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Description  |
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This invention relates to a speech detector for, and to a method of,
detecting the presence of speech in a voice channel signal.
Speech detectors are used in a variety of speech transmission systems in
which speech transmission paths are established in response to the
detection of speech activity on a voice channel. One such system is a TASI
(time assignment speech interpolation) system, such as the TASI system
described and claimed in U.S. patent application Ser. No. 218,683, filed
Sept. 22, 1980, by D. H. A. Black and entitled "TASI System Including an
Order Wire."
A speech detector should be highly sensitive to speech signals while
remaining insensitive to noise. A difficulty arises in distinguishing,
quickly and accurately, between speech signals, particularly at low
levels, and noise. In a TASI system, for example, the speech detector
should be able to detect low level speech signals in order to avoid
excessive speech clipping at the start of speech bursts, but should not
respond to noise alone because this would undesirably increase the
activity of the TASI system.
Various forms of speech detector have been devised in order to distinguish
more effectively between speech signals and noise. For example, Lanier
U.S. Pat. No. 4,008,375, issued Feb. 15, 1977, discloses a digital voice
switch in which speech signal samples are compared with a variable
threshold level which is adapted in dependence upon the noise which is
present. To this end, the samples are also compared with a second
threshold a fixed amount below the variable threshold level, a counter
counts the number of times in a given period that this second threshold is
exceeded, and the variable threshold level is decreased if the count is
less than a predetermined number in two successive counting periods.
Furthermore, the number of times that the samples exceed the variable
threshold level in the given period is counted, and the variable threshold
level is increased in dependence upon the uniformity of this count for
eight successive counting periods. This arrangement is obviously complex
and relatively expensive, is slow to respond to changing noise levels, and
is subject to result in false indications of speech in response to high
noise pulses which may commonly occur.
Some of these disadvantages are reduced by the digital voice switch
disclosed in Jankowski U.S. Pat. No. 4,052,568, issued Oct. 4, 1977. In
this arrangement, speech signal samples are compared with variable speech
and noise threshold levels and with a fixed disabling threshold level. The
number of times that the noise threshold is exceeded in a given period is
used to adaptively adjust the speech and noise threshold levels, which
differ by a fixed amount. When speech has been detected, and for the
duration of a speech hangover period, the adaptive adjustment is prevented
if the disabling threshold level is exceeded. The disabling threshold
level is set relatively high, in order that it is not exceeded by high
noise pulses. However, a result of this is that the adaptive adjustment
may not be prevented during relatively low level speech signals from a
quiet talker, giving rise to maladjustment of the speech and noise
threshold levels. Furthermore, this arrangement is still relatively
complex and expensive, requiring two variable and one fixed threshold
comparators as well as other counting and comparison circuitry.
Accordingly, a need exists to provide an improved speech detector which is
relatively simple but still provides an adaptive threshold level for
effective speech detection. An object of this invention is to provide such
a speech detector, as well as an improved method of detecting the presense
of speech in a voice channel signal.
According to one aspect of this invention there is provided a speech
detector for detecting the presence of speech in a voice channel signal,
comprising: means for producing a control signal in response to the voice
channel signal falling below a first speech threshold; means responsive to
the control signal for determining a noise level of the voice channel
signal while the voice channel signal is below the first speech threshold;
means for determining a second speech threshold in dependence upon the
determined noise level; and means for indicating the presence of speech in
response to the voice channel signal exceeding the second speech
threshold.
Thus in contrast to the prior art discussed above, in a speech detector in
accordance with this invention the noise level can only be determined when
no speech is present, i.e. when the voice channel signal is below the
first speech threshold.
The means for producing the control signal conveniently comprises means for
comparing the voice channel signal with the first speech threshold and
means for producing the control signal in response to a change in the
comparison result. Most conveniently the first speech threshold is a fixed
threshold, the voice channel signal is a digital signal comprising a
plurality of bits, and the means for comparing comprises a gating circuit
to which a plurality of said bits are supplied.
Preferably the means for determining the noise level comprises means
responsive to the control signal for determining a predetermined delay
period, and means for determining the noise level at the end of the delay
period. The latter means conveniently comprises means for averaging the
level of the voice channel signal during a predetermined averaging period
commencing at the end of the delay period.
