Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~%;~573
This invention relates to speech detector systems. the
invention is particularly suitable for use in combination
with a transmitter for voice communication and which is
equipped with a dual-microphone amplifier system arranged so
as selectively to transmit speech signals whilst suppressing
ambient-sound-related signals.
There are a number of applications for speech detector
systems. For example, wireless transmitters, especially of
the portable type, are commonly required to have a VOX
facility, that is, automatic keying of thy transmitter in
response to speech signals.
In previously-known VOX systems which utilise a single
microphone channel, it has been difficult to combine
effective recognition of the operator's speech with adequate
rejection of unwanted background noise or speech. Various
techniques have been employed to improve the voice
discrimination, including band-separation filtering and the
use of noise-cancelling microphones coupled with automatic
gain control, abbreviated as "AGC". However, where a wide
amplitude-dynamic-range is achieved by the use of AGC it is
difficult to avoid false-triggering of the VOX circuit. In
addition, the AGC system tends to amplify background noise
signals and circuit noise to an objectionable level during
periods when the speech is interrupted. This is especially
evident when a transmitter is switched to a manual-keying or
' press-to-talk mode, sometimes abbreviated as "PTT".
i A system which has been used successfully to overcome
the foregoing disadvantages of VOX and AGC systems compriaea
f
-- 2 --
~L2~:~5~3
a two-channel dual-~icrophone arrangement in which one
microphone receives the operator's speech, superimposed on
ambient noise, and the other principally receives ambient
noise. jot only is noise cancellation possible, over at
least the lower part of the frequency range, but the ambient
noise signal may be used to control the speech channel gain
in the absence of speech and, by this method, prevent the
noise signal at the speech channel output from rising above,
for example, a level lOdB below the nominal speech signal
l output level. A significant advantage of this system when
combined with a speech detector is that, at least in the
steady state, there is a well-defined difference in speech
and noise levels which is easily discriminated in a simple
comparator circuit.
Some constraints and disadvantages of the dual-microphone
system are:
A time delay must be included in the speech detector
response to take account oi the AGC attack time which
can be, for example, up to 5 ms.
The two microphones must be closely matched in
sensitivity and frequency response.
Direct noise cancellation is usually only possible at
frequencies up to about 500 Hz because of the transit
time difference for a sound pressure wave travelling to
each of the two microphones, this varying with the
direction of the sound source.
There is a variation of the automatic-gain-controlled
ouput level of either channel with input signal level,
~IL2;~5~3
depending on the loop gain of the AGC loop.
There is an uncertainty in the output reference level of
the GO system because of production tolerances in
components.
There is a need for precise tracking, or equality of
gain, in the control elements of the speech and noise
channels.
There is an uncertainty in the speech detector reference
level owing to production tolerances in components.
A number of the constraints and disadvantages listed
above can produce effects which are additive. This can give
rise to a substantial error in the effective speech detector
threshold. Even with the inclusion in the circuit of a
speech detector threshold adjustment means to take account of
static mismatch with the preceding microphone and AGC
amplifier system, dynamic uncertainties may still give rise
to false-triggering by the speech detector.
An object of the present invention is to provide circuit
means which at least ameliorate some of the above
disadvantages of the prior art and which in preferred
embodiments reduce if not eliminate the effects of variations
in AGC levels, mismatch between speech detector and AGC
reference levels, tracking errors of AGC elements and
transient signals during the AGC attack time. In principle
this is achieved my combining the functions of AGC and speech
detection to eliminate mismatch occurring in separate
circuits and, further, by dynamically comparing the detected
speech level with the detected noise level instead of with a
: - 4 -
constant threshold.
This invention consists in a speech detector system
comprising means for producing a first (speech) signal
representative of speech superimposed on ambient noise and a
second (noise) signal representative of said ambient noise;
comparator means to determine a speech signal threshold
according to peak values of said second signal and to produce
a third (pulse) signal which provides an indication of when
said first signal exceeds said speech signal threshold and
discriminator means responsive to an indication by said
third signal to produce a fourth signal indicating detected
speech when an indication occurs within a second .selected
period which is spaced from a preceeding indication by a
first selected period.
For preference the speech signal and noise signal are
subjected to automatic gain control and noise cancellation
prior to input to the comparison means. For preference also
the ratio of the noise cancelled speech signal to the noise
signal fed to the comparison means is greater than 1:1.
Other aspects of the invention will be apparent from the
description which follows.
An embodiment of the invention will now be described by
way of example only with reference to the accompanying
drawings wherein:
Figure 1 is a schematic block diagram showing part of a
wireless transmitter speech detector system having a two
channel audio system with speech and noise inputs.
Figure 2 shows a first circuit suitable for use as the
~7~3
peak comparator shown as a block in Fig. 1.
Figure 3 shows a circuit suitable for use as the pulse
discriminator shown as a block in Fig. 1.
Figure 4 shows a preferred circuit which combines the
peak comparator and pulse discriminator in a single circuit.
Figure 5 shows a further preferred circuit which combine
the peak comparator and pulse discriminator.
