Circuit and method of double talk detection for use in handsfree telephony terminals

The invention provides an audio subsystem for use in a telephone terminal operable in handsfree mode and comprises a receive path including a receive transducer for generating audio signals from a received signal and a transmit path including a transmit transducer for converting audio signals to electrical signals for transmission to a far-end user. An acoustic echo canceller is communicatively coupled between the receive path and the transmit path and includes a subtractor circuit connected serially with the transmit transducer. The echo canceller is responsive to control signals for controlling the subtractor circuit. The subsystem comprises means for detecting a near-end talking condition including first circuit means for measuring the level of the total energy of the out-of-telephony-band components in the signal from the transmit transducer and circuit means for comparing the energy level represented by an output signal from the first measurement circuit to a first predetermined threshold. If the measured energy level exceeds the predetermined threshold, near-end talking activity is deemed to exist. There is also provided a second measurement circuit for measuring the level of the signal on the receive path and for comparing it to a second predetermined threshold whereby if the latter is exceeded, far-end talking activity is determined to exist. Thus, if both measured signals exceed their respective threshold, a double talk situation is deemed to exist.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to echo cancellers and more particularly 
to a novel circuit and method for detecting near-end talking activity and 
double talk situations thereby to effectively control the convergence 
function of an acoustic echo canceller (AEC). 
2. Description of Background and Related Art 
The presence of echoes in long-distance telephony is a thorny problem. A 
long-distance communication circuit usually comprises four-wire and 
two-wire segments; these are joined at each end by hybrid circuits. 
Impedance mismatch in a hybrid circuit causes a portion of the signal 
received at the hybrid circuit to be reflected back onto the transmit 
four-wire segment whence it came and this reflected signal is perceived as 
echo to the speaker who originated it. Adaptive echo cancellers are thus 
employed to minimize the echo signal created on four-wire transmission 
lines. 
Normally, a four-wire receive signal is at a higher level than its echo 
signal on the four-wire transmit path because there is loss across the 
hybrid circuit. Near end speech on the transmit path will therefore 
typically be stronger than the echo signal. However, near end speech is 
unwanted noise as far as convergence of the echo canceller is concerned 
since it would diverge the canceller if it were to continue updating its 
estimated impulse response while near end speech is present. Various 
techniques and schemes have therefore been developed to provide double 
talk detection in echo cancellers. 
The problem of echo cancellation on the telephone network is exacerbated by 
the connection of a handsfree terminal at one or both ends of the 
transmission line. Double talk detection (DTD) in a handsfree system 
refers to the determination of whether, in the microphone output, there is 
near-end speech mixed with a probably much stronger far-end speech played 
through the loudspeaker. By comparison with double talk detection for 
network echo cancellation applications, the DTD in an acoustic echo 
cancellation handsfree (ECHF) system is more likely to be subjected to 
sudden echo path changes as well as to echo levels that are much above the 
level of near-end speech. This is due to the fact that, in a handsfree 
terminal, the receive transducer or speaker is closer to the microphone of 
the terminal than the near-end user; furthermore, the case of the terminal 
conducts a substantial amount of acoustic energy from speaker to 
microphone. In a typical implementation, it is not unusual for the far-end 
signal from the loudspeaker to be as large as 25 db (decibel) above the 
level of the near-end signal. The near-end activity by a user is therefore 
difficult to ascertain because the far-end signal from the loudspeaker 
will mask at least a portion of the signal from the near-end user. 
Numerous schemes of double talk detection have been devised and usually 
fall into one of three categories. A first category which may be labelled 
the energy comparison scheme usually employs power detectors for detecting 
the average power, the peak power and the residual power of various 
signals to generate the output signal of the double talk detector. Example 
circuits of this type of double talk detectors are described in U.S. Pat. 
Nos. 4,360,712; 5,463,618 and 4,645,883. 
A second category which may be labelled a cross correlation technique is 
basically an extension of the energy comparison category; it adds a 
cross-correlation criterion between various signals to arrive at a control 
decision. This scheme is more complicated than the energy comparison 
technique and requires additional memories and computational power. 
Examples of this type of double talk detection may be found in U.S. Pat. 
Nos. 5,646,990 and 5,193,112. 
Yet a third category is related to the cross correlation technique. It 
monitors the directions of the updating vectors for the echo canceller 
which are given by an adaptation algorithm such as NLMS (Normalized Least 
Mean Square). If the updating vectors over a number of samples all roughly 
point at a common direction, the echo canceller is in the converging mode. 
If, on the other hand, the vectors point at various diverse directions, 
the echo canceller is deemed to have converged. This decision process 
together with the energy of the signals are then used to determine whether 
a double talk condition exists. This scheme may provide a reliable result 
but is very computation intensive. DTD implementations based on monitoring 
updating vectors may be found in U.S. Pat. No. 4,918,727 as well as the 
paper: "A New Double-Talk Detection Algorithm Based On The Orthogonality 
Theorem" by Hua Ye and Bo-Xiu Wu, IEEE Transactions on Communications, 
Vol. 39, No. 11, November 1991. 
