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BACKGROUND OF THE INVENTION
The present invention relates to an adaptive speech signal detector capable
of adaptively setting a threshold value for use in a 4-wire telephone
channel.
To increase the efficiency of the transmission of a speech signal over a
telephone channel, the realization of a speech signal detector with the
excellent detection capability is often desired. For instance, a DSI
(digital speech interpolation) system to improve the channel utilization
efficiency is a system based on the fact that the speech activity, i.e.,
the average length of time occupied by the speech of a subscriber on the
phone is less than 50 percent of the total length. More particularly, in a
given terminal station, the states of the respective channels connected to
a large number of subscribers (hereinafter called subscriber channels) are
supervised to select out of subscriber channels only those subscriber
channels on which speech signals are present, and the transmission is
carried out for another terminal station. Since the speech activity of an
average subscriber is generally 30-40%, the number of the channels for the
transmission between these terminal stations (DSI channels) can be made
about one-half in number of the subscriber channels.
In such DSI system, channel interruption frequently occurs during the
conversation, and so, the detection capability of a speech detector is one
of principal factors to determine the characteristics of the DSI system.
For this reason, if the speech detection takes too much time, the time
required for performing the speech detection for a subscriber channel on
which a speech signal begins to appear and for connecting the subscriber
channel to a DSI channel, is increased. Accordingly, the speech on the
subscriber channel before it is connected to the DSI channel is not
transmitted to a party subscriber with the result that speech-front
mutilation occurs frequently. On the other hand, when the speech detection
rate is increased to reduce the mutilation, the speech signal detector
increasingly malfunctions due to noise, the speech activity is increased,
causing a concern that the probability of the saturation of the DSI
channels is enlarged to cause the speech interruption.
To solve such problems in the DSI system, the use of an adaptive speech
detector in which the threshold value for the speech detection is
adaptively varied depending on the noise level on a channel, or a speech
detector in which in addition to amplitude information of a speech signal,
the zero-crossing information of the speech signal is used, has been
proposed. Such a speech signal detector having the threshold value
adaptively set depending on the channel noise level is proposed in FIG. 1
of the U.S. Pat. No. 4,028,496 or in FIG. 2 of the U.S. Pat. No.
4,052,568. Furthermore, the speech detector using the zero-crossing
information is disclosed in the U.S. Pat. No. 4,001,505.
In such an adaptive speech signal detector, the threshold value is set at a
value as small as possible within the range where noise present on a
channel is not erroneously detected, and if the magnitude of the noise
signal level is gradually varied, the threshold value is also varied
following the variation of the noise signal level. As a result, in case
where the noise amplitude is small, the threshold value takes a small
value so that even low-level speech can be easily detected and the speech
mutilation can be reduced. Whereas in the case of the large noise
amplitude, the threshold value is increased, and as a result, the detector
does not malfunction due to noise of the large amplitude. Although the
speech signal detection is delayed due to the increase of the threshold
value, the degree of the degradation in quality of speech signals caused
by the increase of the speech mutilation is small, if the noise amplitude
is large, because the quality of the original speech signals is already
degraded. In view of these facts, the adaptive speech signal detector
would appear to be a detector suitable for the DSI. However, since the
application of DSI system to channels on which echo signals are present,
is accompanied with the enhancement of the virtual speech activity due to
the echo signals, the DSI efficiency (the degree of the reduction in
number of channels by employing the DSI system) is remarkably lowered. The
conventional telephone channel composed of a 2-wire channel and a 4-wire
channel can not avoid such an echo signal. Accordingly, an echo suppressor
is normally interposed between the DSI system and subscribers.
For details of such an echo suppressor, reference is made to an article by
P. T. Brady and G. K. Helder entitled "Echo Suppressor Design in Telephone
Communications" published in The Bell System Technical Journal, Vol. 42,
No. 6, pp. 2893-2917, November issue, 1963. The operation of the echo
suppressor will be briefly described hereunder.
