuously supplied to the combiner 13 from the D/A converter 12.
The audio signal is not supplied to the combiner 13 when it is not required. Alternatively,
the audio signal from the signal source 2 may be A/D (analog-to-digital) converted
before being combined with the test signal. In such case, the resultant composite
signal is applied to the D/A converter 12.
[0020] A current detecting circuit 14 is connected in series in the output of the power
amplifier 4. The current detecting circuit 14 detects the output current supplied
from the power amplifier 4 to the loudspeaker line 6. Also, a voltage detecting circuit
16 is disposed in parallel in the output of the power amplifier 4. The voltage detecting
circuit 16 detects the output voltage applied from the power amplifier 4 to the loudspeaker
line 6.
[0021] The output signal of the current detecting circuit 14 and the output signal of the
voltage detecting circuit 16 are digitized in A/D converters 18 and 20, respectively,
before being applied to the DSP 10. Hereinafter, the digitized version of the output
signal of the current detecting circuit 14 is referred to as a digital current detection
signal, and the digitized version of the output signal of the voltage detecting circuit
16 is referred to as a digital voltage detection signal.
[0022] The DSP 10 processes the digital current detection signal, the digital voltage detection
signal and the digital test signal, and judges whether the respective loudspeakers
8 and the loudspeaker line 6 are broken or short-circuited, or whether the impedance
of the loudspeakers 8 and the loudspeaker line 6 have significantly decreased. The
result of judgment is notified by a notification device 28. The notification device
may be, for example, a display device, on which the result of judgment is displayed.
[0023] In the DSP 10, each time the successively supplied digital current detection and
digital voltage detection signals are inputted to the DSP 10, the frequency analyzing
processing shown in FIGURE 2 is performed.
[0024] In the frequency analyzing processing, noise frequency components are first removed
from the digital current detection and digital voltage detection signals in a band-pass
filter (Step S2). Then, the digital current detection and digital voltage detection
signals from which noise frequency components have been removed are averaged (Step
S4). Specifically, the DSP 10 is provided therein with memories equal in number to
the digital current detection and digital voltage detection signals in one cycle of
the test signal, and each time the digital current detection and digital voltage detection
signals are supplied to the DSP 10 from the band-pass filter, they are stored in the
corresponding memories over a plurality of cycles. The stored values in the memories
are divided by the number of the plural cycles. The thus averaged digital current
detection and digital voltage detection signals are subjected to cross-spectrum analysis
to determine the correlation between the test signals contained in the digital current
detection and digital voltage detection signals, and an impedance Z1 at the frequency
of 20 KHz, an impedance Z2 at the frequency of 40 Hz, and the coherence of the digital
current detection and digital voltage detection signals in the test signal are computed
(Step S6). It should be noted that when the DSP 10 has high processing ability, Steps
S2 and S4 may be skipped, and only the cross-spectrum analysis in Step S6 is sufficient.
[0025] If it is determined from the coherence that there are many frequency components other
than the test signal frequency components, the DSP 10 raises the voltage of the constant-voltage
test signal.
[0026] Subsequent to the processing shown in FIGURE 2, the root-mean-square values Vrms
and Irms of the digital voltage detection and digital current detection signals are
computed as shown in FIGURE 3 (Step S8).
[0027] Next, as shown in FIGURE 4, using the impedances Z1 and Z2 obtained, by the above-described
cross-spectrum analysis and the root-mean-square value Irms of the digital current
detection signal, judgment is made as to whether any one of short-circuiting, decrease
in impedance (increase of output current of the power amplifier 4) and open-circuiting
has occurred in the loudspeaker line 6 and the loudspeakers 8.
[0028] First, judgment is made as to whether the root-mean-square value Irms of the digital
current detection signal is larger than a predetermined threshold value, e.g. a short-circuiting
current value Isl of the loudspeaker line, or whether the measured impedance Z1 is
smaller than a predetermined threshold value, e.g. a short-circuiting impedance Z1sl
at 20 KHz of the loudspeaker line 6 and the loudspeakers 8, and, at the same time,
the measured impedance Z2 is larger than a predetermined threshold value, e.g. a short-circuiting
impedance Z2sl at 40 Hz of the loudspeaker line 6 and the loudspeakers 8 (Step S14).
