Amplifier with automatic gain control

An amplifier in which the gain (A.sub.i) is automatically controlled to a maximal value comprises means (D.sub.i) for detecting ringing tones and means (V.sub.i) for reducing the gain upon the detection of ringing tones. Also proposed is a circuit (D.sub.i) for detecting ringing tones. A dynamic equilibrium can be obtained by alternately reducing the gain in the event of ringing tones and raising the gain in the absence of ringing tones.

BACKGROUND OF THE INVENTION 
This invention relates to an amplifier with automatic gain control. 
U.S. Pat. No. 4,079,199 describes an automatically controlled amplifier 
arrangement in which the gain is controlled depending on the occurrence of 
oscillations. 
If in such an arrangement one or more microphones are connected to the 
inputs and one or more loudspeakers are connected to the output, 
oscillations may occur in the event of acoustic feedback between one or 
more of the loudspeakers and one or more of the microphones. Such a 
situation may occur when the arrangement is used for amplifying speech 
signals or music signals in an auditorium. The frequencies of the 
oscillations depend, inter alia, on the geometry of the auditorium, the 
temperature and relative humidity in the auditorium, the distance between 
the speaker and the microphone, and the number of listeners in the 
auditorium. Therefore, the oscillation frequencies are of a more or less 
stochastic nature. In order to counteract these oscillations it is 
proposed in U.S. Pat. No. 4,079,199 to detect random oscillations and 
subsequently to reduce the gain of the arrangement until the oscillations 
are eliminated. After a waiting period the gain of the arrangement is then 
increased again automatically. As a result of this, it is always possible 
that in the known arrangement random oscillations occur, for example 
because the characteristics of the auditorium have changed, for example 
when the number of listeners has increased or decreased. These 
oscillations (howling) are annoying to the audience. 
SUMMARY OF THE INVENTION 
It is an object of the invention to eliminate or at least mitigate said 
drawback of the known arrangement and to this end an amplifier of the type 
defined in the opening sentence is characterized in that it comprises 
means for detecting ringing tones and means for reducing the gain upon the 
detection of ringing tones. 
A further improvement of the amplifier in accordance with the invention is 
achieved in that it further comprises means for raising the gain in the 
absence of ringing tones. This enables the gain to be set to a higher 
desired value in the case of a decrease in acoustic feedback. 
Ringing tones occur when the open-loop gain approximates to unity or, in 
other words, when the damping of a resonant circuit decreases excessively 
for the relevant frequency. When such ringing tones are detected, the gain 
is reduced, in accordance with the invention, so that these ringing tones 
are damped again. Ringing tones are hardly, or not at all perceptible by 
the audience although they can already be detected by the detection system 
and they can subsequently be eliminated before howling (oscillation) 
occurs. This is a significant advantage in comparison with the arrangement 
in accordance with U.S. Pat. No. 4,079,199. 
Ringing tones have the property that their frequency is constant and their 
decay time is comparatively long. Suitably, the invention utilises these 
two properties for the detection of such ringing tones. 
The invention also provides a means for detecting ringing tones which is 
characterized in that it comprises one or more frequency discriminators, 
preferably phase-locked loops, to which at least a portion of the signal 
to be amplified is applied, a clock means for defining a time interval T, 
a period counter for counting a number of periods n of the output signal 
of the phase-locked loop with a frequency f0, and a detection counter for 
counting the number of times N that the period counter has counted n 
oscillations within the time interval T. 
A further disadvantage of the arrangement in accordance with U.S. Pat. No. 
4,079,199 is that the gain cannot be optimized. This is because the gain 
is reduced until the oscillation just ceases at the frequency for which 
the maximum gain is obtained. This means that the gain may be too low for 
other frequency bands. 
In order to preclude this problem it is proposed in accordance with the 
invention that the means for reducing the gain are constructed in such a 
way that the gain is reduced selectively in that frequency band in which a 
ringing tone is detected. 
