Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a speech detector for, and t~ 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 detect10n of speech actlvity on a voice channe1. One such
system is a TASI (time ass7gnment speech interpolation) system, such as
the TASI system described and claimed in Canadian Patent Application
No. 361,499, filed October 3, 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 re~aining insensitive to noise. A difficulty arises in
distinguishing, quickly and accurately, between speech signals,
particularly at low 1evels, 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 thls would undesirably increase
the activity of the TASI system.
Various for~s cf speech detector have been devised in order
to distinguish more effectively between speech signals and noise. For
example, Lanier U~S. Patent No. 4,008,375 issued February 15, 1977,
discloses a digital vo~ce 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
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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 variab7e threshold level is increase~ in dependence upon
the uniformity of this count for eight success~ve 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.
IO Some of these disadvantages are reduced by the digital voice
switch disclosed in Jankowski U.S. Patent No. 49052,568 issued October 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.
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Accordingly, d need exists to provide an improved speech
detector which is relatively slmple but still provides an adaptive
threshold level for effective speech detection. An object of thls
invention is to provide such a speech detector, as well as an improved
method of detecting the presence 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.
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Preferably the means for determining the noise level
comprises means responsive to the control signal ~or 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 i~ 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 i~ 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 presence 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 dis~urb
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 ~alse indication that speech is
present, pre~erably the ~oice 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 ~or detecting the presence of speech in digital signal samples on
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a transmit path of a voice channel also having digital s1gnal samples on a
receive path, the speech detector comprising:- means for averag1ng 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 prod~cing a timlng
trigger signal in response to the transmit path average digital s~gnal
falling below a speech threshold; means for producing a timing abort
signal in response to either the trans~it path average digital signal
exceeding said speech ~hreshold 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 produclng 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 fGr storing the determined adaptive
digital speech threshold at the end of each predetermined averaging period
timed by said timing means; and digl~al 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 ~he
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 ~he
voice channel signal falling below, and remaining below, a first speech
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threshold; determining and storing d second speech threshold a
predetermlned 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 whlch:-
Fig. 1 shows a block diagram of a speech detector inaccordance with the invention; and
Fig. 2 illustrates in more detail parts of the speech
detector shown wlthin 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 ls is an 8-bit digital slgnal sample,
the voice channel signal being sampled at a frequency of 8kHz.
In addition to the transmit path signal, in a bidirectional
transmission system such as a TASI system there is a voice channel slgnal
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.
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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 avera~ed over a period of 4ms by an
averager 16, which produces on a line 18 an averaged transmit path slgnal
magnitude every 4~s. The period of 4ms is not critical, but is selected
for convenience and simp1iclty of the averaging clrcuitry~ Similarly, the
receive path signal sample magnitudes are averaged over 4ms 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 detall here.
The averaged magnitude on the line 18, this being a 7-bit
digital signal, 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 detall
in Fig. 2. This circuitry includes the averager 25, 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
o~ the noise level of the transmit path signal, this average being taken
over a period of 2S6ms. Again, this period is not critical but is
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se1ected 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 3dB, above the average noise level. The PROM 32 is used here,
rather than an adder, because the transmit path signal is typ~cally 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 a~fect the averaging process and result ln an unduly high threshold,
the control clrcuit 28 is controlled by comparators 36 and 38 which
compare the average transmit and receive path signal magnitudes,
respectively, with a fixed threshold of for example -40dBmO. 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
256ms, timed by the timer the control circuit enables the averager 26 to
start the averaging process. At the end of the 256ms averag~ng period,
also timed by the ~imer, 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.
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Thus the noise level averaging process ls not started until
a certain time after the transmit path signal aYerage 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 slgnal, the noise 1evel averaging ~s inhibited. Similarly,
if speech occurs in ~he receive path signal the nolse 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 nolse level averaging takes place
only when no speech i5 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 -40dBmO 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 inputs of the OR gate 46, whose output is a logic 1 if the
thréshold is exceeded. Similarly, the three most significant bits of the
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receive path avera~e from -the averager 20 are supplied -to inputs of the OR
gate 4B, ~hose output is a logic 1 if the threshold is exceeded. lhe
outputs of the gates 46 and 48 are combined in the OR gate 58, whose
OlltpUt signal nn a line 60 is supplied to the timing circuit -to inhibit or
abort the timing process when speech is present on 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 4ms, i.e. until the output from the gate 46 is available in
ln respect of the next transmit path average. The current output of the gate
4fi, 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
O 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 b2 is supplied to the timing
circuit 50 and, assuming that the abort signal on the line 60 is a logic O
and does not change, triggers the timing circuit 50 to commence timing a
period of 256ms. At the end of this period the timing circuit 50 starts
to time another period of 256ms, 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 ~rite input W. At the
start of the averaging period the timing circuit 50 supplies a signal via
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a line 64 to a clear input CL of the latch 44, so that ini~ially the
accumulated average is 7ero.
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 amourlt above the average noise levelO The threshold on
the line 66 is stored in the RAM 34 in response to a wrike 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 256ms per~od 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 ls
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
O 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.
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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 de~ect speech
in a plurality of voice channel signals. To this end the RAMs 34, 42, and
52 and the timing circuit 50, and simllarly RAMs in the averagers 16 and
20 and the timing circuits In the comparator and hangover clrcuit 22, are
conveniently addressed with address signals identifying each channel in
turn in a time division multiplexed manner. Accordingly, ~he 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. FurthermoreJ
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 ~he 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,
20 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.