An echo suppressor performs switching operations for suppressing an echo
signal in such a manner that a large loss is provided or the interruption
is made at a transmitter when a received signal is larger than a certain
fixed value and is also larger than a transmission signal. For this
reason, as soon as a speech signal is given to a receiver, the echo
suppressing switch is actuated to prevent the echo signal from being
outputted and at the same time, the noise level at the output terminal of
the echo suppressor changes momentarily. If the echo suppressor is
interposed between subscribers and the DSI system, then in response to the
operation of the echo suppressing switch, the noise level on the channel
inputted to the transmitter of the DSI system similarly changes
immediately. Consequently, the adaptive speech detector used in the DSI
system malfunctions at the time point when the noise level has changed
abruptly from a small amplitude to the normal one.
Furthermore, in an echo suppressor, when a speech signal of a near end
subscriber which is deemed not to be an echo signal is detected at a
transmitter, the attenuation of 5 to 6 dB (decibels) is inserted into a
receiver in order to relatively intensify (in order to put a preference
on) the speech signal of this subscriber against a speech signal of a
remote end subscriber. In this case also, there is not only the
possibility of the noise level change on the transmitter-side subscriber
channel but also the possibility of the malfunction of the adaptive speech
signal detector.
Thus, the adaptive speech signal detector cannot be directly used in the
DSI system. On the other hand, though the above-mentioned detection system
employing the zero-crossing information improves the detection capability
for periodic speech signals or signals having a number of high frequency
components, there still remains the problem that the detection for signals
of a small amplitude and of many low frequency components are difficult.
The conventional adaptive speech signal detector, therefore, has a
disadvantage in that its interposition in 4-wire channels between the DSI
system and subscribers does not permit the achievement of the expected
performance.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide an adaptive speech
signal detector for use in a 4-wire telephone channel in which if the
channel noise is at a low level, a speech signal can be detected in a very
short period of time, while if the noise is at a high level, the speech
can be detected with a small malfunction rate caused by noise and the
detecting operation can be achieved adaptively so that the malfunction due
to an echo suppressor may not arise.
The present adaptive speech signal detector interposed in a 4-wire
telephone channel for performing an adaptive threshold value setting
operation depending on the channel noise level on a transmitter-side
channel to detect a speech signal present at the transmitter, comprises
first speech signal detector means having a transmission signal inputted
thereto, threshold value setting means for setting a threshold value for
the speech signal detection and giving the set value to said first speech
signal detector means, a first hangover circuit for extending the output
signal of said first speech signal detector means by a first predetermined
period of time, second speech signal detector means having a reception
signal inputted thereto, and a second hangover circuit for prolonging the
output signal of said second speech signal detector means by a second
predetermined period of time to control said threshold value setting
means.
Also, the present adaptive speech signal detector to be interposed in a
4-wire telephone channel for performing an adaptive threshold value
setting operation in accordance with the channel noise level, comprises
first speech signal detector means having a transmission signal inputted
thereto, threshold value setting means for setting a threshold value for
speech signal detection and giving the set value to said first speech
signal detector means, a first hangover circuit for extending the output
signal of said first speech signal detector means by a first predetermined
period of time to control said threshold value setting means, second
speech signal detector means having a reception signal inputted thereto,
and a second hangover circuit for extending the output signal of said
second speech signal detector means by a second predetermined period of
time to control said threshold value setting means.
BRIEF DESCRIPTION OF THE DRAWINGS
Now, the present invention will be described in greater detail in
conjunction with the accompanying drawings, in which:
FIG. 1 is a block diagram showing a conventional DSI system;
FIG. 2 is a block diagram of the DSI system using an adaptive speech signal
detector of the present invention;
FIG. 3 shows one embodiment of the present invention;
FIG. 4 shows a second embodiment of the present invention;
FIG. 5 is a waveform diagram for explanation of the present invention; and
FIG. 6 is a block diagram showing one example of a hangover circuit 302
used in the present speech detector.
