Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND OF THE INVENTION
Echo cancellation circuits which are ~7ell known
in the telephone communications environment usually
employ a form of adaptive finite impulse response
digital fil~er. In a typical system the echo- sup-
pression apparatus at the near subscriber functions
to disable the outgoing path from that subscriber
when signals from the far end subscriber appear on
the incoming path. Echoes due to incoming signals
on the receive path are, therefore, prevented from
returning to the far end subscriber over the out-
going path. Double talking refers to the condition
when ~he near end subscriber breaks in and both
subscribexs are talking simultaneously. Prior art
32 echo suppressors include do~ble talk detectors which
distinguish between speech signals generated on the
outgoing path by the near end subscriber (double
talk signals) and echo signals returning on the
outgoing path due to the far end subscriber speech
signals on the incoming path. If the outgoing path
signal exceeds the incoming path signal it is
assumed that the near end subscriber is transmitting
and the echo suppression is disabled. When the
opposite condition occurs it is assumed that the
near end subscriber is not transmitting and the echo
is suppressed.
The patents to May, Jr. -(U.S. Patent No.
3,973,086), Helder (U.S. Patent No. 3,992,594) and
Geigel e~ al (U.S. Patent No. 4,029,912) discuss the
problem of distinguishing between an echo and double
talk. The above systems work by initially providing
an approximated echo using a pair of storage memo-
ries, one following decreasing magnitude siynals to
give an output corresponding to the anticipated (or
worst case) echo end delay, while the other memory
is used to approximate echo as long as i-ts output is
less than the ou~put o~ the first memory. Othex-
wise, the first memory is used for the echo approxi-
s mation. The second memory follows increasing magni~tude siynals, less a certain transmission loss, and
holds the value of the last signal peak while the
signal magnitude is decreasing. These approxima-
tions are compared with the actual signals on the
echo return path. Double talk detection by this
method is evidenced in all three of the above-
mentioned patents. This method is different than
that of the subject disclosure in which a third
detector is used to control the signal path through
the other detectors during initial adaptive filter-
ing when the residual echo is less than the true
echo.
The patent to Araseki et al (U.S. Patent No.
4,005,277) describes an echo controller comprising a
mode switch for switching an echo suppressor and
canceller. This switch normally supplies the echo
cancelled signal to the outgoing path and further
evaluates characteristics of the echo path and
operation of the echo canceller to substitute the
echo suppressed signal for the echo cancelled signal
only when the path characteristics are questionable.
The mode switch thus monitors the double talk and
carries out the switching between the echo cancelled
and suppressed signals in the absence of double
talk.
Other patents to Araseki et al (~.S. Patent No.
4,012,603) and Ochiai et al (~.S. Patent No.
3,787,645) discuss various aspects o~ this type of
double talk detection system.
~6'~;89
A "Twelve-Channel Digital Echo Canceller" which
uses a digltal filter and a center clipper is de-
scribed by D. Duttweiler in IEEE Transactions on
Communications, Vol. Com-26, No. 5, May 1978.
BRIEF DESCRIPTION OF THE DRAWINGS
_
Figure 1 is a block diagram of a prior art echo
cancellation circuit.
Figure ~ is a schematic diagram illustrating a
double talk detection circuit which can be used in a
preferred embodiment of the present invention.
Figure 3 is a block diagram of a preferred
e~bodiment of the pres~nt invention.
The prior art, such as that discussed above, is
further illustrated in Figure 1, which shows an echo
canceller circuit at the near talker location. A
received signal rom the distant talker Dt is passed
throllgh insulating amplifier 7 t.o the receiver
output ~o. Path 2 represents the arbitrary path
.aken by the received signal causing an echo thereof
to be fed back into the transmitting portion of near
talker eguipment. The echo signal Ec is similar to
the signal Dt but attenuated ~y the propagating
medium 2.
The use of adaptive filter 1 for echo cancella-
tion in long distance telephone circuits is well
known and descxibed in detai.l by S. J. Campanella et
al, "ANALYSIS OF AN ~DAPTIVE IMPULS~ RESPONSE ECHO
CANCELL~.~", Comsat Technical Review, Vol. 2, No. 1,
Sprin~ 1972, pp. 1-36, and by O. A. Horna, "ECHO
C~NCELLER UTILIZING PSUEDO LOGARITHMIC COATING", NTC
1977 Conference Record, Vol. 1, pp. 04:7-1 through
04:7-8. Such adaptive filters compute an estimate
E'c of the true echo signal Ec and apply it to
subtractor c~.-cu~ r. 3 . After an initial adaptive
period, the proper transfer characteristic is devel-
oped in filter 1 and only a small rcsidual echo
sig~al Re remains at the output subtractor 3. The
residlla1 he lS t.hen easily suppressed by a non-
ear center cli~er circuit.