Most of the known techniques and schemes of the prior art were developed 
for use in network echo cancellations and probably perform adequately in 
that environment; however, their performance in ECHF applications is not 
entirely satisfactory. The main reason, as mentioned above, is that in a 
handsfree environment, the portion of the far-end signal from the 
loudspeaker appearing as echo at the microphone of the terminal is usually 
much stronger than the near-end signal, and the difference in a typical 
implementation can be as large as about 25 db. The far-end signal from the 
loudspeaker tends to mask the signal from the near-end user and makes the 
determination of double talk conditions very difficult using the known 
techniques. 
SUMMARY OF THE INVENTION 
An analysis of the environment of handsfree telephony indicates that the 
far-end speech is a relatively narrow-band signal because it is received 
over the telephone network; it therefore contains only frequency 
components in the telephony band. The near-end speech, on the other hand, 
has a wider bandwidth than the telephony band. Therefore, if the 
telephony-band signals are filtered out of the mixed signal at the output 
of the microphone, the existence of near-end user activity may effectively 
be ascertained. 
It is therefore an object of the invention to provide a relatively simple 
and effective DTD circuit particularly adapted for use in ECHF 
applications. 
It is a further object of the invention to provide a circuit and method for 
the detection of near-end activity by a user of a handsfree telephone 
terminal. 
It is thus a still further object of the invention to provide a circuit for 
the detection of double talk conditions based on the total energy content 
of the out-of-telephony-band components in the mixed signal at the output 
of a microphone in a handsfree telephone terminal. 
Therefore, from a first aspect, the invention provides an audio subsystem 
for use in a telephone terminal operable in handsfree mode and comprising 
a receive path including a receive transducer for generating audio signals 
from a received signal and a transmit path including a transmit transducer 
for converting audio signals to electrical signals for transmission to a 
far-end user. An acoustic echo canceller is communicatively coupled 
between the receive path and the transmit path and includes a subtractor 
circuit connected serially with the transmit transducer. The echo 
canceller is responsive to control signals for controlling the subtractor 
circuit. The subsystem comprises means for detecting a near-end talking 
condition including first circuit means for measuring the level of the 
total energy of the out-of-telephony-band components in the signal from 
the transmit transducer and circuit means for comparing the energy level 
represented by an output signal from the first measurement circuit to a 
first predetermined threshold. If the measured energy level exceeds the 
predetermined threshold, near-end talking activity is deemed to exist. 
From another aspect of the invention, there is provided a second 
measurement circuit for measuring the level of the signal on the receive 
path and for comparing it to a second predetermined threshold whereby if 
the latter is exceeded, far-end talking activity is determined to exist. 
Thus, if both measured signals exceed their respective threshold, a double 
talk situation is deemed to exist. 
From yet another aspect, the invention provides a method for detecting a 
near-end talking condition in an acoustic echo canceller for use in a 
telephone terminal operable in handsfree mode. The method comprises the 
steps of measuring the level of the total energy of the 
out-of-telephony-band components in the signal from the transmit 
transducer of the terminal and comparing the measured energy level with a 
first predetermined threshold. If the measured energy level exceeds the 
first predetermined threshold, near-end talking activity is determined to 
exist. In a similar manner, the energy level on the receive path of the 
terminal may be measured and compared to a second predetermined threshold. 
If both measured energy levels exceed their respective threshold, a 
double-talk situation is deemed to exist.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates a simplified block diagram circuit for an echo canceller 
system adapted for use in a handsfree telephone terminal. It shows a 
receive path RX including a receive signal variolosser RXV, a digital to 
analog converter 10 having a sampling frequency of 8 Khz, and a power 
amplifier 11 for driving a receive transducer such as speaker 12. A 
transmit path also includes a variolosser TXV and a microphone 13. As is 
well known in the art, variolossers such as RXV and TXV serve to attenuate 
the amplitude of both the receive and transmit signals. The amount of loss 
provided by the respective variolossers is usually controlled by the 
system's digital signal processor (DSP--not shown) responsive to a voice 
signal activated switch. As the loss provided by the receive variolosser 
increases, the loss provided by the transmit variolosser is decreased 
proportionately. This arrangement performs quite adequately in a 
half-duplex system, that is, a system in which any party to a connection 
cannot hear and be heard at the same time. 
It is also known in the art to incorporate both receive and transmit 
variolossers in conjunction with an acoustic echo canceller AEC in order 
to meet the echo return loss targets mandated by operational standards of 
the telephone network. The AEC, also controlled by the DSP, makes an 
estimate of the echo on the receive path and subtracts that amount from 
the transmit path. 
FIG. 1 also shows an AEC 14 communicatively coupled between the transmit 
and receive paths in a known manner. The AEC 14 includes a substractor 
circuit 15 serially connected in the transmit path. The various signals 
associated with the AEC are conventionally denoted as X(n), Y(n), e(n) and 
d(n). The notation LDS denotes the loudspeaker signal, MCS the microphone 
signal and MCSLEE the microphone signal less the echo estimate made by the 
AEC 14. 
