Patent Application
20070286404
With reference to
A short default coefficients filter 120 (Default_Length=N, number of coefficients representing the estimated length of the direct echo path) represents the direct echo path captured by the long adaptive filter 100. Filter 120 is non-adaptive such that it does not track echo path changes and also does not diverge during double-talk, as is known from published Canadian Patent Application 2,451,417, referred to herein above. Subtractor 125 outputs an error signal resulting from echo cancellation via filter 120.
According to an aspect of the invention, a further short adaptive filter 130 (Short_Length=Default_Length) is provided for modeling only the direct echo path and adapting quickly whenever a reference signal is present. This is in contrast with filter 100 which adapts slowly when the reference signal is present. The filter 130 is not used for echo cancellation, but only for echo path detection. Specifically, the filter 130 quickly diverges during double-talk and is able to provide an early indication of any echo path changes. A subtractor 135 outputs an error signal resulting from filter 130.
Decision logic 140 distinguishes between echo path changes and double-talk based on the reference signal and the estimated error signals from all three filters 100, 120 and 130, as discussed in greater detail below.
Finally, a non-linear processor 150 (NLP) is provided, as is conventional in adaptive echo cancellers.
Applying the short adaptive filter 130 to the major (direct) echo path changes minimizes the impact on normal FDHF (Full Duplex Hands Free) behavior. Typically, in FDHF applications, the direct echo path reflection is the major contributor to the echo. Since the secondary echo path changes are much smaller due to attenuation in the room, their impact on FDHF performance is not critical such that NLP 150 is capable of handling them.
One difference between non-adaptive default filter 120 and the short adaptive filter 130 is that filter 130 adapts whenever the reference signal (FE_signal) is present, whereas the default filter 120 statically models the previously captured echo path and never adapts. On the other hand, a difference between the short and long adaptive filters 130 and 100 is that the short filter 130 is an under modeled system (i.e. the short adaptive filter 130 only covers the direct echo path, and never converges as deeply as the long NLMS filter 100 when it is in the stable/converged state, or when there is no echo path change). Therefore, in a single talk scenario a measurable difference will exist in the ERLE between short and long adaptive filters 130 and 100 (i.e. the long filter 100 performs better than the short filter 130). By monitoring the difference between the two error energies from filters 100 and 130 the EPC detector logic 140 indicates a change of state from single talk to one of either an echo path change (EPC) or double talk (DT).
During an echo path change the short adaptive filter 130 converges more quickly to the new echo path than the long adaptive filter 100, as discussed above. Consequently the difference between the two error signals output from filters 100 and 130 will be small or even negative, as the short filter 130 becomes better converged than filter 100.
Turning to
An EPC timer is set to a predetermined value (EPC_DECISION_HOLD) whenever an echo path change (EPC) is detected, as in step 250. The EPC detection logic 140 remains in an EPC detected state until this timer expires. The NLP 150 remains turned on such that the echo is masked, during the time between when the EPC is detected (the timer is set) and the timer has expired. The value EPC_DECISION_HOLD is used to hold the EPC detected state. In a successful prototype, the constant was chosen to be 600 samples (i.e. or 75 ms).
In the event the state hold timer has expired at step 210 (either expired, or never set since it last expired), the EPC_decision is set to FALSE (step 225). This makes sure that the default-state of the EPC detection logic 140 is one in which no echo path change has been detected.
Next, at step 230, a determination is made as to whether the reference signal is present (i.e. the energy of the reference signal exceeds a Threshold (e.g. −32 dBmo) and the measured ERLE of the long filter 100 exceeds a predetermined value (e.g. 12 dB). If either of these conditions fails, the algorithm exits (step 220), indicating either an absence of echo or that the algorithm has not yet converged, such that there no good condition to decide about the echo path change.