~oth adaptlle fiiter 1 and center clipper 5 may
-einain ln an act~e state dS lonq as de~ired to
effect ca cellation c,f the echo signal Ec. However,
n ~he event that the r.ear talker breaks into the
conversation, a "double talk" condition occurs. Both
the echo Ec and the near talker speech Nt appear at
the input to the transmitting portion of the near
talker equipment. Under these conditions, two
changes must take place: (1) the adaptation process
of the filter 1 must be "frozen`' so that it will not
be contaminated by the uncorrelated near talker
si~nal Nt and (2) the center clipper 5 must be
disabled in order not to distort the near talker
speech. The residual echo signal Re, being much
smaller than Nt is completely masked by the speech
signal.
The function of the circuit in Figure 1 is
based on -the assumption that during double talk, the
volume of reclr talker speech Nt is higher than the
volu~e of the echo, i.e., Nt is greater than Ec.
The tripping point o~ double talk detector 6 can be
adjusted to indicate a double talk condition when,
Si = Ec ~ Nt ' 0.5 Dt.
Detector 6 under these conditions disables the
center clipper 5 and the correction loop of filter 1
over lines 9 and 8, respectively.
3(~ Certain conditions will degrade performance of
the latter echo cancellation circuit. One such
coindition, known as false double talk, can occur
when the distant speaker pauses. The echo, delayed
due to the prcpagation delays inherent in path 2,
will be greater than 0.5 Dt thereby causing the
double talk detector to falsely indicate a double
talk conditioll. Center clipper 5 wlll be turned off
resulting in a burst of residual echo Re on the
send-out line.
The opp~site situation exists when the near talker speech
is of a lo~er amplitude than the echo signal. This can occur in
low quality, long two-wire cïrcuits. In this case, detector 6
is unable to detect the double talk condition and the impulse
response o~ filter 1 is contamïnated and the center clipper
distorts the speech sïgnal Nt.
SUMMARY OF THE INVENT I ON
T~e degradatïon of the echo canceller performance as
descrïbed with respect to the prior art above is avoided by the
use of a plurality of double talk detectors.
In one aspect the invention pertains to an echo cancellation
circuit having a reception line for providing a distant party
signal received from a distant party~ a transmit line having
a near party signal from a near party and an echo signal applied
thereto for providing a composite signal. A filter is provided
for receiving the distant party signal at a filter input and
producïng a signal which estimates the echo signal, the estimated
signal being produced after an initial adaptive period. Sub-
tractïon means provide for substracting the estimated signal from
the composite signal to produce an intermediate signal along the
transmission line. Signal suppression means provide for receiv-
ing the intermediate signal and for producing a transmitted sig-
nal. A first detector receives the distant party signal and the
composite signal and provides a first output signal when a first
predetermined quantitative relationship exists between the dis-
tant party signal and the composite signal. A second detector
receives the distant party signal and the intermediate signal
and provides a second output signal when a second predetermined
quantitative relationship exists between the distant party sig-
nal and the intermediate signal. A third detector receives thecomposite s;gnal and the intermediate signal and provides a
third output signal when a third predetermined quantitative re-
lationship exists between the composite signal and the inter-
mediate signal. Logic means receive output signals from each of
the first, second and third detectors, the logic means selec-
tively enabling and disabling the fil-ter means and suppression
means under the control of the first, second and third detector
output signals.
~' ~
d~
Another aspec-t of the invention pertains to an echo can-
cellation circuit having a recep-tion line for providing a dls-
tant party signal received from a distant party, a -transmit
line having a near party signal from a near party and an echo
signal applied thereto for providing a composite signal. A
filter is provided for receiving the distant party signal at a
filter input and producing a signal which estimates the echo
signal, the estimated signal being produced after an initial
adaptive per;od. Subtraction means subtract the estimated sig-
nal from the composite signal to produce an intermediate signalalong the transmission line and signal suppression means receive
the intermediate signal and produces a transmitted signal. A
first detector generates a first detection signal in response to
the existence of a first quantItative relationship between the
distant and near party signals and first means enable and dis-
able at least one of the ~ilter and signal suppression means in
response to -the first detection signal. Means are provided for
determining the end of the intitial adaptive period and a second
detector is provided for generating a second detection signal in
response to the existence of a second quantitative relationship
between the intermediate signal and the distant party signal,
the first means disabling at least one of the filter and the
signal suppression means in response to the second detection
signal only after the end of the initial adaptive period is
determined.