In accordance with the invention, the amplifier may further comprise means 
for raising the gain, which means are constructed in such a way that the 
gain is increased selectively in those frequency bands in which ringing 
tones are absent. Thus the gain can always be optimized for each of the 
frequency bands. 
Preferably, the ringing-tone detector in accordance with the invention 
comprises a plurality of coupled phase-locked loops whose centre 
frequencies are offset relative to one another so as to cover a maximal 
frequency range. 
In accordance with the invention the value of T lies suitably between 20 
and 40 s, the value of n between 60 and 100, and the value of N between 1 
and 4. This enables a maximum value for the gain setting to be obtained 
and the effect of ringing tones for the audience to be minimized. An 
optimum effect is obtained when T/N.apprxeq.10 s and n.apprxeq.80.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 microphones M1-M.sub.1 are connected to separately controllable 
amplifiers V1-V.sub.1. The outputs of the amplifiers V1-V.sub.1 are 
connected to a signal summing device S having an output coupled to the 
inputs of the filters F1-Fn. The filter F1 is a low-pass filter and the 
filter Fn is a high-pass filter, F2 . . . Fn-1 being bandpass filters for 
intermediate frequency bands. The output signals of the filters F1-Fn are 
applied to variable attenuators A1-An and to ringing detectors D1-Dn. If a 
ringing tone is detected by one of the detectors D1-Dn, an output signal 
is applied to a corresponding one of the gain controls V'1-V'n. This gain 
control then varies the gain by a desired number of steps by means of the 
corresponding attenuator A1-An. The output signals of the attenuators 
A1-An are added to each other and applied to a variable amplifier A0, 
whose output signal is applied to loudspeakers L1-Lm. 
In the block diagram of FIG. 2 of a detection channel of an amplifier in 
accordance with the invention with simple gain control (detection being 
effected in different frequency bands but the gain being controlled 
simultaneously for the entire frequency range) the reference numeral 1 
denotes a phase-locked loop to which the signal to be amplified is applied 
via a variable attenuator 7 and an automatic gain control 8. The 
phase-locked loop 1 has two outputs. One output indicates that the 
oscillator is in lock and the other output supplies the oscillation signal 
of the frequency f0 in the non-locked condition. When the phase-locked 
loop is in lock, i.e. when oscillations of a frequency near the frequency 
f0 of the phase-locked loop occur on the input of this phase-locked loop, 
the periods of the output signal of the phase-locked loop are counted by 
the period counter 2. When the count of the counter 2 exceeds the value n 
detection is effected by the threshold detector 3 and a pulse signal is 
transferred to the detection counter 4. When the count of the counter 4 
exceeds the value N during a time interval T, this is detected by the 
threshold detector 5, which supplies an output signal to the OR gate 6. 
The output signal of the OR gate 6 then controls a variable attenuator, 
which reduces the gain of the arrangement by a desired number of steps. 
The time interval T is defined by means of the clock 24. The counter 2 is 
reset by means of the threshold detector 3 when the threshold n is 
exceeded or by means of the signal L/NL when the threshold n is not 
exceeded (see also FIG. 3), NL being the logic "1" signal. In the manner 
shown in FIG. 3 the counter 4 is reset (signal R'). The counter 4 is reset 
(R') in the same way as illustrated in FIG. 3 for the ringing detector 20. 
The output signal of the OR gate 6 can be used for controlling the gain in 
the same way as the output of 20 (FIG. 3). The reference numeral 26 
indicates a switch which can be opened automatically when the audio signal 
is not adequate. 
In FIG. 3, which shows a channel of the amplifier of FIG. 1 in further 
detail, the reference numeral 17 denotes a band-pass filter, the reference 
numeral 18 a variable attenuator, the numeral 19 an automatic gain 
control, the numeral 20 a ringing detector, such as for example shown in 
FIG. 2 or FIG. 4, the reference numeral 21 an up-down counter, the 
reference numeral 22 a digitally controlled attenuator, the reference 
numeral 23 a gate circuit, the reference numeral 24 a clock, and the 
reference numeral 25 a switch. When the signal to be amplified is not 
strong enough, the switch 25 is opened automatically and the clock 24 is 
reset automatically, so that no gain control is applied. The output signal 
of the clock 24 and the output signal of the ringing detector 20 are 
applied to the gate circuit 23. When N ringing tones have been detected in 
the time interval T defined by the clock 24, the gate circuit 23 supplies 
a control signal to the up/down counter 21, causing the count of the 
counter 21 to be changed by one step and the setting of the digitally 
controlled attenuator 22 to be changed accordingly. Moreover, the counter 
21 then resets the clock 24 and the ringing detector 20. 