In the event of a “Yes” decision at step 230, a determination is made (step 235) as to whether Elong>=(Thresh_activity*Eshort), where Thresh_activity is a threshold of, for example, −6 dB in a successful implementation of an embodiment of the invention. This condition is based on the fact that the long filter 100 cancels echo much better than short filter 130 in a stable/converged single-talk scenario. However, in either a double talk or EPC scenario the long filter 100 does not achieve as good ERLE as in the stable/converged single-talk state. This makes the relation between long filter 100 and short filter 130 change significantly so that the long filter 100 does not achieve 6 dB better than the short filter 130. A “No” event at step 235 indicates there is no double talk or EPC. The algorithm exits (step 220).
In the event of a “Yes” decision at step 235, a determination is made (step 245) as to whether Eshort<=(Thresh_epc*Edef), where Thresh_epc is a threshold of, for example, −5 dB in a successful implementation of an embodiment of the invention. This condition is based on the fact that the short filter 130 will quickly adapt to the new echo path while the default filter 120 does not. In the event of an echo path change the short filter 130 achieves better ERLE than the default filter 120, by, e.g. 5 dB. A “No” event at step 245 indicates that the activity detected from step 235 is not for EPC. The algorithm exits (step 220).
In the event of a “Yes” decision at step 245, then at step 250 the EPC_decision is set to TRUE and an Echo Path Change (EPC) is detected. As described above, this state will be held for at least EPC_DECISION_HOLD samples (e.g. 600 samples). and the NLP 150 is set to mask the error (i.e. provide full attenuation of the signal), and the algorithm ends (step 220). Alternatively, rather than control the NLP 150 for masking unwanted echo due to echo path changes, the EPC detection logic 140 may be used to control the NLMS adaptation. Specifically, the EPC detection logic 140 may be used to freeze or slow down the adaptation of the long filter 100 when double-talk is detected and to speed up the adaptation of the long filter when an echo path change (EPC) is detected.
As shown in
In a double talk scenario the default coefficients continue to be valid and may be used to cancel the echo signal. The short adaptive filter 130, on the other hand, updates its coefficients based on the NE_signal (echo+near-end speech) and thus causes a divergence of the coefficients. If the adaptive filter error is consistently worse relative to the fixed filter error over a period of time a double talk condition is identified.
During double talk, the near-end (NE) speech contributes to the residue echo. That is:
Echo=Real_Echo+NE_Speech; and
Energy_residue_echo=Energy_of_Echo−Estimated echo of the NLMS filter 100.
The presence of the near-end signal results in a decrease of ERLE for both long and short adaptive filters 100 and 130, so that the ratio of the error energies between the two filters can no longer achieve the aforementioned measurable difference (i.e. the difference, in dB, between the two filters in the single talk scenario). By monitoring the difference between the two error energies the EPC detector logic 140 is able to identify one of either an echo path change (as discussed above in connection with
During an echo path change, the default coefficients in the fixed filter 120 are no longer valid, while the short adaptive filter 130 converges to the new echo path change. Consequently, the error from the short adaptive filter 130 becomes much smaller than the error from the default fixed filter 120. Therefore, when the error energy ratio between the adaptive filter 130 and fixed filter 120 is less than Thresh epc, (e.g. −5 dB) an echo path change is flagged, as discussed above in connection with
On the other hand, if EShort<=(Thresh_epc*Edef) is not true, then the EPC_decision is set to FALSE and a further determination is made (step 330) as to whether EShort>=(Thresh_dt*Edef), where Thresh_dt is +1 dB according to a successful prototype of the invention. To make a reliable decision for double talk this condition has to be consecutively fulfilled for at least DECISION_TIMER_THRESH times. The DECISION_TIMER_THRESH is chosen for 16 samples, according to a successful prototype. If a double talk state is finally detected this state will at least be held for DT_DECISION_HOLD samples. To hold the DT state a DT_hold_timer is set to DT_DECISION_HOLD. The timer is checked at step 300 and updated at step 310 if necessary.
The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention that fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.