The short-circuiting current Isl and the short-circuiting impedances Z1sl and Z2sl
are predetermined in view of the protection of the loudspeaker line 6 and the loudspeakers
8. If the answer to the query in Step S14 is YES, from which it is judged that there
is short-circuiting in the loudspeaker line 6 etc., such short-circuiting is indicated
on the display device (Step S16), and this judgment processing is ended.
[0029] When the answer to the query in Step S14 is NO, judgment is made as to whether the
measured impedance Z1 is smaller than the lower limit value Z1 inc for 20 KHz or whether
the impedance Z2 is smaller than the lower limit value Z2inc for 40 Hz (Step S18).
The lower limit values Z1inc and Z2inc are explained later. When the answer to the
query in Step S18 is YES, which means that, while the loudspeaker line current has
not yet increased to the value indicating short-circuiting, the output current from
the power amplifier 4 has increased to some extent and requires some caution, current
increase is displayed (Step S20), and this judgment is ended.
[0030] If the answer to the query in Step S18 is NO, judgment is made as to whether the
measured impedance Z1 is larger than the upper limit value Z1open for 20 KHz or whether
the impedance Z2 is larger than the upper limit value Z2open for 40 Hz (Step S22).
The upper limit values Z1open and Z2open are explained later. When the answer to the
query in Step S22 is YES, from which it is judged that open-circuiting has happened
in the loudspeaker line 6 and the loudspeakers 8, open-circuiting is displayed (Step
S24), and this judgment is ended.
[0031] In making the above-described judgments, the upper limit values Z1open and Z2open
and the lower limit values Z1inc and Z2inc are used. These values are determined,
based on a reference impedance Z1 ave at 20 KHz and a reference impedance Z2ave at
40 Hz of the loudspeaker line 6 and loudspeakers 8, respectively. The reference impedances
Z1 ave and Z2ave are set by a worker when the worker initializes the public address
system on the first use after its installation, or are set by the worker when the
public address system is re-initialized for some reason. Sometimes, however, it may
happen that there are large differences between these reference impedances Z1 ave
and Z2ave and the impedances Z1 and Z2 measured afterwards in a normal condition where
there is no line breakage or short-circuiting in the loudspeaker line 6 and loudspeakers
8. For example, if the impedances Z1 and Z2 are measured during the usual operation
of the public address system, with an audio signal having various frequencies and
the test signal being supplied to the loudspeaker line 6, there is a possibility that
the measured impedance Z1 is different from the reference impedance Z1 ave and the
impedance Z2 is different from the reference impedance Z2ave. The differences in value
are significant particularly when the level of the audio signal is larger than that
of the test signal. Then, as shown in FIGURE 5, after the above-described judgment
is done, the upper limit values Z1open and Z2open and the lower limit values Z1 inc
and Z2inc, which are prepared based on the reference impedances Z1ave and Z2ave, are
subjected to revising processing.
[0032] In the revising processing, judgment is first made as to whether the root-mean-square
value Vrms of the digital voltage detection signal is larger, by a predetermined factor,
e.g. 1.2, or more, than the root-mean-square voltage value Vtest of the digital test
signal (Step S26). If the answer is YES, it is judged that many components at the
same frequencies as the test signal are contained in the audio signal. Then, the computation
processing for revising the upper limit values Z1open and Z2open and the lower limit
values Z1 inc and Z2inc is executed (Step S28). It should be noted that the predetermined
factor is not limited to 1.2.
