Electrical audio signal processing systems and devices – Feedback suppression
Reexamination Certificate
1998-08-10
2001-07-31
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Feedback suppression
C381S083000, C379S413020, C379S413020
Reexamination Certificate
active
06269165
ABSTRACT:
This invention relates to a method of and apparatus for reduction of unwanted feedback in a system.
Unwanted feedback naturally follows from the use of amplifiers. It has the effect of making uncontrolled changes to the frequency response of a system. As a result it sets a limit to the gain which can be used before oscillation or unacceptable degradation of the frequency response occurs.
A well-known example of such feedback is with public address systems, where a microphone used by a person speaking or playing a musical instrument picks up the output of a nearby loudspeaker, giving rise to a howl which drowns out the speech or music and renders it unintelligible. This is sometimes termed ‘howl-around’. The system which gives rise to this is illustrated in outline in
FIG. 1
, which is a plan of a stage system
10
used by a speaker
12
to address an audience. His speech is picked up by a microphone
14
, and the resultant signal amplified by an amplifier
16
. A loudspeaker
18
broadcasts the sound, some of which is picked up by the microphone
14
, either directly or after reflection from the walls of the building or stage etc. Such a sound path is indicated by the reference
20
.
It is possible to draw an equivalent circuit to this, as shown in FIG.
2
. In
FIG. 2
is seen a signal source
30
, in this case the speaker
12
of
FIG. 1
, and an input path
32
representing the sound path between the speaker and the microphone
14
. At the microphone the sound from the speaker through input path
32
is combined in circuit
34
with sound from the unwanted feedback path
36
which corresponds to the sound path
20
of FIG.
1
. The combined signal from combiner
34
is applied to the amplifier
38
in
FIG. 2
, corresponding to the amplifier
16
and loudspeaker
18
in FIG.
1
. The output
39
of amplifier
38
both constitutes the desired output and provides the input to the feedback path
36
.
A similar situation arises in a rebroadcast transceiver which is designed to receive RF signals, amplify them, and retransmit them onward on the same frequency. While steps are taken, e.g. by using highly directional antennas, to reduce unwanted feedback, there is inevitably some unwanted feedback from the transmitting antenna back to the receiving antenna.
FIG. 3
is a generalised diagram based on
FIG. 2
, showing the transfer functions of the various parts of the system. It is assumed that the input signal
30
has spectrum I(f), the input path
32
has a frequency response H(f), the unwanted feedback path
36
has a response B(f), the amplifier
38
has a response A(f), and the output signal
39
has spectrum O(f). The resulting overall transfer function is:
O
(
f
)
I
(
f
)
=
A
(
f
)
H
(
f
)
1
-
A
(
f
)
B
(
f
)
(
1
)
As the complex-valued loop gain A(f)B(f) approaches (1+jO), the system becomes unstable. If the loop contains a dominant delay which is significant compared with the reciprocal of the system bandwidth, then the frequency response contains regular ripples.
To remove the effect of this feedback it needs to be cancelled out. This can be done using either of the circuits of
FIG. 4
or FIG.
5
. In
FIG. 4
, a compensating circuit
40
with transfer function C(f) has its input connected to the output of the amplifier
38
. The output of the compensating circuit
40
is combined in a combining circuit
42
with the output of the existing combiner
34
, and the output applied to the input of the amplifier
38
. Such a circuit cancels out the unwanted feedback so long as C(f)=−B(f). This is known as neutralisation.
The overall transfer function now becomes:
O
(
f
)
I
(
f
)
=
A
(
f
)
H
(
f
)
1
-
A
(
f
)
{
B
(
f
)
+
C
(
f
)
}
(
2
)
Commonly, some simple technique is used to ensure that C(f)=−B(f) at some spot frequency which is most critical. Depending on the exact circuit used to ensure that the deliberate feedback has the right amplitude and phase this will give a more or less narrow-band solution.
Once feedback cancellation has been achieved, the final output is what would be expected, given that the signal has travelled the input path H(f):
O
(
f
)
I
(
f
)
=
A
(
f
)
H
(
f
)
(
3
)
However, the circuit of
FIG. 4
has certain disadvantages. For example, it might not be desired to tap off some of the amplifier output signal into the compensating path. Also, in the context of RF transmission, if the processing is done at an intermediate frequency (or at baseband), the circuit of
FIG. 4
requires two down-converters. This is particularly disadvantageous if the processing in the compensating path is digital, since each down-converter must also then be accompanied by filtering and analogue-to-digital conversion.
The circuit of
FIG. 5
is therefore preferred. In this circuit the compensating path
44
with transfer function D(f) has its input coupled in parallel with the input of the amplifier
38
, rather than being connected to its output. The compensating path
44
together with the combining circuit
42
now form a pre-corrector
46
for the amplifier
38
.
The transfer response in this case is:
O
(
f
)
I
(
f
)
=
A
(
f
)
H
(
f
)
1
-
D
(
f
)
-
A
(
f
)
B
(
f
)
(
4
)
To cancel the unwanted feedback D(f) is chosen so that D(f)=−A(f)B(f).
An example of a circuit of the type shown in
FIG. 5
is described in UK Patent Application GB-A-2 065 421. This describes a rebroadcast transceiver which compensates at baseband prior to amplification by a power amplifier. The circuit described in GB-A-2 065 421 is presented in
FIG. 6
of the present application. This figure is not as such in GB-A-2 065 421, but is based on its contents. The rebroadcast transceiver
300
is used to receive the off-air broadcast from source
330
through channel
332
, combined at
334
with feedback from the transceiver output through path
336
. The main amplifier
319
of the transceiver is preceded by a pre-corrector
346
which is fed by a signal on an antenna line
301
from combiner
334
.
The pre-corrector comprises a series circuit containing a down-converter
304
, a combining circuit
342
, a low-pass filter
309
, an amplifier
313
, a delay
314
, and an up-converter
311
. A multiplicative mixer
324
is connected to receive and multiply together the input and the output of the delay
314
. The output of the multiplicative mixer
324
is applied through a low-pass filter
326
. A second multiplicative mixer
325
receives and multiplies together the output of the delay
314
and the output of the low-pass filter
326
. The output of the second multiplicative mixer
325
is applied to the second input of the combining circuit
342
(actually represented by a simple junction in the patent specification).
Because of the down-conversion operation, it is important to retain both phase and amplitude information in the signal. For this purpose the pre-corrector
346
operates with complex signals, but this complexity is not shown in
FIG. 6
, for simplicity of explanation.
The circuits
324
,
325
and
326
are together referred to as a “correlator”
320
, the purpose of which is to compensate for feedback signals through path
336
. The “correlator”
320
is stated to “correlate” unwanted frequencies sensed after filtering by the low-pass filter
309
with the unwanted frequencies present at the output of the down-converter
304
so as to effect cancellation. In fact, however, the “correlator” generates only a single output coefficient. The delay
314
is stated to be necessary in order to distinguish between the wanted signals and the feedback signal, so that the “correlator” operates only to cancel the unwanted signal. However, other known correlators may be used to produce a similar result.
This circuit can however only provide proper compensation at a single frequency, and thus is an extreme example of a narrow-band solution to the problem. It may be adequate for, e.g., military communications
Stott Jonathan Highton
Wells Nicholas Dominic
British Broadcasting Corporation
Dike Bronstein, Roberts & Cushman LLP
Isen Forester W.
Neuner George W.
Pendleton Brian Tyrone
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