Automatic gain control device for a single-sideband receiver

A circuit determines whether the signal transmitted by a variable-gain amplifier comprises information or not. If it comprises information, its level is regulated, by comparison with a first reference signal, in order to give a predetermined correct listening level at the output of the receiver. If it does not comprise any, its level is regulated, by comparison with a second reference signal, in order to give at the output of the receiver the listening level corresponding to the hearing threshold.

The present invention relates to an automatic gain control device for a 
single-sideband receiver comprising at least one variable-gain amplifier, 
a discriminator circuit for determining whether any information is present 
in the signal received by the receiver and a control circuit, coupled to 
the discriminator circuit, for regulating the output level of the 
variable-gain amplifier. 
Devices of this type are known, in particular in receivers in which the 
information consists of speech; these devices make it possible on the one 
hand to regulate the variable-gain amplification to a value corresponding 
to a constant listening level from the s.s.b. receiver, and on the other 
hand to prevent the receiver from delivering a signal for listening when 
the discriminator circuit indicates that the received signal does not 
comprise any speech. However, since speech is not recognised without error 
by known discriminator circuits, this results in losses of speech in the 
signal and untimely increases in noise. 
It is possible to manufacture such an automatic gain control device which 
is not attended by the aforementioned disadvantages but which, at the same 
time, retains the aforementioned advantages. 
This is done by regulating the output level of the variable-gain amplifier 
so as to bring the output signal of the s.s.b. receiver towards the normal 
level of hearing when the signal contains information, and towards the 
threshold of hearing when it contains only noise. 
According to the invention, an automatic gain control device for a 
single-sideband receiver, comprising at least one variable-gain amplifier 
having an output, a discriminator circuit for delivering a first or a 
second signal according to whether information is present or not in a 
signal received by said receiver, and a control circuit, coupled to said 
discriminator circuit, said control circuit comprising controlling means 
for bringing the output level of said variable-gain amplifier to a value 
V.sub.a or V.sub.b, according to whether said first or second signal is 
present, V.sub.a and V.sub.b being two different values of said output 
level enabling said receiver to be heard.