[0033] In the revising computation processing in Step S28, a degree of measurement accuracy
Ra of the measured impedances Z1 and Z2 is used. The unit of the degree of measurement
accuracy Ra is percent (%). The smaller the value, the degree of measurement accuracy
of the impedance Z1, Z2 is higher, and the larger the value, the degree of measurement
accuracy Ra of the impedance Z1, Z2 is lower. The degree of measurement accuracy Ra
is set to the smallest value, for example, 5 %, when Vrms is equal to Vtest. As shown
in FIGURE 6, judgment is made, in the revision computation processing in Step S28,
as to whether the digital voltage detection signal Vrms is larger than the digital
voltage detection signal ΔVrms used in the previous revision computation processing
(Step S30). When the answer to the query made in Step S30 is YES, which means that
audio signal components, except the test signal, at the same frequencies as the test
signal, have increased from the previous revision computation processing, the value
of the degree of measurement accuracy Ra must be revised to a great extent. For that
purpose, computation,
is carried out. In this equation, α and β are predetermined factors, and there are
relationship between α and β, that α + β = 1 and α > β. The function f(Vrms/Vtest)
is a function with an argument Vrms/Vtest, and its value increases when the value
of Vrms/Vtest is increasing and decreases when the value of Vrms/Vtest is decreasing.
Thus, since the value of Vrms/Vtest is changing greatly and α > β, the proportion
of αf(Vrms/Vtest) in the revised degree of measurement accuracy Ra is large, and,
the revised degree of measurement accuracy Ra increases rapidly when the value of
Vrms/Vtest is increasing, as shown in the first half portion of FIGURE 7.
[0034] When the answer to the query made in Step S30 is NO, which means that audio signal
components at the same frequencies as the test signal, except the test signal, present
when the previous revision was performed, have decreased, the degree of measurement
accuracy Ra is revised to have a smaller value. The rate of change in the decreasing
direction, however, is smaller. For that purpose, a computation,
is performed. Since the value of Vrms/Vtest is smaller, f(Vrms/Vtest) is also smaller.
Since f(Vrms/Vtest) is multiplied by β, which is smaller than α, the proportion of
βf(Vrms/Vtest) in the revised Ra is small, and the value of the revised Ra gradually
decreases when the value of Vrms/Vtest is decreasing, as is seen in the latter half
portion of FIGURE 7.
[0035] Using the thus revised degree of measurement accuracy Ra, the computations of Z1open,
Z2open, Z1inc and Z2inc are performed (Step S36). For use in the next execution of
Step S30, Vrms is memorized as ΔVrms (Step S38).
[0036] The computation of Z1open and Z1inc in Step S36 is done in the manner shown in FIGURE
8. First, judgment is made as to whether the degree of measurement accuracy Ra is
larger than an impedance open-circuiting proportion initial value Rul (Step S40).
The impedance open-circuiting proportion initial value Rul is expressed in percent
(%), and is a proportion of the upper limit impedance to the reference impedance (Z1ave,
Z2ave).The upper limit impedance is the impedance at which the loudspeaker line 6
etc. can be considered to have been open-circuited, with the degree of measurement
accuracy Ra being highest, or, in other words, with Ra having the smallest value.
The impedance open-circuiting proportion initial value Rul is set by the worker at
the time of initialization or re-initialization of the system, and is used for both
Z1open and Z2open. When the answer to the query made in Step S40 is NO, it is not
necessary for the degree of measurement accuracy Ra to be increased above the impedance
open-circuiting proportion initial value Rul, and, therefore, Z1open is computed according
to
(Step S42).
[0037] If the answer to the query made in Step S40 is YES, it is necessary to revise Z1open
based on the degree of measurement accuracy Ra, and Z1open is computed (Step S44)
according to
[0038] Following Step S42 or S44, judgment is made as to whether Z1open is larger than an
upper limit value Z1ul of the impedance at 20 KHz (Step S46). The upper limit impedance
value Z1ul is the upper limit value of the impedance expected to actually occur at
20 KHz when the loudspeaker line 6 etc. are open-circuited. The upper limit value
Z1ul is manually set by the worker at the time of initialization or re-initialization
of the system. Alternatively, Z1ave measured by DSP 10 at the time of initialization
or re-initialization of the system is multiplied by a factor greater than 1, and the
resultant product is set as the upper limit value Z1ul. The reason why the judgment
in Step S46 is done is that it is sometimes possible for the value of Z1open revised
based on the degree of measurement accuracy Ra to be an impossible value. When the
answer to the query made in Step S46 is YES, Z1open is used as Z1ul (Step S48) since
it is impossible that Z1open is greater than Z1ul.