The means for determining the noise level preferably comprises means for
inhibiting the determination of the noise level if the voice channel
signal exceeds the first speech threshold during said delay period or
during said averaging period. The speech detector preferably further
comprises means for inhibiting the determination of the noise level if
during said delay period or during said averaging period the level of a
voice channel signal, in the opposite direction of transmission from that
of the voice channel signal in which the presense of speech is to be
detected, exceeds a third speech threshold. Thus echoes of speech signals
on a receive path, which may occur in the voice channel signal but may be
insufficient to exceed the first speech threshold, can not disturb the
correct noise level determination. The first and third speech thresholds
can be the same or different.
In order that the adaptive second speech threshold is not exceeded by high
short-duration noise pulses which may occur in the voice channel and which
could give rise to a false indication that speech is present, preferably
the voice channel signal is an averaged signal, the speech detector
including means for averaging individual voice channel signal samples to
produce the averaged voice channel signal.
According to another aspect this invention provides a speech detector for
detecting the presence of speech in digital signal samples on a transmit
path of a voice channel also having digital signal samples on a receive
path, the speech detector comprising: means for averaging the transmit
path digital signal samples over a predetermined period to produce a
transmit path average digital signal; means for averaging the receive path
digital signal samples over a predetermined period to produce a receive
path average digital signal; means for producing a timing trigger signal
in response to the transmit path average digital signal falling below a
speech threshold; means for producing a timing abort signal in response to
either the transmit path average digital signal exceeding said speech
threshold or the receive path average digital signal exceeding a speech
threshold; timing means responsive to the timing trigger signal to time a
predetermined delay period and an immediately following predetermined
averaging period and responsive to the timing abort signal to abort said
timing; means for producing an average noise level of the transmit path
digital signal samples during each predetermined averaging period timed by
said timing means; means for determining an adaptive digital speech
threshold a predetermined level above said average noise level; means for
storing the determined adaptive digital speech threshold at the end of
each predetermined averaging period timed by said timing means; and
digital comparator means for comparing the transmit path average digital
signal with the stored adaptive digital speech threshold and indicating
the presence of speech in response to the average signal exceeding the
adaptive threshold.
The invention also extends to a method of detecting the presence of speech
in a voice channel signal, comprising the steps of: determining a noise
level of the voice channel signal in response to the voice channel signal
falling below, and remaining below, a first speech threshold; determining
and storing a second speech threshold a predetermined level above the
determined noise level; and comparing the voice channel signal with the
second speech threshold and indicating that speech is present in response
to the voice channel signal exceeding the second speech threshold.
The invention will be further understood from the following description
with reference to the accompanying drawings, in which:
FIG. 1 shows a block diagram of a speech detector in accordance with the
invention; and
FIG. 2 illustrates in more detail parts of the speech detector shown within
a dashed line box II in FIG. 1.
The speech detector shown in FIG. 1 serves for producing a speech decision
on an output line 10 in response to speech being present in a voice
channel signal, referred to herein as the transmit path signal and present
on a line 12. The speech detector is for example for use in a TASI system
such as that described in the patent application by D. H. A. Black already
referred to. It is assumed here that, as is typical in such a system, the
voice channel signal is an 8-bit digital signal sample, the voice channel
signal being sampled at a frequency of 8 kHz.
In addition to the transmit path signal, in a bidirectional transmission
system such as a TASI system there is a voice channel signal for the
opposite direction of transmission. This is referred to herein as the
receive path signal and is present on a line 14. The reason for supplying
the receive path signal, which is also assumed to be an 8 -bit digital
signal sampled at a frequency of 8kHz, to the speech detector will become
clear from the following description.
In order to reduce triggering of the speech decision by high level noise
pulses which commonly occur in the transmit path signal, the magnitudes of
the signal samples are averaged over a period of 4 ms by an averager 16,
which produces on a line 18 an averaged transmit path signal magnitude
every 4 ms. The period of 4 ms is not critical, but is selected for
convenience and simplicity of the averaging circuitry. Similarly, the
receive path signal sample magnitudes are averaged over 4 ms periods by an
averager 20. The averagers 16 and 20 have a similar form to an averager 26
described in detail below, except that they are supplied with different
timing signals and have a division factor of 32. Accordingly the averagers
16 and 20 are not described in further detail here.