In another aspect, the invention pertains to an echo can-
cellation circuit having a reception line for providing a dis-
tant party signal received from a distant party, a transmit line
having a near party signal from a near party and an echo signal
applied thereto for providing a composite signal, a filter for
receIving the distant party signal at a filter input and pro-
ducing a signal which estimates the echo signal, the estimated
si~nal being produced after an initial adaptive period. Sub-
traction means are provided for subtracting the estimated signal
from the composi-te signal to produce an intermediate signal
along the transmission line, and signal suppression means receive
the intermediate s;gnal and produces a transmitted signal. The
method of controlling the echo cancellation circuit, is of the
type comprisîng the s~eps of generating a first comparison signal
in accordance with the comparison of the composite signal to the
distant party signal and further includes generating a second
comparison signal in accordance with the comparison of the second
intermediate signal and the distant party signal, and controlling
at least one of the filter and the suppression means in accordance
with both the first and second comparison signals.
A still further aspect of the invention pertains to an echo
cancellation circuit having a reception line for providing a
di5tant party signal received from a dïstant party, a transmit
line having a near party signal from a near party and an echo
signal applied thereto for providing a composite signal and a
filter having an adaptation process having enabled and disabled
states for receiving the distant party signal at a filter input
and producing a signal which estimates the echo signal, the
estimated signal being produced after an initial adaptive period.
Su~traction means are provided for subtracting the estimated
signal from the composite signal to produce an intermediate sig-
nal along the transmission line and signal suppression means
having enabled and disabled states receives the intermediate sig-
nal and produces a transmitted signal. The method of selectively
enahling and disabling the adaptation process and suppression
means comprises the steps of, generating a first signal when a
first quantitative relationship exists between the distant party
~5 signal and the composite signal, generating a second signal
~hen a second quantitative relationship exists between the dis-
tant party signal and the intermediate signal, generating a
third s;gnal when a third quantitative relationship exists
between the composite signal and the intermediate signal, and
selectively disabling and enabling the filter adaptation process
and the suppression means in accordance with predetermined com-
binatîon~ of the first, second, and third signals.
~ ~d~
More particularly, thR invention as disclosed provides a
first double talk detector to measure the signal transmitted by
the near talker station, which signal includes near talker
speech (when present) and the echo signa]. The first double
talk detector compares this signal with the signal received
from the distant talker station. A second double talk detector
is provided to measure a signal comprising the near talker
speech ~when present~ and the residual echo remaining after the
suhtracti`on of the synthesized echo signal produced by the
1~ adaptive filter from the true echo signal. ~ouhle talk detector
2 compares this composite signal to the signal received from
the distant talker station.
A third double talk detector is provided to compare
the composïte s;gnal compris;ng the near talker speech (when
present) and the echo signal with a second composite signal
comprising the near talker speech and the residual echo signal.
The third double talk detector functions to determine the
extent of the initïal adaptive period~ that is, the period of
time it takes for the adaptive filter to drive the residual
echo signal to an arbitrarily small value. The third double
talk detector also determines the presence of a false double
talk cond;tion.
.~ i
The outputs of each of the double talk detec-
tors are logically combined to effect control of the
adaptive filter and the center clipper. The combi-
natorial logic specifically provides for efective
control of the filter and clipper by the first
double talk detector during the initial adaptive
period as determined by the third double talk detec-
tor. Control is subseguently swi~ched to ~he second
double talk detector upon the transition rom the
initial adaptive period to a steady state condition
where the adaptive filter is producing an accurate
estimated echo signal. The third double talk detec-
tor furthermore over~ees the operation of the second
double talk detector to insure that the second
double talk detector does not respond to a false
double talk condition.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the invention, as
shown in Figure 3, includes adaptive filter 31 and
center clipper 35 to cancel the echo signal in a
manner similar to the circuit of Figure 1. Control
of the filter and clipper, however, is fundamentally
different than the Figure 1 system.
, The incoming signal rom a distant talker is
received on the receive line and applied to adaptive
filt~r 31 and insulating amplifier 37. The output
signal, Ro from amplifier 37 is applied to first and
second double talk detectors 36 and 38. The echo
path 32 represents an arbitrary path taken by the
.