[0039] Subsequent to Step S48, or when the answer to the query made in Step S46 is NO, judgment
is made as to whether the degree of measurement accuracy Ra is larger than an impedance
increase proportion initial value RII (Step S50). The impedance increase proportion
initial value RII is a value resulting from subtracting 1 (unity) from the reciprocal
of the proportion of the reference impedance (Z1ave or Z2ave) to the impedance at
which the impedance of the loudspeaker line 6 and the loudspeakers 8, when the degree
of measurement accuracy Ra is highest, can be considered to have decreased. The impedance
increase proportion initial value Rll is expressed in percent (%). The impedance increase
proportion initial value Rll is set by the worker when the system is initialized or
re-initialized, and is used for both of Z1 inc and Z2inc. If the answer to the query
in Step 50 is NO, Z1 inc is computed (Step S52) according to
since it is not necessary to decrease the degree of measurement accuracy Ra below
Rll.
[0040] If the answer to the query made in Step S50 is YES, it is necessary to revise Z1
inc according to the degree of measurement accuracy Ra, and, therefore, Z1 inc is
computed (Step S54) according to
[0041] Subsequent to Step S52 or S54, judgment is made as to whether Z1inc is smaller than
a lower limit value Z1ll of the impedance Z1 at 20 KHz (Step S56). The impedance lower
limit value Z1ll is the lower limit value at 20 KHz at which impedance decrease is
expected to actually occur while no short-circuiting has occurred in the loudspeaker
line 6 or the loudspeakers 8. The lower limit value Z1ll is manually set by the worker
at the time of initialization or re-initialization of the system. Alternatively, the
product of 21ave measured by the DSP 10 at the time of initialization or re-initialization
of the system multiplied by a factor smaller than 1 (unity) is set as the lower limit
value Z1ll. Step S56 is executed since Z1inc revised in Step S54 sometimes takes a
value which it cannot actually take. If the answer to the query made in Step S56 is
YES, Z1inc is adopted as Z1ll (Step S58) since it is impossible for Z1inc to be smaller
than Z1ll. When the execution of Step S58 is finished or if the answer to the query
made in Step S56 is NO, the processing for computing Z1open and Z2open is ended.
[0042] By the processing similar to the ones described above, Z2open and Z2inc are computed,
using the impedance open-circuiting proportion initial value Rul, the impedance increase
proportion initial value RII, an upper limit value Z2ul of the impedance Z2 at 40
Hz, a lower limit value Z2ll of the impedance Z2 at 40 Hz, and the reference impedance
Z2ave of the impedance Z2 at 40 Hz. Description of this processing is not made.
[0043] Let it be assumed, for example, that Z2ave is 1,000 Ω, Z2ul is 2,000 Ω, Z2ll is 500
Ω, Z2sl is 20 Ω, Z1ave is 1,500 Ω, Z1ul is 3,000 Ω, Z1ll is 750 Z2, Z1sl is 30 Ω,
Isl is 5 A, Rul is 10 %, Rll is 10 %, Ra is 5 %, and Vtest is 5 V. The Ra of 5 % is
the highest degree of accuracy. A state in which Vrms is 5 V is maintained, with the
above-assumed values maintained, since Ra<Rul, Z1open and Z2open are:
Also, since Ra>Rll, Z2inc and Z1 inc are:
[0044] In this condition, if the measured impedance Z2 is 1,000 Ω and the measured impedance
Z1 is 1,500 Ω, it is judged by the processing shown in FIGURE 4 that the loudspeaker
line is in the proper state. Similarly, if the measured impedances Z2 and Z1 are 1,100
Ω and 1,500 Ω, respectively, it is judged by the processing shown in FIGURE 4 that
open-circuiting is present. If the measured impedances Z2 and Z1 are 1,100 Ω and 1,600
Ω, respectively, it is judged by the processing shown in FIGURE 4 that open-circuiting
is present. If the measured impedances Z2 and Z1 are 1,000 Ω and 1,400 Ω, respectively,
it is judged by the processing shown in FIGURE 4 that increase has occurred. If the
measured impedances Z2 and Z1 are 15 Ω and 10 Ω, respectively, it is judged by the
processing shown in FIGURE 4 that short-circuiting has occurred.