The averaged magnitude on the line 18, this being a 7-bit digital singal,
is compared in a comparator and hangover circuit 22 with an adaptive
digital threshold supplied on a line 24 and produced as described below.
The circuit 22 comprises a digital comparator and a timing circuit which
is responsive to the comparator output to produce the speech decision on
the line 10 when the magnitude on the line 18 exceeds the threshold on the
line 24 and for a following hangover period. The circuit 22 can be of a
known form and accordingly is not further described here.
The adaptive threshold is produced on the line 24 by circuitry within a
dashed line box II and which is shown in more detail in FIG. 2. This
circuitry includes the averager 26, which is supplied with the averaged
transmit path signal magnitude from the line 18 and serves to produce,
under the control of a control circuit 28, an average of the noise level
of the transmit path signal, this average being taken over a period of 256
ms. Again, this period is not critical but is selected for convenience.
The average noise level, produced on a line 30, is used to address a PROM
(programmable read only memory) 32 to read out to a RAM (random access
memory) 34 a threshold which is a fixed level, for example 3 dB, above the
average noise level. The PROM 32 is used here, rather than an adder,
because the transmit path signal is typically a non-linearly encoded
signal. The threshold from the PROM 32 is stored in the RAM 34 under the
control of the control circuit 28, and is read from the RAM 34 to
constitute the adaptive threshold on the line 24.
In order to ensure that the averager 26 only averages noise in the transmit
path signal, and that no speech signals are included which would affect
the averaging process and result in an unduly high threshold, the control
circuit 28 is controlled by comparators 36 and 38 which compare the
average transmit and receive the path signal magnitudes, respectively,
with a fixed threshold of for example -40 dBmO. In response to the output
of the comparator 36 changing in response to the average on the line 18
falling below the fixed threshold, a timer in the control circuit 28 is
started. After a predetermined delay period, for example 256 ms, timed by
the timer the control circuit enables the averager 26 to start the
averaging process. At the end of the 256 ms averaging period, also timed
by the timer, the control circuit enables the threshold produced by the
PROM 32 to be stored in the RAM 34, so that the threshold in the RAM 34 is
updated, or adapted, in accordance with the prevailing noise level of the
transmit path signal. However, if either of the comparators 36 and 38
produces, during these timing periods, an output which represents that
either the transmit path or the receive path average exceeds the fixed
threshold, then the timing and averaging are aborted and the threshold
stored in the RAM 34 is not changed.
Thus the noise level averaging process is not started until a certain time
after the transmit path signal average has fallen below the fixed
threshold, to ensure that no speech signal is present at the start of the
noise level averaging. If speech subsequently occurs in the transmit path
signal, the noise level averaging is inhibited. Similarly, if speech
occurs in the receive path signal the noise level averaging is inhibited,
because speech in the receive path signal generally produces some echo in
the transmit path signal. Such echo may not be sufficiently great as to
cause the average on the line 18 to exceed the fixed threshold, but
nevertheless can be sufficient to adversely affect the noise level
averaging.
Accordingly, the arrangement of the comparators 36 and 38 and the control
circuit 28 ensures that noise level averaging takes place only when no
speech is present, so that a reliable and accurate noise level measurement
is obtained, so that the adaptive threshold is also reliably and
accurately determined.
Referring to FIG. 2, the averager 26 is constituted by a 12-bit adder 40, a
RAM 42, and a latch 44; the comparators 36 and 38 are constituted by OR
gates 46 and 48 respectively, and the control circuit 28 is constituted by
a timing circuit 50, a RAM 52, an inverter 54, an AND gate 56, and an OR
gate 58. FIG. 2 also shows the PROM 32 and the RAM 34.