The block diagram of Figure 1 shows schematically a
wireless transmitter speech detector system having a two
channel audio system with speech and noise inputs, and AGC.
The rectified speech and noise signals are compared to detect
the presence of speech.
The AGC system operates in substantially the known
manner to control the peak noise level at the output of the
noise channel, to a predetermined value equal to VREF/ .
The high-frequency components of noise in the speech channel,
which are not removed by the low-frequency noise-cancellation
circuit, will generally be o similar amplitude to the output
of the noise channel. The foregoing control mode is
over-ridden when the level of signal in the speech channel
output exceeds VREF, thus suppressing the noise further and
enabling an effective signal-to-noise ratio for speech of
lOdB to be maintained in noisy environments.
It has been found that, in a practical system, it is
desirable to have the VOX control switched at a speech-to-
noise ratio of approximately 6dB, based on peak values. In
previously-known speech detectors it has been usual,
therefore, to detect, by means of a comparator amplifier, the
757~3
instances when a signal in the speech channel exceeds a
predetermined value ox 2VREF/ ~0.
In the present invention, the peak value of the
rectified noise-channel signal is itself used, after
amplification, as the comparator reference as shown in Fig. 2.
Figure 2 shows a peak comparator circuit for use in the
arrangement shown in Figure 1. Rectified speech signal Is
representative of speech superimposed on ambient noise (a
first signal) and rectified noise signal nIN representative
of ambient noise pa second signal) are fed to the inputs as
shown after amplification by amplifiers shown in figure 1,
where n is the ratio of the amplification of the noise
channel to the amplification of the speech channel.
The current signals Is and nIN respectively generate
voltages Vs and VN across the input resistors Rl and
R2 .
Voltage VN generated by signal nIN appears at the
base of transistor Ql which has its collector connected to a
positive power supply V . The emitter of transistor Ql is
connected to an R-C combination of resistor R4 and capacitor
C2. Transistor Ql, capacitor C2 and resistor R4 comprise
peak detecting means. The transistor Ql operates as a
voltage follower when VN is greater than the voltage on
capacitor C2 and is switched off when VN is less than the
voltage on capacitor C2. Current to charge capacitor C2 is
drawn from supply Vcc. The charging current is therefore
independent of the current drawn by resistor R2 thereby
avoiding response lag. Resistor R4 is in parallel with
7S73
capacitor C2 and the decay time constant ~4C2 is long enough
for acceptable smoothing of random noise.
Voltage Vs generated by signal Is charges capacitor
Cl through diode Dl only when the voltage stored across
capacitor Cl is less than Vs. That is, when a positive
pulse or peak occurs in the speech signal Is. The time
constant of resistor Rl and Capacitor Cl is short enough to
allow Cl to be charged by peaks in Is corresponding to
speech "glottal" pulses, but long enough to filter out
transient noise. Resistor R3 is in parallel to capacitor Cl
and its value sets the decay time of the charge on capacitor
Cl. The decay time constant resistor R3 and capacitor Cl is
made equal to the time constant o resistor R4 and capacitor
C2 to provide good dynamic tracking.
The voltages appearing across capacitors Cl and C2 are
fed to the inputs ox a differential amplifier OPl which
produces a positive output when the voltage across Cl exceeds
the voltage across C2 and a negative output in the reverse
situation. what is, the voltage across capacitor C2
constitutes a speech signal threshold and the output of
amplifier OP1 comprises a third signal which provides an
indication of when the speech signal exceeds the speech
signal threshold.
The rectified noise signal amplification in Figure 1 is
usually twice the rectified speech signal amplification so
that a positive peak in the speech signal of at least twice
the level of ambient noise is xequired to produce a positive
output from differential amplifier OPl.
75~3
Fig. 3 shows one kind of pulse discriminator circuit,
the principle purpose of which is to generate an output VOX
control pulse only if a series of speech pulses is received
but not when a single pulse or short burst of pulses is
received. This is achieved by discriminating between glotta
puls0s comprising speech, which typically occur at 6-8 ms
intervals, and single pulses or short bursts of pulses
separated by less than 3 ms generated by noise. In this way
the system provides immunity to transient noise such as that
caused by an impact, which is typically too fast for AGC
response to be effective.
In the system shown, a bistable latch is used to
generate a VOX control or "speech detected" signal. When a
trigger pulse is received from the peak comparator, a timer
comprising a double pulse generator generates two control
pulses, A and B, with a first selected period Tl and a second
selected period T2 respactively. Control pulse A and the
trigger pulse from the peak comparator are fed to an AND
gate. The output of the AND gate is connected to the SET
input of the bistable latch. A delay is included in the
trigger pulse connection to prevent a trigger pulse reaching
` the AND gate before control pulse A. In this way control
pulse A inhibits the setting of the bistable latch by either
the initial trigger pulse or any subsequent pulse occurring
within the period Tl, typically 5 ms. However, a trigger
pulse occurring within the period T2 is able to sex the
' bistable latch as well as re-starting the timing period T2 of
control pulse B. The latter function ensures that the output
_ 9 _
i
,
~2~2~573
VOX control pulse has at least a period equal to the initial
or minimum value of T2, 10 ins for example. This period is
determined by the requirements of any ensuing transmitter
circuit. At the end of the period T2 after the last trigger
pulse, the bistable latch is reset and control pulse A
trigger is enabled.