[0045] Let it be assumed that a condition in which the audio signal contains components
at the same frequencies as the test signal to some extent, in addition to the test
signal, and Vrms is larger than Vtest, for example, Vrms is 10 V, continues for some
time, resulting in rapid increase of Ra to, for example, 50 %, due to the processing
shown in FIGURE 6. Since Ra>Rul,
Also, since Ra>RII,
[0046] In this condition, if the measured impedance Z2 is 1,100 Ω and the measured impedance
Z1 is 1,600 Ω, it is judged by the processing shown in FIGURE 4 that the loudspeaker
line is in the normal state. If the measured impedances Z2 and Z1 are 2,300 Ω and
1,000 Ω, respectively, it is judged by the processing shown in FIGURE 4 that open-circuiting
has occurred. If the measured impedances Z2 and Z1 are 1,400 Ω and 600 Ω, respectively,
it is judged by the processing shown in FIGURE 4 that current has increased. If the
measured impedances Z2 and Z1 are 15 Ω and 10 Ω, respectively, it is judged by the
processing shown in FIGURE 4 that short-circuiting has occurred.
[0047] Let it be assumed that a condition in which the audio signal contains, in addition
to the test signal, a large amount of components at the same frequencies as the test
signal, and Vrms is significantly larger than Vtest, for example, Vrms is 30 V, continues
for some time, resulting in rapid increase of Ra to, for example, 300 %, due to the
processing shown in FIGURE 6. Since Ra>Rul,
However, since Z2open>Z2ul and Z1open>Z1ul, Z2open and Z1open are changed to:
Also, since Ra>RII,
Since Z2inc<Z2II and Z1inc<Z1II, Z2inc ad Z1 inc are changed to:
[0048] In this condition, if the measured impedance Z2 is 1,000 Ω and the measured impedance
Z1 is 1,600 Ω, it is judged by the processing shown in FIGURE 4 that the loudspeaker
line is normal. If the measured impedances Z2 and Z1 are 2,500 Ω and 2,800 Ω, respectively,
it is judged by the processing shown in FIGURE 4 that open-circuiting has occurred.
If the measured impedances Z2 and Z1 are 400 Ω and 1,000 Ω, respectively, it is judged
by the processing shown in FIGURE 4 that current has increased. If the measured impedances
Z2 and Z1 are 15 Ω and 10 Ω, respectively, it is judged by the processing shown in
FIGURE 4 that short-circuiting has occurred.
[0049] Even if the state in which Vrms is 30 V returns to the state in which Vrms is 10
V, for example, Ra does not change to 50 %, but only decreases slightly, due to the
processing of Step S34 in FIGURE 6. Accordingly, Z2open slightly decreases from 4,000
Ω, Z1open slightly decreases from 6,000 Ω, Z2inc slightly increases from 500 Ω, and
Z1 inc slightly increases from 750 Ω. With this arrangement, if, for example, the
loudspeakers 8 generate heat due to a large value of Vrms, and it takes a long time
for the temperature of the loudspeakers 8 to return to the temperature before they
began to generate heat, erroneous judgment can be avoided since it takes a long time
for Z1open, Z2open, Z1 inc and Z2inc to return to their values before the heat generation
occurred.