The FIGURE illustrates a variable-gain amplifier 1 of a high-frequency 
s.s.b. receiver. The amplifier receives a low-frequency signal V.sub.e and 
delivers a signal V.sub.s whereof the signal-to-noise ratio will by 
hypothesis be at least 10 dB for correct transmission. The amplifier 1 
comprises a control input to which are connected: the first terminal of a 
condenser C whereof the second terminal goes to earth, and the output of a 
negative-current generator 70 and of a positive-current generator 71; 
these current-generators are respectively intended to enable the condenser 
C to be discharged and charged, and constitute the current-generator 
circuit 7 of the device of the FIGURE. 
The signal V.sub.s is the output signal of the device of the FIGURE; this 
is the signal which, after amplification, gives the listening signal of 
the receiver. 
The signal V.sub.s is also used in the device of the FIGURE as the input 
signal of a comparison circuit 2, and of a speech-discriminator circuit 3. 
The circuit 3 determines, with a certain risk of error which is inherent 
in circuits of this kind, whether the signal V.sub.s comprises speech or 
not; it delivers a signal P and a signal P, respectively exhibiting the 
logic levels "1" and "0" in the case in which speech is detected in the 
signal V.sub.s, and the logic levels "0" and "1" in the opposite case. 
The comparison circuit 2 comprises a peak-detector circuit 20 and two 
voltage-comparators 21 and 22. The signal V.sub.s is applied via the 
circuit 20 to the "+" input of the comparator 21 and to the "-" input of 
the comparator 22. By its constitution, the circuit 20 introduces a delay 
of 1 ms into the transmission, with respect to its output signal, of the 
variations in the peak value of its input signal V.sub.s. 
A voltage-generator 4 delivers two d.c. reference voltages V.sub.1 and 
V.sub.2, which are applied to the "-" input of the comparator 21 and to 
the "+- input of the comparator 22 respectively; the values of the 
voltages V.sub.1 and V.sub.2 are those assumed by the output signal of the 
peak-detector circuit 20 when the level of the signal V.sub.s is 
respectively the predetermined listening level considered as normal (which 
will be by convention a level of 0 dB) and the level of the threshold of 
hearing, situated NdB below the normal listening level; in the example 
described, N is taken as equal to 20. 
The output signals of the comparators 21 and 22 are applied to a group of 
logic circuits 5, which receives in addition the signals P and P worked 
out by the speech-discriminator circuit 3 and the signals H.sub.1, H.sub.2 
and H.sub.3 formed from periodic pulses supplied respectively by the three 
outputs of a frequency-generator 6. 
The group of logic circuits 5 is intended to supply the current-generators 
70 and 71 in non-simultaneous manner with pulses during which these 
generators will respectively be able to charge or discharge the condenser 
C. The time required to charge or discharge the condenser, which will for 
simplicity be called the "charge (or discharge) time constant" is a 
function of the value of this condenser, of the output impedance of the 
current-generator which supplies a charging or discharging current for the 
condenser, and of the width and frequency of the pulses which control the 
operation of the current-generators 70, 71; it is by varying the frequency 
of the pulses which it supplies to the current-generators 70 and 71 that 
the group of logic circuits will vary the charging time constant of the 
condenser C; the reasons for these variations will be explained 
hereinafter. 
The automatic gain control devices of the FIGURE is provided in order to: 
return the level of the signal V.sub.s very rapidly to 0 dB when it exceeds 
this value; this makes it possible to avoid disagreeable listening, due to 
too much sound power inside the output headphone of the receiver, 
make the level of V.sub.s approach normal listening level when the 
speech-discriminator circuit determines that speech is present, and that 
the level of the signal V.sub.s is lower than 0 dB, 
make the level of V.sub.s approach the threshold of hearing when the 
discriminator circuit does not determine that any speech is present in the 
signal V.sub.s, or when the level of the signal V.sub.s is lower than 
-NdB; on the one hand this avoids any annoyance, during the absence of a 
speech signal, by a high-level (0 dB) noise signal, and on the other hand 
it enables the speech signal to be heard if the speech-discriminator 
circuit gives an erroneous result (signal P=0 when the signal V.sub.s 
contains speech), 
arrange that if the level of the signal V.sub.s is between 0 and -NdB the 
approach to the levels of 0 dB or -NdB shall not be too rapid should the 
speech-discriminator circuit give an erroneous result for a short time, so 
that this erroneous result shall involve only slight disadvantages; in 
fact, at the instant when the result is again correct the gain of the 
amplifier has not had time to vary appreciably, 
arrange that the noise level rises slowly when the level of the signal 
V.sub.s is lower than -NdB; in fact, the signal V.sub.s is practically 
unable to exhibit a level lower than -NdB except when it comprises no 
speech signal and when the signal-to-noise ratio of the transmission is 
very good (greater than 20 dB), and there is therefore no advantage in 
this case in rapidly increasing the gain of the variable-gain amplifier, 
since when the speech signal appears its level will be too high (greater 
than 0 dB). 
In order to obtain these objectives, the group of logic circuits 5 
comprises: 
two AND gates 51 and 52 which respectively receive at one of their two 
inputs the signals H.sub.1 and H.sub.3, and at their other input the 
output signals of the comparators 21 and 22, 
an inverting OR gate 50 which receives the output signals of the 
comparators 21 and 22, 
two AND gates 53 and 54 receiving at their first input the output signal of 
the inverting OR gate 50, at their second input the signal H.sub.2, and at 
their third input the signals P and P respectively, 
two OR gates 55 and 56 which receive at their first input the output 
signals of the gates 51 and 52 respectively, and at their second input the 
output signals of the gates 53 and 54 respectively; the output signals of 
the gates 55 and 56 respectively constitute the control signals for the 
current-generators 70 and 71. 
Thus when the signal V.sub.s has a level greater than 0 dB (that is to say 
when the output voltage of the peak-detector circuit 20 is greater than 
the voltage V.sub.1 supplied by the generator 4) the comparator 21 
supplies a logic level "1" which enables the signal H.sub.1 to be sent via 
the AND gate 51 and the OR gate 55. The frequency of the pulses in this 
signal H.sub.1 is so adjusted that the negative current-generator 70 
discharges the condenser C with a short time constant .tau..sub.1 of the 
order of 3 ms; the result of this is that the signal V.sub.s rapidly 
approaches 0 dB. 
When the signal V.sub.s has a level of between 0 and -NdB, and the signal P 
from the circuit 3 represents the logic level "1" (speech signal detected 
in the signal V.sub.s), the AND gate 54 allows the signal H.sub.2 to pass, 
and this signal will control the positive current-generator 71 via the OR 
gate 56. The frequency of the pulses in this signal H.sub.2 is so adjusted 
that the generator 71 charges the condenser C with a mean time constant 
.tau..sub.2 of 30 ms; this results in V.sub.s approaching the level of 0 
dB with this time constant .tau..sub.2. 
When the signal V.sub.s has a level of between 0 and -NdB, but the signal P 
from the circuit 3 exhibits the logic level "1" (no speech signal detected 
in the signal V.sub.s), the signal H.sub.2 passes through the gates 53 and 
55; this results in the negative current-generator 70 being so controlled 
that the condenser C discharges and causes the level of the signal V.sub.s 
to approach -NdB with the same time constant .tau..sub.2 as before, given 
that in the example being described the current-generators 70 and 71 have 
the same output impedance. 
And when the signal V.sub.s has a level lower than -NdB the comparator 22 
supplies a logic level "1" which enables the signal H.sub.3 to be sent via 
the AND gate 52 and the OR gate 56 to the control input of the positive 
current-generator 71. The frequency of the pulses in the signal H.sub.3 is 
so adjusted that the current-generator 71 charges the condenser C in such 
a manner as to cause the level of V.sub.s to approach -NdB with a time 
constant .tau..sub.3 of 100 ms. 
Other automatic gain control devices for a s.s.b. receiver may be envisaged 
within the scope of the invention. Thus for example when the level of the 
voltage V.sub.s is between 0 and -NdB the time constant in order to bring 
it to 0 dB may be different from the time constant in order to bring it to 
-NdB. Likewise, the time constants which have been discussed were obtained 
by virtue of signals formed from pulses whereof the frequency was chosen 
as a function of these time constants; this could be replaced by a group 
of resistances capable of being switched over and intended to alter the 
output impedance of the current-generators 70 and 71, the change-over of 
these resistances being controlled for example with the aid of the output 
signals of the comparators 21 and 22 and the inverting OR gate 50. 
The signal V.sub.e of the FIGURE is a low-frequency signal; it should be 
noted that this signal could be an intermediate-frequency signal, in which 
case the circuit 1 would be made up of a group of amplifiers and 
frequency-changer circuits, and the control signal, taken off across the 
terminals of the condenser C, would control the gain of all or some of 
these amplifiers. 
It should be noted that the invention is more particularly applicable to 
s.s.b. h.f. receivers because of the problems of fading which affect the 
received signal. 
Of course, the invention is not limited to the embodiment described and 
shown which was given soley by way of example.