Note: Descriptions are shown in the official language in which they were submitted.
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SQUELCH CIRCUlT
This invention relates to the processing of audio
signals in relation to deriving a squelch function for use
with radio receivers. The term 'squelch' refers to the
muting of or significant reduction of the level of audio
output of a receiver when there is no intelligence being
received, thereby relieving the aural strain of an operator
who may have to maintain 2 listening watch for long periods.
A variety of solutions to this requirement have
been proposed. Australiar Patents 500,961 and 509,~53 each
deal with a squelch facility to reduce backcround noise
by comparison of energy levels in the frequency range out-
side the audio range. Other techniques which have been
used are :
1. Comparative assessment of energy levels
present in narrow sub-bands of the received audio band;
2. Real-time analysis of the audio signal;
3. The use of coded preambles and postambles
on the transmitted signal;
4. The use of a tone transmitted continuously
with the signal; and
5. Real-time analysis of the zero-crossings
present in the audio signal.
The technique described in the U.S. Patent
3,939,425 takes its input from the intermediate frequency
(IF) signal of the receiver, to derive the muting signal.
It also demodulates the IF signal to recover the audio out-
put for the receiver.
It appears, from the description, that the
technique is applicable only to amplitude modulated (AM)
signals Thus, the technique requires access to a signal
(the IF signal) which is not normally available as a
receiver output, and is applicable only to Al signals.
The above approaches have varying degrees of
complexity and perform their function with varying degrees
of success. In contrast the technique which is the subject
of this invention has the object of being simple to
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receiver when the V.CØ output frequency is higher than a
pre-set frequency for more than a pre-set time.
A preEerred embodiment of this invention will
now be described with reference to the circuit diagram shown
in Figure 1.
The audio input is shown at 1 a muting gate at 2
and the output at 3.
The present embodiment uses a limiting amplifier 4
to process the audio signal, and uses a phase-locked loop
(PLL~ 5 to determine the average zero-crossing rate of the
limited signal.
If the audio output from a receiver is passed
through a limiting amplifier, thus removing all information
that is not near the centre of the waveform, the resultant
spectrum has the dominant spectra enhanced, thus providing
a low (100 to 700 Hz) average zero-crossing rate for speech
and a higher (800 to 3000 Hz) average zero-crossing rate
for noise The PAL 5 attempts to track the average freq-
uency Gf the clipped audio signal.
The PLL device contains a voltage-controlled
oscillator (VCO) 7, an input amplifier 8, a type I phase
comparator 9, a type II phase comparator l a source
follower 11, and a zener diode lla. The VC0 signal from 7
is compared in frequency with the clipped audio signal by
the phase comparator 10 and an error voltage derived. The
type I phase comparator 9, the source follower 11 and the
zener diode lla all of which are components of this standard
PLL component, are not used in this application of the
invention.
The feedback provided by the error voltage gener-
ated by the phase comparator 10 attempts to maintain the
YC0 frequency equal to the instantaneous frequency of the
clipped audio. Thus, the VC0 control voltage corresponds
to the average zero-crossing rate of the input signal.
The error voltage is fed through a network 12 which provides
fast attack when the input frequency drops, and a slower
decay when the frequency rises.
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implement, while providing excellent perform2nce, and uses
as its input only the audiG signal from the receiver, thus
not requiring access to a receiver IF signal.
To this end the present invention provides a
squelch circuit for muting audio output of a receiver except
when a desired audio signal is received, comprising : an
input terminal connected to a limiting amplifier and a
muting gate; a phase locked loop which recedes the lim-
iting amplifier output and which contains an oscillator
and a phase comparator for comparing the frequency of said
oscillator and said limitirg amplifier output to produce
an error voltage which is fed back to control said oscill-
ator through a network to enable the oscillator to match its
frequency to that of the limiting amplifier output, the
match occurring faster when the limiting amplifier output
frequency drops than when the limiting amplifier output
frequency rises; said muting gate being connected to the
output of said phase locked loop to mute the output of the
receiver when the limiting amplifier output frequency is
~0 higher than that desired.
The device of this invention performs an analysis
of a receiver audio output in terms of the zero-crossings
of the signal and provides an output signal kick can be
used to control the muting of the system.
Examination of thy spectra of audio-signals prod-
uced by a radio receiver shows that for speech, the fre-
quency of the dominant spectral components lies in the range
from about 100 Hz to 300 Hz for male vo;ces and from about
200 Hz to 700 Hz for female voices. These figures hold
true for the majority of English speech, the exception being
with sibilants and fricatives. With noise, however, the
frequency of the dominant spectral components covers a much
wider range, limited by the band-width of the receiver.
Preferably the osc;llator in the phase locked loop
is a voltage controlled oscillator ~V.CØ). The muting
gate output is preferably derived from the V.CØ control
voltage of the phase locked loop to mute the output of the
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receiver when the V.CØ output frequency is higher than a
preset frequency for more than a preset time.
A preferred embodiment of this invenLion will now
be described with reference to the circuit diagram shown in
S Figure 1.
The audio input is shown at 1, a muting gate at 2
and the output at 3.
The present embodiment uses a limiting amplifier
4 to process the audio signal, and uses a phase-locked loop
(PLL) 5 to determine the average zero-crossing rate of the
limited signal.
If the audio output from a receiver is passed
through a limiting amplifier, thus removing all information
that is not near the centre of the waveform, the resultant
spectrum has the dominant spectra enhanced, thus providing
a low (100 to 700 Hz) average zero-crossing rate for speech
and a higher (800 to 3000 Hz) average zero-crossing rate Eor
noise. The PLL 5 attempts to track the average frequency of
the clipped audio signal.