[0050] In this examination system, the reference impedances Z1 ave and Z2ave at 20 KHz and
40 Hz of the loudspeaker line 6 and the loudspeakers 8 are measured prior to the application
of the audio signal, and judgment is made as to whether the reference impedance Z1
ave is between predetermined allowable aging upper and lower limit values Z1UL and
Z1LL for 20 KHz, or whether the reference impedance Z2ave is between predetermined
allowable aging upper and lower limit values Z2UL and Z2LL for 40 Hz. Specifically,
as the public address system is operated for a long time, the impedance of the loudspeakers
8 changes due to aging, and the reference impedances Z1 ave and Z2ave also change
as the impedance of the loudspeakers 8 changes. Judgment is made as to whether the
reference impedance Z1 ave is within an allowable range defined by the allowable upper
limit Z1 UL and the allowable lower limit Z1LL, which are the limits for 20 KHz indicating
the necessity for replacement of the loudspeakers, or whether the reference impedance
Z2ave is within an allowable range defined by the allowable upper limit Z2UL and the
allowable lower limit Z2LL, which are the limits for 40 Hz indicating the necessity
for replacement of the loudspeakers. If the reference impedance Z1ave is outside the
allowable range defined by the allowable upper limit Z1 UL and the allowable lower
limit Z1 LL, or if the reference impedance Z2ave is outside the allowable range defined
by the allowable upper limit Z2UL and the allowable lower limit Z2LL, an indication
to recommend the replacement of loudspeakers is displayed on the notification device
28. Such judgment is made at a time when the public address system is not in use.
For example, if the public address system is installed in a store, the judgment is
made everyday at a given time within a time period after the store is closed and before
the store is opened.
[0051] As shown in FIGURE 9, whether the time for examination comes or not is judged (Step
S60). If the answer to the query made in Step S60 is NO, the processing is ended.
If the answer is YES, the DSP 10 provides the test signal (Step S62). Then, the reference
impedances Z1 ave and Z2ave are measured in the manner described with reference to
FIGURE 2 (Step S64). Judgment is made as to whether Z1 ave is within the above-described
allowable range defined by Z1UL and Z1LL and, at the same time, whether Z2ave is within
the above-described allowable range defined by Z2UL and Z2LL (Step S66). If the answer
to the query made in Step S66 is NO, an error notification is displayed on the notification
device 28 (Step S68) to recommend replacement of a loudspeaker. If the answer is YES,
the measured Z1 ave and Z2ave are stored (Step S70). The execution of the processing
of Step S70 renews Z1 ave and Z2ave for use in computing Z1open, Z2open, Z1 inc and
Z2inc in the processing shown in FIGURE 8 in later stages. This prevents erroneous
judgment which would be caused by influence given by changes in impedance caused by
aging.
[0052] According to the described embodiment, cross-spectrum analysis is used to determine
the impedances Z1, Z2, Z1ave and Z2ave, but the impedances may be determined by using
a band-pass filter having a narrow band capable of deriving the test signal to derive
current and voltage of the test signal, and determine the impedances from the derived
current and voltage, for example. Also, according to the described embodiment, the
test signal used has frequencies of 40 Hz and 20 KHz, but a test signal at either
one of 40 Hz and 20 KHz, for example, may be used instead. Further, according to the
described embodiment, the digital test signal from the DSP 10 is digital-to-analog
converted and the resultant analog test signal is applied to the combiner 13. Instead,
an analog test signal source is additionally used and a test signal from this analog
test signal source may be applied to the combiner 13. In such case, the analog test
signal is analog-to-digital converted and the resultant digital signal is applied
to the DSP 10. According to the described embodiment, open-circuiting and decrease
in impedance of the loudspeaker line and loudspeakers are determined, but only one
of them may be determined, instead.