.~
signal Ro which causes it to be fed back into the
transmitting portion of the near talker equipment.
The echo signal ~c ~echo) is similar to the signal
Ro but attenuated by the propagating medium 32. The
signal Ec is combined with any signal from the near
talker (Nt) to produce signal input Si as shown.
The signal Si is applied to each of first and third
double talk detectors 36 and 40, respectively, as
well as to subtractor means 33. The adaptive filter
31 provides the second input to subtractor 33 so
that the replica of the signal Ec, generated by the
adaptive filter can ~)e subtracted from the signal Si
to produce the output signal So which comprises the
residual echo signal Re plus the near talker signal
Nt. The signal So ~s applied to the filter 31 to
complete the servo lc)op of the adaptation process of
the filter 31. The signal So is also applied to
second and third double talk detectors 38 and 40,
respectively, as well as to center clipper 35. The
outputs of each of the detectors 36, 38 and 40 are
applied to logic means ~4. Under the control of the
three detectors, logic 44 provides control outputs
41 and 42 to adaptive filter 31 and to center clip-
per 35 in order to selectively enable or disable the
adaptive filter 31 and the clipper 35.
One particular type of double talk detection
circuit ~hich may be utilized in the present inven-
tion is shown in Figure 2. The two signals are
coup.ed by capacitors 16 and 26 to double diode
detectors 17, 18 and 27 and 28, and filtered by
lowpass RC filters 10 and 11, and 20 and 21. The
time constant is determined by resistors 19 and 29.
The d.c. voltag~ across capacitors 11 and 21 are
compared in a comparator 12 and its output 13 indi-
cates in binary form which of the two inputs is
higher. It should be noted that the Figure 2 cir-
cuit is merely illustrative of a particular type of
detection circuit which can be used in the system
shown in Figure 3. Other detector circuits which
~unction to determine the relative average or RMS
signal value over a given time can be readily
adapted for use in the echo cancellation circuit.
The triggering point of each of the detectors
36, 38, and 40 is adjusted to detect a particular
condition. Specifi~:ally, the detectors can be
provided with a well-known biasing arrangement at
comparator 12 to detect and trigger on the following
conditions:
DTD 1: Ro 6~2si (Similar to prior art
detector 6)
DTD 2: Ro < 4 So
DTD 3: Si > 5 So
These particular choices of coefficients ~2, 4,
and 5) are merely e~emplary. The coefficients which
can be employed will always be a matter of design
choice based on system noise, desired sensitivity
and other considerations which will become apparent
from the description of the system operation below.
The operation of the system will be described
by showing how the present invention overcomes the
problems associated with the prior art system of
Figure l. As mentioned above, a condition (Condi-
tion D, Table I belot~) which causes degradation of
the prior art system occurs when the near ~alker
signal Nt is of a lower amplitude than the echo
signal Ec, as can occur when low quality transmis-
sion lines are used, or where the near talker is
B9
12
talking very softly. When this condition occurs,
DTD 1 will not trigger even when the near talker is
speaking since Nt is of a very low amplitude. As a
result, and as discussed with regard to Figure 1,
above, the detector fails to detect a true double
talk condition.
The present system, however, provides a second
detector, DTD 2, which avoids this problem. The
latter detector compares Ro with So. After the
initial adaptation period of filter 31, Re will be
extremely small and therefore, when near talker
speech is present, So ~ill be substantially the same
as Nt. This being the case, the detector can be
adjusted to trigger when So approaches a small
fraction of Ro, i.e. Ro ~ 4So. In this manner,
detector 38 will detect the presence of signal Nt
even when it is of low amplitude due to the near
talker's low voice or less than perfect transmission
conditions. Detector 38 will accordingly control
clipper 35 and adaptive loop of filter 31 in order
not to distort the near talker's transmission or
contaminate the adaptive loop.
The above-described detection in DTD 2 of a
true double talk condition when Nt is very weak is
only effective, however, after the signal Re is
driven to a very low value by the adaptive loop in
filter 31 ("full adaptation"). In the initial
adaptive period before the filter can fully adapt,
the signal Re will typically be sufficiently large
so as to trig~er DTD 2 even in the absence of a near
talker signal Nt. To avoid false control of filter
31 and clipper 35 by DTD 2 during this adaptive
period, the logic 44 operates to exclude DTD 2 from
control during this period. Instead control is by
DTD 1, which operates in a manner similar to the
detector 6 in the system of Figure 1.