DETAILED DESCRIPTION OF THE INVENTION
In the following description, it should be noted that unless any reference
is specifically given to a party station, the description is made in
connection with the structure (in the local station) shown in FIGS. 1 and
2.
In a conventional DSI system illustrated in FIG. 1, only one terminal
station (a local station) is shown. In this DSI system, transmitter-side
subscriber channels and receiver-side subscriber channels connected to a
large number of subscribers are respectively connected to input terminals
11, 12, . . . , li, . . . , 1 N and input terminals 41, 42, . . . 4i, . .
. , 4 N. A PCM encoder 110 converts the signals from the respective
subscribers into a time-division multiplexed digital signal. The output of
the PCM encoder 110 is fed to a transmitter-side speech signal detector
140 and a delay circuit 120. The speech signal detector 140 supervises the
presence of speech signals on the respective subscriber channels through a
signal line 51, and gives the supervised result to a channel assignment
circuit 150 through a signal line 52. The channel assignment circuit 150
writes in a memory 130 only assigned signals among the respective
subscriber channel signals delayed through the delay circuit 120. Although
the delay circuit 120 has been conventionally used to compensate for the
delay in speech detection as well as the delay in calculation relating to
the assignment determination in the channel assignment circuit 150, the
degradation in quality is little even without the delay circuit 120 if the
speech detection capability is high and consequently, the detection time
is short as is the case with the illustrated example in the prior art. An
assignment signal delivered from the channel assignment circuit 150 and
the output of the memory 130 are mixed by a multiplexer 160 and outputted
from a transmission output terminal 2. The DSI system on the party side
also has the same structure as that shown in FIG. 1. A signal received
from the party station is given through a reception terminal 3, and the
channel assignment signal is selected out of the received signals by a
demultiplexer 210 and is fed to a receiver-side channel assignment circuit
220. The receiver-side channel assignment circuit 220 writes the received
signal at a position in a memory 230 corresponding to a predetermined
channel, and also controls a gate circuit 240 so that at a predetermined
time a signal on a corresponding subscriber channel may be produced. The
reproduced time-division multiplex signal is sent to a PCM decoder 250 so
that at the output terminals 41, 42, . . . 4i, . . . 4 N connected to the
respective subscribers, speech signals are decoded and outputted.
For details of the method for realizing the DSI system, reference is made
to the U.S. Pat. No. 3,644,680 (especially FIG. 4) and to Japanese Patent
Application Disclosure No. 114011/1976 (corresponding to the U.S. patent
application Ser. No. 560,423), and therefore, the following description
will be made only with respect to the parts of the DSI system directly
related to the present invention.
In FIG. 2 which shows a first embodiment of the present invention, the
present detector is composed of a transmitter-side speech signal detector
140 and a receiver-side speech signal detector 300.
Since the DSI system does not operate efficiently in a channel on which an
echo signal is present as described above, an echo suppressor is needed
for the use of the DSI system. In case that the echo suppressor is
interposed between subscribers and the DSI system, the noise level on a
subscriber channel given to a transmitter-side of the DSI system is varied
by switching operations in the echo suppressor. Therefore, the adaptive
speech signal detector in the DSI system operates erroneously. More
particularly, if an input signal appears at the receiver as shown in FIG.
5(a), at the transmitter appears an echo signal (ECHO) of the received
signal as well as a speech signal (NET) of a near end subscriber as shown
in FIG. 5(c). At this time point, the echo suppressor interrupts the
channel during the time period (T1 in FIG. 5(a)) when the received signal
is present plus a predetermined period of its extension (T3) for the
purpose of suppressing the echo signal. Accordingly, at the output of the
echo suppressor there results a soundless state in which even noise is not
present during the period of the channel interruption (T4) as shown in
FIG. 5(d), and this output is given to the DSI system. The
transmitter-side speech signal detector 140 in the DSI system tends to
erroneously acknowledge noise as a speech signal at the time point C shown
in the middle of FIG. 5(d) when the signal changes from the soundless
state to a state where noise is present. As a result, during that period
the adapting operation of the speech signal detector must be stopped.