The fixed threshold of -40 dBmO corresponds to the 7-bit digital value
0001111. Accordingly, this threshold is exceeded if any of the three most
significant bits of the 7-bit average on the line 18 is a logic 1. The
three most significant bits of the average on the line 18 are supplied to
inuts of the OR gate 46, whose output is a logic 1 if the threshold is
exceeded. Similarly, the three most significant bits of the receive path
average from the averager 20 are supplied to inputs of the OR gate 48,
whose output is a logic 1 if the threshold is exceeded. The outputs of the
gates 46 and 48 are combined in the OR gate 58, whose output signal on a
line 60 is supplied to the timing circuit to inhibit or abort the timing
process when speech is present in either of the receive and transmit
paths.
The output of the gate 46 is also supplied to the RAM 52, which is
controlled in known manner by timing means not shown to delay this output
by 4 ms, i.e. until the output from the gate 46 is available in respect of
the next transmit path average. The current output of the gate 46,
inverted by the inverter 54, and the delayed previous output of the gate
46 are supplied to the inputs of the gate 56, whose output is a logic 1
trigger signal only in response to the gate 46 output changing from 1 to 0
for successive transmit path averages. Thus this trigger signal is
produced on a line 62 in response to the transmit path signal average
falling below the fixed threshold.
The trigger signal on the line 62 is supplied to the timing circuit 50 and,
assuming that the abort signal on the line 60 is a logic 0 and does not
change, triggers the timing circuit 50 to commence timing a period of 256
ms. At the end of this period the timing circuit 50 starts to time another
period of 256 ms, this being the averaging period. During the averaging
period, every 4 ms the latch 44 is clocked by a timing signal supplied to
its clock input CK to store a 12-bit accumulated average from the RAM 42,
the current transmit path average is added to this by the adder 40, and
the resultant new accumulated average is written into the RAM 42 by a
timing signal applied to its write input W. At the start of the averaging
period the timing circuit 50 supplies a signal via a line 64 to a clear
input CL of the latch 44, so that initially the accumulated average is
zero.
At the end of the averaging period the 6 most significant bits of the
12-bit accumulated average in the RAM 42, which equal the accumulated
average divided by 64, constitute a true average noise level of the
transmit path signal. These 6 bits are used to address the PROM 32 to read
out to a line 66 the desired, for example 4-bit, adaptive threshold a
fixed amount above the average noise level. The threshold on the line 66
is stored in the RAM 34 in response to a write signal which the timing
circuit 50 produces at the end of the averaging period and which is
supplied via a line 68 to a write input W of the RAM 34. Consequently, the
newly updated stored threshold is subsequently supplied to the line 24.
If during the timing of either 256 ms period the signal on the line 60
becomes a logic 1, the timing is aborted and no write signal is produced
on the line 68, so that the threshold stored in the RAM 34 is not changed.
The timing processes described above are then started again in response to
the next logic 1 trigger signal on the line 62 with a logic 0 abort signal
on the line 60.
As described above, in operation of the speech detector the average noise
level of the voice channel is determined. It should be appreciated that,
in a TASI system, this average noise level can also be transmitted to the
far end where it can be used to adaptively adjust the level of a locally
generated noise signal which in known manner is inserted during
disconnected periods of the voice channel in order to reduce noise signal
contrast.
Although the speech detector has been described above in relation to a
single voice channel signal, as is known in the art the speech detector
can be operated in a multiplexed manner to detect speech in a plurality of
voice channel signals. To this end the RAMs 34, 42, and 52 and the timing
circuit 50, and similarly RAMs in the averagers 16 and 20 and the timing
circuits in the comparator and hangover circuit 22, are conveniently
addressed with address signals identifying each channel in turn in a time
division multiplexed manner. Accordingly, the described speech detector
can operate in all respects contemporaneously in respect of a plurality of
voice channels.
Numerous other changes may be made in the speech detector described above.
For example, the averaging and comparison of the receive path signal could
be dispensed with, the trigger and abort signals being produced solely in
dependence on the transmit path signal. Furthermore, the averaging
periods, the delay period between the occurrence of the trigger signal and
the start of the noise level averaging period, the fixed thresholds, and
the difference between the adaptive threshold and the monitored noise
level, produced in the PROM 32, may all be varied from the values given
above. The manners of effecting the averaging, monitoring the noise level,
and timing may also be different from those described. Accordingly,
numerous variations, modifications, and adaptations may be made to the
embodiment of the invention described above without departing from the
scope of the invention, as defined in the claims.
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