~' ~
~ 2 ~9
The third detector, DTD 3, provides an indica-
tion of when the filter 31 has fully adapted to the
echo signal. The condition of full adaptation (th~
converse of the initial adaptive period) is defined
as the condition where the filter 31 is producing an
accurate estimate of the echo signal Ec so as to
drive ~le residual echo Re to an arbitrarily small
value. In the example described above, this condi-
tion is deemed to occur when Si > 5 So. Again, this
particular condition is merely exemplary and it
should be understood that the condi~ion used to
determine the trigger point of each of the three
detectors is-a matter of choice. The detector, DTD
3, therefore, generally indicates whether or not the
system is in the initial or fully adaptive period in
order to shift the control of the clipping and
filter from DTD 1 to DTD 2. Specific combinations of
outputs from DTD 1, DTD 2 and DTD 3 which lead to
the various control outputs from logic means 44,
will be discussed below.
A further function of the echo canceller is to
overcome the second abovementioned shortcoming of
the prior art systems, specifically, the "false
double talk" condition. In the situation where the
near talker is silent and the distant talker has
been speaking but momentarily pauses in speech
(Condition E, Table I), the pause in the distant
talker speech will, in effect, propagate through
insulating amplifier 37, through echo path 32, and
back into the near talker's send circuit. As the
distant talker's pause in speech propagates through
the receive line of the near talker circuit, but
before it has propagated through echo path 32 to
send side of near talker's circuit, the situation
exists where the siynal Ro at the input of detector
~6'~
14
38 will be zero but ~he signal si will have a small,
b~t finite, value even in the absence of Nt. l'his
condition would trigger DTD 2 causing a false indica-
tion of double talk. In order to avoid the latter,
DTD 3 discriminates between the true and false
double talk conditions. The residual echo Re will be
- much less than Ec after the initial adaptive period.
Thus, when the near talker is silent (Nt = 0) DTD 3
will be triggered to its second state because the
signals will satisfy the condition Si > 5 So. On
the other hand, when near talker speech is present
(true double talk), the signal Nt is the major
portion of Si and So, the condition Si > 5 So will
no longer be satisfied and DTD 3 will revert to its
first state. The combination of outputs from DTD 2
and DTD 3 can therefore, generally distinguish
between a true and false double talk condition.
In response to the detected false double talk,
the clipper will be maintained in an enabled state
so that a burst of residual echo on the send-out
line will not occur. In the preferred embodiment of
the invention, however, the adap~ive loop of filter
31 is disabled since, by definition, there is a
"pause" or lack of any signal cn the receive line
during false double talk. Since the lack of signal
is not indicative of the quality of the far talker
speech, the loop is disabled so that the aaaptation
prccess is not contalninated by the pause.
Another situation (Condition F, Table I) where
selective control of the clipper and filter can be
employed is where there is double talk in the fully
adapted period with short breaks in the near talker
speech. Since a center clipper such as clipper 35
may typically be analog in nature as opposed to
~ilter 31 which may be digital, it is apparent that
the analog clipper will not react as quickly as the
digital filter to an enable/ disable command. If
this is the case, logic 44 can be designed to main-
tain the clipper 35 in a disabled state during
double talk when there are very short breaks in the
near talker speech, while filter 31 can be made ~o
react to the short breaks so that it is enabled
during the breaks in order to resume the adaptation
process based on the far talker signal. Detector 36
is provided with a relatively long release time
constant for the purpose of detecting this particu-
lar condition. The clipper will remain under the
control of detector 36 and will not react to the
short breaks in spe,~ch due to the long time con-
stant. Filter 31, on the other hand, reacts todetectors 38 and 40 which have relatively fast time
constants to thereby enable it to react to the short
breaks in speech.
It should be noted that the above situation is
~0 merely illustrative and that either an analog or
digital filter ar.d clipper can be used in any
desired combination as a matter of design choice.
The necessary changes to the functions performed by
logic 44 can be determined through subjective test-
ing of the canceller in an operational environment.
The existence of any of the above describedconditions is determined in combinatorial logic
means 44 which receives inputs from each of the
dete~tors 36, 38, and 40. Table ] is an example of
one particular logic scheme to combine the outputs
of the three detectors to effect the control of the
filter loop and the clipper in accordance with the
various conditions described above. It should be
~oted that modifications of the logic to effect
3s slightly different control of the filter and clipper
.
-~t ~
39
16
for different system characteristics and require-
ments can obviously be made by those of ordinary
skill in the art.