As shown in FIG. 3, the speech signal detector 140 is composed of a
threshold value setting section 142 for successively measuring the noise
signal level to determine a threshold value, a speech signal detector
section 141 for detecting a speech by making use of the threshold value
determined by the section 142, and a hangover circuit 143 for holding a
detection signal from the section 141 for a predetermined period of time
after the detection signal has disappeared. The section 142 is
constructed, for instance, in such a manner that if an input signal is
larger than the threshold value appropriately set, an integrator contained
therein (not shown) is increased by one, whereas if it is smaller, the
integrator is reduced by one, and as the above operations are done
repeatedly, if the threshold value is set at a value smaller than the
input signal, the integrator continues to increase until it reaches a
predetermined value when the threshold value is raised, whereas if the
threshold value is set at a value larger than the input signal, the
integrator continues to decrease until it takes a value lower than a
predetermined value when the threshold value is lowered. With regard to
the construction of the section 142, besides the circuit construction
described above, the constructions disclosed in FIG. 1 of the U.S. Pat.
No. 4,028,496 and in FIG. 2 of the U.S. Pat. No. 4,052,568 can be
employed. The input signal to the transmitter-side speech signal detector
140 is supplied to the speech signal detector section 141 and the section
142 through a signal line 51. If the noise signal level present in the
input signal is low, the section 142 lowers the threshold value to such an
extent that the noise does not cause the speech signal detector section
141 to operate erroneously. However, in case where the noise signal level
begins to rise gradually, the section 142 raises the threshold value so
that the detector section 141 may not operate erroneously.
On the other hand, the receiver-side speech signal detector 300 is composed
of a speech signal detector section 301 and a hangover circuit 302, and
when a speech signal as shown in FIG. 5(a) has been given to the receiver
and the signal level on the receiver side is higher than a preset
threshold value .theta., the receiver-side speech signal detector 300
transmits an instruction for stopping the threshold value adapting
operation (INST in FIG. 5(b)) to the transmitter-side speech signal
detector 140. More particularly, the hangover circuit 302 is a circuit for
extending the state of "1" at the output of the speech signal detector 301
for a predetermined period of time (T2 in FIG. 5(b)) when the output has
turned from "1" to "0," and is composed of a count-T2 counter having a
full count corresponding to said time period T2 and a flip-flop as shown
in FIG. 6. More in detail, immediately after the input signal for the
hangover circuit 302 is "1," the flip-flop is set at "1." Furthermore,
when the input signal has turned from "1" to "0," a predetermined value T2
is set in the counter, and subsequently while the input signal continues
to take the state "0," the counter is successively reduced by one at a
predetermined time interval until the count in said counter becomes zero,
when said flip-flop is reset to "0." It is noted that the hangover
circuit 143 also has a construction similar to the circuit 302.
The decision of the hangover time in the hangover circuit 302 in this case
is made in the following manner. The interruption of the adapting
operation in the threshold value setting section 142 must continue over
the entire period when an echo suppressor switch in the echo suppressor
interposed between the DSI system and subscribers operates. Accordingly,
it is necessary to select a value longer than the hangover time of the
echo suppressor (T3 in FIG. 5(d)) as the hangover time of the hangover
circuit 302 (T2 in FIG. 5(b)). Assuming that the hangover time of the echo
suppressor conforms to the CCITT recommendation, it is 350 milliseconds or
less. For this reason, it is necessary to select the hangover time of the
hangover circuit 302 at 350 milliseconds or more.
Now description will be made on a second embodiment of the present
invention. As shown in FIG. 4, the present adaptive speech signal detector
is composed of a transmitter-side speech signal detector 140 and a
receiver-side speech signal detector 300.