Fig. 4 shows a system which combines the peak comparator
and pulse-discriminator functions in one circuit, resulting
in a substantial saving in components which can be important
in the envisaged applications.
The transistors Ql and Q2 act as peak detectiny means for
the noise and speech rectified inputs respectively, and because
transistors Ql and Q2 share a common connection to ground via
the parallel combination of resistor R4 and a storage element
comprising CapaCitGr C2, the collector-emitter current of
either transistor Oll input signal peaks is dependent on the
previous peak value to which capacitor C2 has been charged.
By suitable choice of the decay time of capacitor C2 via
resistor R4, the transistor pair Ql, Q2 can therefore also
act to supress second and subsequent peaks separated by less
than the normal "glottal" period.
This is achieved because the total charge flowing into
the collector of transistor Q2 during a detected speech
pulse input IS is dependent on the instantaneous charge of
capacitor C2, which is arranged to decay via resistor R4. It
J' follows that, if a rapid burst of pulses of similar amplitude
occur in Is, capacitor C2 will charge up rapidly and only
one or two large pulses of charge will flow into the
,, .
-- 1 0
/
7573
collector of transistor Q2, followed by small charge pulses
which are sufficient to keep capacitor C2 charged. In
contrast, speech signals typically have large impulses, or
glottal pulses, spaced apart at 6-8 ms intervals with smaller
amplitude pulses in between. By choice of decay time
capacitor C2 charge decays enough for each glottal pulse to
produce a large current impulse in the collector lead of
transistor Q2. It is therefore quite a simple matter to
discriminate between speech and either continuous or impact
noise by integrating the collector charge impulses and
comparing this voltage with a suitable reference value using
a Schmitt trigger circuit.
Integration is achieved in the circuit of Figure 4 by
means of the R-C combination comprising resistor R5 and
capacitor C3. Charge impulses from the collector lead of
transistor Q2 result in a voltage appearing across capacitor
C3 with respect to constant voltage supply Vcc. The
reference voltage is chosen so that it is exceeded by the
voltage across C3 when three consecutive pulses occur in the
collector lead o transistor Q2 without substantial decay of
the charge on capacitor C3 between pulses. This is achieved
by suitable choice of resistor R5 so that "staircase"
integration of current pulses is obtained for pulses
occurring within a selected period.
The Schmitt trigger is preferred to a comparator, for
example, to ensure clean switching and an output pulse width
in excess of some arbitrary value, dependent on the nerds of
ensuing circuitry. This is related partly to the decay
~~
:~27~;;73
time-constant of capacitor C3 which is controlled by resistor
R5. The decay time-constant is principally determined by the
need for recovery between typical impact-noise occurrence,
but must be long enough for effective "staircase" integration
of glottal pulses. A resistor R6 is included in the
collector lead of transistor Q2 to limit thy amplitude of
charge impulses in the event of the AGC system being
overloaded momentarily by a large impact-noise signal, thus
ensuring that effective discrimination is maintained.
As will be apparent to those skilled in the art, the
amplification of detected noise inputs IN by a greater
factor n than speech input Is further enhances the
discrimination of the circuit by, firstly, generating a
comparator threshold proportional to the ambient noise level
and thereby reducing the frequency of Q2 collector impulses
resulting from random noise and, secondly, because of the
I previously-discussed properties of the two-microphone system,
permitting operator speech to be distinguished from more
distant "ambient" speech, the effectiveness of the approach
being improved because of the reduced reliance on AGC control
accuracy.
Figure 5 shows a further circuit combining the peak
comparator and pulse discriminator functions. The circuit
!~ operates in substantially the same manner as the circuit of
Figure 4 however the transistors Q4 and Q5 act as a current
mirror in a known manner to produce a current flowing through
collector of transistor Q5 proportional to the collector
current of transistor Q2. This allows R-C combination of
.
',
~Z;~i73
resistor R5 and capacitor C3 to be connected to ground
thereby eliminating the need for maintaining the voltage
supply Vcc in Figure 4 constant.
It will be apparent that resistor R5 and capacitor C3
effectively integrate charge impulses flowing from transistor
Q5 collector in substantially the same manner as described
above to control the voltage at the input of Schmitt trigger
Sl which generates a VOX control or "speech detected" signal
when a predetermined threshold voltage is reached.
The amplitude of charge impulses from the collector of
Q5 is limited by the shunting action of Q3 in response to the
voltage developed across R7 by the current flowing into Q2
collector.
As will be apparent to those skilled in the art the
invention hereof may be embodied in other circuits which
function in an equivalent or analagous manner and such
embodiments are within the scope hereof.
- 13