1. Ein Prüfsystem für Lautsprecherleitungen zur Verwendung in einer Lautsprecheranlage,
genannte Lautsprecheranlage umfassend:
eine Signalquelle (2) eines Audiosignals,
einen Verstärker (4) zur Verstärkung des genannten Audiosignals,
eine Lautsprecherleitung (6), über die ein Ausgangssignal des genannten Verstärkers
übertragen wird, sowie
eine Vielzahl von Lautsprechern (8), parallel zueinander mit der genannten Lautsprecherleitung
verbunden,
das genannte Prüfsystem für Lautsprecherleitungen bestimmt für die Prüfung der genannten
Lautsprecherleitung und umfassend:
eine Quelle eines Prüfsignals (10), welches eine oder beide einer Frequenz nahe an
einer niedrigsten Frequenz eines menschlichen Audiofrequenzbandes und einer Frequenz
nahe an einer höchsten Frequenz des menschlichen Audiofrequenzbandes enthält,
einen Kombinierer (13) zum Kombinieren des genannten Prüfsignals mit dem genannten
Audiosignal und zum Anlegen eines resultierenden Signals an den genannten Verstärker,
eine Impedanz-Bestimmungseinrichtung (10, 14, 16) zum Ableiten einer Komponente des
in einem Ausgangssignal des genannten Verstärkers enthaltenen genannten Prüfsignals
und zum Bestimmen, aus der abgeleiteten Prüfsignalkomponente, einer Impedanz, betrachtet
vom Ausgang des genannten Verstärkers in Richtung des genannten Lautsprechers,
eine Beurteilungseinrichtung zum Vergleichen der ermittelten Impedanz mit einem vorbestimmten
Schwellenwert, um zu beurteilen, ob zumindest ein Öffnen des Schaltkreises oder eine
Abnahme der Impedanz der genannten Lautsprecherleitung und der Lautsprecher eingetreten
ist, und
eine Schwellenwertänderungseinrichtung zur Änderung des genannten Schwellenwerts in
eine Richtung zur Verringerung der Beurteilungsgenauigkeit der genannten Beurteilungseinrichtung,
wenn ein Verhältnis des genannten resultierenden Signals zum genannten Prüfsignal
größer als ein vorgegebener Wert ist und das genannte resultierende Signal zunimmt.
2. Lautsprecherprüfsystem nach Anspruch 1, bei welchem die genannte Schwellenwertänderungseinrichtung,
sobald der Zustand des genannten resultierenden Signals von einem Zustand, in welchem
das genannte resultierende Signal zunimmt, in einen Zustand wechselt, in welchem das
genannte resultierende Signal abnimmt, die genannte Beurteilungsgenauigkeit erhöht,
welche abnimmt, wenn das genannte resultierende Signal ansteigt, wobei eine Änderungsrate
in einer Richtung zur Verringerung der genannten Beurteilungsgenauigkeit größer und
eine Änderungsrate in einer Richtung zur Erhöhung der genannten Beurteilungsgenauigkeit
kleiner ist.
3. Prüfsystem für Lautsprecherleitungen nach Anspruch 1, bei welchem:
die genannte Impedanz-Bestimmungseinrichtung die genannte Impedanz in einem Zeitraum
bestimmt, in welchem die genannte Signalquelle des Audiosignals nicht in Betrieb ist,
und
bei welchem das genannte Prüfsystem für Lautsprecherleitungen Folgendes umfasst:
eine Schwellenwert-Einstelleinrichtung für das Einstellen des genannten Schwellenwerts
auf der Grundlage der ermittelten Impedanz, sowie
eine zweite Beurteilungseinrichtung zum Beurteilen, ob die genannte ermittelte Impedanz
innerhalb eines vorgegebenen zulässigen Bereichs liegt.
4. Prüfsystem für Lautsprecherleitungen nach Anspruch 1, bei welchem die genannte Impedanzbestimmungseinrichtung
Folgendes umfasst:
eine Stromfeststellungseinrichtung zum Feststellen von durch die genannte Lautsprecherleitung
fließendem Strom,
eine Spannungsfeststellungseinrichtung zum Feststellen einer an der genannten Lautsprecherleitung
anliegenden Spannung,
eine Frequenzkomponenten-Feststellungseinrichtung für das Feststellen einer Frequenzkomponente
des im Strom enthaltenen genannten Prüfsignals, wie es von der genannten Stromfeststellungseinrichtung
festgestellt wird, und des in der Spannung enthaltenen Prüfsignals, wie es von der
genannten Spannungsfeststellungseinrichtung festgestellt wird, sowie
eine Berechnungseinrichtung zur Berechnung der genannten Impedanz aus der Prüfsignal-Frequenzkomponente
in dem/der genannten festgestellten Strom/Spannung.