The clock ensures that the gain for a channel in FIG. 3 is incremented by 
one step for every period T at least when less than N ringing tones have 
been detected in this period T. In this way the gain of this channel is 
balanced in such a way that the acoustic gain lies between two selected 
values. These values depend on the selected T, n and N and on the 
magnitude of the steps of the attenuators (22 in FIG. 3) in the audio 
amplifiers. A variation of T means a variation of the average howling 
margin. 
FIGS. 4 and 5 illustrate an efficient use of a number of phase-locked loops 
in accordance with the invention. For a specific frequency band three 
phase-locked loops are employed. FIG. 5 illustrates the locking 
sensitivity (U) of each of the phase-locked loops of FIG. 4 as a function 
of the frequency f of a toneburst which is used as a model for the input 
signal applied to a phase-locked loop. The centre frequency of each 
phase-locked loop is equal to (1+x) times the centre frequency of the 
preceding phase-locked loop (where x depends on the type of phase-locked 
loop used and may have a typical value of approximately 0.08). When the 
configuration shown in FIG. 4 is employed and a suitable choice is made 
for the centre frequency in accordance with FIG. 5 it is possible to use a 
comparatively small number of phase-locked loops for a specific frequency 
band. The output signal L/NL of the phase locked loops 9, 10 and 11 is 
applied to one input of the OR gate 12, the output signal of the AND gate 
13 being applied to the other input. (The signal L/NL becomes "0" when 
one of the phase-locked loops is locked. A negligible counting error 
occurs because f0 is always counted and not always the frequency of the 
actually locked PLL.) When one of the circuits 9, 10 or 11 is in lock, the 
number of oscillations of the output signal f0 of the phase-locked loop 10 
is counted by the period counter 15. When the count of the period counter 
15 exceeds the value n the AND gate 13 supplies a signal to the detection 
counter 16, so that the counter 15 is also reset (OR gate 12). When the 
count of the detection counter 16 exceeds the value N the AND gate 14 
generates a signal by means of which the gain can be reduced. 
This signal may be employed, for example as shown in FIG. 3, for reducing 
the gain in a specific channel. The counter 15 is reset in the manner as 
indicated in FIG. 3 (R'). 
A complication is that not all the ringing tones result from excessive 
acoustic feedback of the amplifier. The original spech contains 
preferential tones which may be interpreted as ringing tones by the 
arrangement. In addition, the passive space itself has preferential 
frequencies (resonances). These ringing tones are also detected. However, 
when n is selected to be approximately 80, T to be approximately 30, and N 
to be approximately 3, these ringing tones are detected less frequently on 
the average than ringing tones resulting from excessive acoustic feedback. 
The invention is not limited to the embodiments shown in the Figures. Other 
variants also fall within the scope of the invention. For example, for a 
channel corresponding to a specific frequency band (a specific filter), 
depending on the bandwidth of the filter, a plurality of ringing-tone 
detectors may be used, whose output signals are applied to an OR gate (as 
described with reference to FIG. 2). For each channel there is then 
provided an up/down counter 21 to which the output signal of the OR gate 
is applied, a clock 24 and a gate circuit 23, which elements are arranged 
as indicated in FIG. 3. The output signal of this OR gate may then be used 
for gain control in a manner as described with reference to FIG. 3. In 
principle, the invention also relates to an arrangement comprising only 
one channel in which ringing-tone detection over the entire frequency 
range is effected by means of a single ringing-tone detector and the gain 
for the entire frequency range is controlled by means of a single variable 
amplifier.