17
TABLE I
CONmITION DTD 1 DTD 2 DTD 3 FILTER CLIPPER
(Tl) I (T2) (T3) 0 = 0 =
(Lon~ T ) disable disable
A. Initial Adaptive Ro > 2Si RoC4So Si c 5So
Period Far end l 0 0
talk only (no
double talk)
B. Full Adaptation Ro > 2Si Ro > 4So Si ~5So
Far end talk only
(No double talk)
C. Initial or Full Ro < 2Si Ro c 4So Si < 5So
Adaptation Near 0 0 0 0 0
end talker (normal
double talk)
.
D. Full adaptation Ro > 2Si Ro > 4So Si c 5So
- double talk 1 l 0 0 0
with soft near
end talker or bad
trabsmission lines
-
E. Full adaptation Ro < 2Si Ro < 4So Si > 5So
with pause in Ear 0 0 l O
talker speech and
no near talker speech
(false double talk)
F. Full adaptation, Ro c 2S:i Ro > 4So Si > 5So
douhle talk with 0 l l l 0
short breaks in
near talker speech
-
18
Table I tabulates the six conditions which may
be expected to occ~r in voice communlcations
systems. Condition A indicates tho inti~l adaptive
period with far-end talk only. DTD 1 indicates that
Ro > 2 Si to define the condition Tl = 1. DTD 2
detects Ro < 4 so to indicate the condition T2 - O.
DTD 3 detects Si ~ S So to indicate that T3 = 0.
Each of the conditions T1 = 1, T2 = 0 and T3 = 0 are
delivered to logic means 44. In response to the
three inputs, the clipper and the adaptive loop of
the filter are enabled by the logic. Condition B is
detected when T1 = 1, T2 = l and T3 = 1. Under this
condition (full adaptation, far end talk only),
logic 44 enables both the filter loop and the clip-
per. Condition C ~normal double talk) is detectedwhen Tl = 0, T2 = 0, and T3 = 0. Logic 44 responds
to condition C by disabling both the filter loop and
the clipper. Condition D indicates that there is
double talk in the adaptive period but the near
talker signal is weak. The detectors provide sig-
nals Tl - 1, T2 = l and T3 = 0 to the logic which in
turn disables both the clipper and filter loop.
Condition E where T1 = 0, T2 = 0 and T3 - l, indi-
cates a false double talk condition. Logic 44
~5 disable the filter loop but enables the clipper in
response thereto. Finally, Condition F indicates
full double talk with short breaks in Nt by provid-
ing Tl = O, T2 = 1 and T3 = 1 to the logic. The
filter loop is ac~ordingly enabled during the breaks
in Nt but the clipper is disabled.
Logic 44 can be designed around the scheme
shown in Table I by constructing Table II as fol-
lows:
19
TABLE II
CONDITION Tl T2 T3 FILTER CLIPPER
C O O O O O
E 0 0 1 0
X 0 1 0 X X
F 0 1 1 1 0
A 1 0 0
X 1 0 1 X X
D 1 1 0 0 0
B
The "don't care" conditions indicated by X are
used to show conditions which are not anticipated to
occur in the system for which the cancellation
circuit is designed. The particular "don't care"
conditions in Table II are indicated for the system
described above in accordance with the scheme of
present invention using Table I.
Karnaugh maps for the filter and clipper are
shown in Table III in order to reduce the logic0 required to generate the enable/disable siynals.
TABLE III
FILTER
Tl
T3 T2 00 01 11 10
O X
X
CLIPPER
Tl
T3 T2 0001 11 10
O X
X
___
The maps in Table III xeduce the xequired logic
as follows:
FILTER: T2 T3 + Tl T2 = ENABLE
CLIPPER: T2 T3 + Tl (T2 ~ T3) - ENABLE
The logic 44 can be implemented by standard
AND/OR or NAND gates, or alternatively by a ROM
having a look-up table programmed to provide the two
logical functions shown above.
Thus it is seen that the combination of detec-
tors 36, 38 and 40 and logic means 44 effects double
talk control of the filter and cllpper during the
initial adaptive period ~Conditions A or C) by using
detector 36. After the filter has fully adapted to
~le echo signal, however, as determined by de'ector
38, control is effectively shifted to the more
sensi~ive detector 40 (Condition D, for example).
The combination logic further provides detection of
particular conditions to ensure that khe system does
not react to a "alse double talk" condition ~Condi-
tion E) or a rapidly changing double talk condition
(Condition F) by monitoring the outputs of each of
the detectors 36, 38 and 40.