Also, connected serially between the microphone 13 and the substractor 15, 
is an analog filter 16 usually denoted as an anti-aliasing filter having a 
bandpass of between 40 Hertz and 7 Khz. Connected between the filter 16 
and the subtractor 15 is an analog to digital converter 17 operating at a 
sampling frequency of 16 Khz and a factor-of-two down sampling circuit 18 
providing signals at a sampling frequency of 8 Khz to the subtractor 15. A 
bandstop filter 19 is connected at the junction of circuits 17 and 18 and 
provides an output signal to an energy level circuit 20. 
FIG. 2 is a table illustrating example parameters of a filter 19 useful in 
the realization of the invention. The filter 19 may be of the infinite 
impulse response (IIR) type and its design may be generated with the 
software package "QEDesign" available from Momentum Data Systems, Costa 
Mesa, Calif., USA. FIG. 2 indicates that the example bandstop loss was set 
at 50 dB in the stopband of 250 Hertz to 4 Khz for a sampling frequency of 
16 Khz. Also, the low passband cutoff frequency of the filter is set at 
200 Hertz to ensure that the fundamental frequency of the near-end talker 
is within the lower passband for a majority of users. A limited collection 
of speech samples from a variety of humans indicated that the fundamental 
frequency of male users was about 100-160 Hertz and that of female users 
ranged from about 160 to 220 Hertz. A larger sampling of speech samples 
would likely yield a different range of fundamental frequencies. It is 
thus evident that the parameters of the filter 19 should be selected to 
suit the target population. 
As discussed above, the far-end signal from the loudspeaker is usually much 
stronger than the near-end signal, and the difference in a typical 
implementation may be as large as about 25 dB. The filter 19 must 
therefore have a stopband loss large enough to ensure that the far-end 
speech from the loudspeaker is suppressed enough so as not to trigger a 
false detection of near-end activity. Although a stopband loss of 50 dB 
was selected in the described embodiment, it is quite possible that a 
different amount of loss may be used and still provide adequate results. 
The energy level circuit 20 functions to measure the filtered signal from 
the microphone 13 and thus provides a measure of the total energy of the 
out-of-telephony-band components in the mixed signal. The circuit 20 
measures the level of the signal by finding a temporally averaged version 
of the squares of the corresponding input. An exponential weighting 
function is used to find the energy levels, with more emphasis on recent 
samples. The input/output relationship of the circuit 20 is 
output(n)=(1-1/L) output(n-1)+(1/L) input.sup.2 (n). In this equation L is 
the number of samples corresponding to a 4 millisecond interval and thus 
equals 64 for the 16 Khz sampling frequency of the A/D converter 17. 
A circuit 21 compares the output level at the output of circuit 20 with a 
predetermined threshold level represented by circuit block 22. A useful 
threshold may be determined by looking at the average level of the sum at 
the output of circuit 20 while there is not speech and the average level 
of the sum while there is speech present. The threshold can then be 
selected to be a level somewhere between the two levels. If the output 
signal from circuit 20 exceeds the predetermined threshold level of 
circuit block 22, near-end talking activity is determined to exist. 
The level of the received signal LDS is measured in a circuit 23 in a 
manner identical to that described for circuit 20. The digital signal on 
the receive path is the result of voice sampled at 8 Khz as is 
conventional in the telephone network; therefore, in this case, L equals 
32 samples. The output signal of circuit 23 is compared to a threshold 
represented by circuit block 24. This threshold may be determined in a 
manner similar as that of circuit block 22. If the signal at the output of 
circuit 23 exceeds the threshold level of circuit 24, the comparison 
circuit 21 determines that far-end talking exists. If the level of 
circuits 20 and 23 each exceed their respective threshold, double talk is 
deemed to exist and the AEC is controlled accordingly. Under some 
circumstances, it may be desirable to also use the output of circuit 21 to 
control the function of variolossers RXV and TXV. 
FIG. 3 is a flow chart illustrating the operation of FIG. 1. The echo path 
signal from the receive transducer 12 together with the near-end signal 
from a user is filtered in circuit 16 and converted to a digital signal in 
A/D converter 17 using a sampling frequency of 16 Khz. The output of the 
converter 17 is then down-sampled to 8 Khz and fed to the subtractor 16 in 
the conventional manner. The output of the converter 17 is also fed to the 
bandstop filter 19 that effectively removes all telephony-band frequency 
components from the mixed signal. The signal level at the output of filter 
19 is then measured and compared to a predetermined threshold. At the same 
time, the LDS signal on the receive path is measured and compared to a 
respective threshold. If the levels at the output of circuits 20 and 23 
each exceed their respective threshold, a double talk situation is 
determined to exist. 
Because the disclosed DTD implementation extracts its input samples before 
the subtraction point of the AEC, its performance is completely 
independent of the convergence status of the latter. In other words, the 
convergence status of the AEC or the occurrence of an abrupt echo path 
change does not affect the operation of the DTD scheme. Therefore, the 
invention provides a reliable and relatively simple DTD scheme for use in 
ECHF applications. 
Although a particular embodiment of the invention has been illustrated and 
described, it is apparent that various changes can be introduced. For 
example, the parameters of the bandstop filter can be modified and the 
level measurement techniques changed without departing from the scope and 
spirit of the invention.