In this second embodiment, in addition to the function of stopping the
adapting operation for setting a threshold value as is the case with the
first embodiment, the function of stopping the adapting operation in the
threshold value setting section 142 is provided by giving an inhibit
signal (FIG. 5(e)) to the threshold value setting section 142 through a
signal line 60, taking into consideration the fact that in case where a
speech signal of a near end subscriber present on at the transmitter (NTS
in FIG. 5(d)) is detected by the speech signal detector 140, a loss may be
inserted at the transmitter of said echo suppressor on the grounds as will
be described later.
The operations of the speech signal detector 300 and the speech signal
detector 140 will be described in more detail referring to FIG. 4.
The input signal to the speech signal detector 140 is given to the speech
signal detector section 141 and the threshold value setting section 142
through a signal line 51. If the noise signal level present in the input
signal is low, the section 142 lowers the threshold value but not to such
an extent that the noise causes the speech signal detector section 141 to
operate erroneously. However, if the noise signal level begins to rise
gradually, the threshold value setting section raises the threshold value
so that the section 141 does not operate erroneously. On the other hand,
the speech signal detector 300 is comprised of a speech signal detector
section 301 and a hangover circuit 302, and if the signal level at the
receiver has become higher than a preset threshold value .theta., the
detector 300 outputs an instruction for stopping the threshold value
adapting operation to the speech signal detector 140. Here, it is
essential to select the hangover time of the hangover circuit 302 at 305
milliseconds or more, similarly to the first embodiment.
In the above-described manner, the present adaptive speech signal detector
does not malfunction by the operation of the echo suppressor. In some echo
suppressors, as described previously, when speech signals of near end
subscribers are present on the subscriber channels connected to the
transmitter-side input terminals 11, . . . , 1 N of the DSI, a loss of
several decibels is inserted at the receiver in order to put preference on
the speech signals supplied from these subscribers. In this case,
depending on the channel state, the variation in the noise level of said
subscriber channel on the transmitter side of the DSI system such as the
noise level in the period T6 shown in FIG. 5(c) appears (the level
decrease appears). As a result, at the end of the period T6 the speech
signal detector 141 erroneously detects the noise as a speech signal, and,
therefore, in this case also it is necessary to stop the adapting
operation of the threshold value setting section 142. More particularly,
taking into consideration the hangover time of the echo suppressor, it is
necessary to output a control signal (CS in FIG. 5(e)) from the hangover
circuit 143 through a signal line 60 to the section 142 as shown in FIG. 4
so that the adapting operation of the section 142 may be stopped during
the time period T8 in FIG. 5(e) (the time period T4 when a speech signal
of a near end subscriber is present plus the hangover time T7 of the
hangover circuit 143).
While the echo suppressor has been assumed to be interposed between the DSI
system and the subscribers in the first and second embodiments, it is
desirable to incorporate the echo suppressing function in the DSI system
by the reasons such that by integrating the DSI system and the echo
suppressor the speech signal detectors of both can be used in common. In
that case, the interruption of the adapting operation of the threshold
value setting section 142 is carried out on the basis of the operating
state of the integrated echo suppressor section. It is evident that the
speech signal detectors 141 and 301 can be realized by making use of the
construction shown in FIG. 1 of the U.S. Pat. No. 4,001,505.
In the case where the subscriber channels connected to the DSI system are a
primary group or a group of higher order of a PCM system, the PCM encoder
110 and the PCM decoder 250 become unnecessary. In addition, even in the
case of an analog TASI (time assignment speech interporation), the
adaptive speech signal detector of the present invention can be likewise
realized in an analog fashion.
Although the description has been made above in connection to the
embodiments of the present invention in which the DSI system is interposed
in 4 wire telephone channels, the present invention can be practiced for
the purpose of carrying out the speech processing in general 4-wire
telephone channels not employing the DSI system at a high performance
without errors.
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