The PLL device contains a voltage-controlled
oscillator ~VCO) 7, an input amplifier 8, a type I phase
comparator 9, a type II phase comparator 10, a source follower
11, and a zener diode lla. The VCO signal`from 7 is compared
in frequency with the clipped audio signal by the phase
comparator 10 and an error voltage derived. The type I phase
comparator 9, the source follower 11 and the zener diode lla
all of which are components of this standard PLL component,
are not used in this application of the invention.
The feedback provided by the error voltage generated
by the phase comparator 10 attempts to maintain the VCO
frequency equal to the instantaneous frequency of the clipped
audio. Thus 7 the VCO control voltage corresponds to the
average zero-crossing rate of the input signal. The error
voltage is fed through a network 12 which provides fast
attack when the input frequency drops, and a slower decay
when the frequency rises.
Thus, when the average zero-crossing rate suddenly
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reduces, corresponding to the appearance of speech components
on the received signal, the VC0 frequency will quickly attempt
to follow, rapidly lowering the VC0 control voltage which is
used to unmute the receiver. If the zero crossing rate later
increases, as would occur at the end of a transmission when
the carrier ceases9 the VC0 frequency will rise slowly, and
will only mute the receiver when it has risen above a specific
threshold voltage. This fast attack and slow decay allows
the muting to be held off between words of the speech, even
though the zero-crossing rate is high during this time.
he VC0 control voltage is fed to a comparator 13
to derive the mutiny signal. When the VC0 control voltage
is lower than the comparator reference (half the supply in
this case) the muting signal is high. When the VC0 control
voltage is greaLer than the reference, the muting signal is
low. The comparator signal is further smoothed and modified
in its attack and decay characteristics by a nonlinear filter
14 on the comparator output, and fed to a muting switch 2 in
the form of an analogue gate for the control of the audio
output signal. The output of the comparator is also fed to
a light emitting diode 15 which gives visual indication that
the VC0 frequency is below the specified limit, corresponding
to the detection of speech.
Since the spectral nature of speech and the noise
output of a receiver both vary, an adjustment 16 is provided
to set the optimum centre frequency and tuning range of the
VC0. The adjustment of the frequency characteristics of the
VC0 is implemented in a manner somewhat diferent to that
generally used, and provides an essentially constant ratio
between the centre frequency and the tuning range of the ~C0
as the centre frequency is adjusted. It has been demonstrated
in tests involving a wide range of speech and noise spectra
that the constant ratio between centre frequency and tuning
range has advantage over the usual characteristic of the VC0
where this ratio increases significantly with centre fre-
quency.
l~ith strong signals, the centre frequency setting
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is not critical, but to detect speech în very poor signal/
noise ratios, the optimum adjustment range is narrower.
Because of the finite response time of the signal
processing, unmuting will occur slightly after speech is
first received, resulting in the partial loss of the first
syllable of speech. In this en-bodiment, the reaction time
of the system is such that very little degradation results
from the delay. However, should this degradation be unaccept-
able, it can be overcome by incorporating a suitable delay
in the path between the audio input and the muting switch.
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Thus, when the average zero-crossing rate suddenly
reduces, corresponding to the appearance ox speech compon-
ents on the receiver signal, the VC0 frequency will quickly
attempt to follow, rapidly ]owering the VC0 control voltage
which is used to unmute the receiver. If the zero-crossing
rate later increases, as would occur at the end of a trans-
mission when the carrier ceases, the VC0 Frequency will rise
slowly, and will only mute the receiver when it has risen
above a specific threshold voltage. This fast attack and
slow decay allows the muting to be held off between words of
the speech, even though the zero-crossing rate is high
during this time.
The VC0 control voltage is fed to a comparator 13
to derive the muting signal. When the VC0 control voltage
is lower than the comparator reference (half the supply in
this case) the muting signal is high. When the VC0 control
voltage is greater than the reference, the muting signal is
low. The comparator signal is further smoothed and modified
in its attack and decay characteristics by a non-linear
filter 14 on the comparator output, and fed to a muting
switch 2 in the form of an analogue gate for the control of
the audio output signal. The output of the comparator is
also fed to a light emitting diode 15 which gives visual
indication that the VCO frequency is below the specified
limit, corresponding to the detection of speech.
Since the spectral nature of speech and the noise
output of a receiver both vary an adjustment ~6 is provided
to set the optimum centre frequency and tuning rahge of the
VC0. The ad3ustment of the frequency characteristics of the
VC0 is implemented in a manner somewhat different to that
generally used, and provides an essentially constant ratio
between the centre frequency and the tuning range of the VC0
as the centre frequency is adjusted. It has been demon-
strated in tests involving a wide range of speech and noise
spectra that the constant ratio between centre frequency and
tuning range has advantage over the usual characteristic of
the VC0 where this ratio increases significantly with centre
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frequency.
With strong signals, the centre frequency setting
is not critical, but to detect speech in very poor signal/
noise ratios, the optimum adjustment range is narrower.
Because of the finite response time of the signal
processing unmuting will occur slightly after speech is
first received, resulting in the partial loss of the first
sy].lable of speech. In this embodiment, the reaction time
of the system i.s such that very little degradation results
from the delay. However, should this degrada_ion be un-
acceptable, it can be overcome by incorporating a suitable
delay in the path between the audio input and the muting
switch.
