US 7792280 B2
The present invention relates generally to tone detection in echo canceling systems, and more particularly to a method of monitoring oscillations of adaptive filter coefficients for narrow band tone detection in echo cancellation systems. The invention is directed at improving communication in a telephone communication network having one or more hybrid connections. A method for detecting continuous tones in an echo canceling system having an adaptive filter is provided. Continuous tones have a deleterious effect on the performance of adaptive filters. In the present invention, the adaptive filter coefficients are monitored. Oscillation of those coefficients indicates continuous tone presence.
1. For use in an echo canceling system having an adaptive filter, the improvement comprising (i) detecting oscillation of coefficients of said adaptive filter, (ii) establishing a tone trigger threshold for each coefficient of said adaptive filter; and, in response, (iii) indicating to said echo canceling system that a tone is present, wherein each said tone trigger threshold is a function of a maximum value of said corresponding one of said coefficients of said adaptive filter.
2. The improvement of
(a) establishing a tone trigger count;
(b) measuring echo return loss enhancement (ERLE);
(c) comparing said adaptive filter coefficients to corresponding ones of said tone trigger thresholds, and accumulating a count value representing how many of said adaptive filter coefficients exceed corresponding ones of said tone trigger thresholds;
(d) comparing said count value to said tone trigger count and comparing said ERLE to an ERLE threshold; and
(e) in the event that said count value exceeds said tone trigger count and said ERLE is greater than said ERLE threshold, then indicating detection of said oscillation of said coefficients, and otherwise, indicating an absence of detection of oscillation of said coefficients.
3. A narrow band tone detector for use in an echo canceling system comprising an adaptive filter, comprising:
detecting means configured to detect oscillation of coefficients of said adaptive filter;
threshold setting means configured to establish a tone trigger threshold for each coefficient of said adaptive filter; and
indicating means configured to respond to said detection by providing an indication that a tone is present, wherein each said tone trigger threshold is a function of a maximum value of said corresponding one of said coefficients of said adaptive filter.
4. The narrow band tone detector according to
threshold setting means configured to establish a tone trigger threshold for each coefficient of said adaptive filter,
a counter arranged to count tone triggers;
Echo Return Loss Enhancement (ERLE) measurement means;
comparing means arranged to compare said adaptive filter coefficients to corresponding ones of said tone trigger thresholds and to accumulate a count value representing how many of said adaptive filter coefficients exceed corresponding ones of said tone trigger thresholds;
count comparing means arranged to compare said count value to said tone trigger count and to compare said ERLE to an ERLE threshold; and
output means configured to indicate detection of said oscillation of said coefficients in the event that said count value exceeds said tone trigger count and said ERLE is greater than said ERLE threshold and otherwise, to indicate an absence of detection of oscillation of said coefficients.
The present invention relates generally to tone detection in echo canceling systems, and more particularly to a method of monitoring oscillations of adaptive filter coefficients for narrow band tone detection in echo cancellation systems.
The signal path between two telephones, involving a call other than a local one, requires amplification using a four-wire circuit. The-cost and cabling required discourages extending a four-wire circuit to a subscriber's premise from the local exchange. For this reason, the four-wire trunk circuits are coupled to two-wire local circuits, using a device called a hybrid.
Hybrid echo, the primary source of echo generated from the public-switched telephone network (PSTN) is created as voice signals are transmitted across the network via the hybrid connection at the two-wire/four-wire PSTN conversion points.
Unfortunately, the hybrid is by nature a leaky device. As voice signals pass from the four-wire to the two-wire portion of the network, the energy in the four-wire section is reflected back, creating an echo of the speech signal. Provided that the total round-trip delay occurs within just a few milliseconds, the echo generates a sense that the call is live by adding sidetone, thereby making a positive contribution to the quality of the call.
In cases where the total network delay exceeds 36 ms, however, the positive benefits disappear, and intrusive echo results. The actual amount of signal that is reflected back depends on how well the balance circuit of the hybrid matches the two-wire line. In the vast majority of cases, the match is poor, resulting in a considerable level of signal being reflected back.
The effective removal of hybrid echo is one key to maintaining and improving perceived voice quality on a call. This has led to intensive research into the area of echo cancellation, with the aim of providing solutions that can reduce echo from hybrid. By employing this technology, the overall speech quality is improved significantly.
It is known in the art to employ adaptive filtering to address hybrid echo cancellation. In an adaptive filter, the filter coefficients are based, in part, on feedback of filter output. Normalized Least Mean Square (NLMS) adaptive filtering is one method, popular in echo cancellation, to address reflections in the telephony system.
Where an excitation is highly correlated, e.g. a continuous tone, the NLMS algorithm performs poorly. Where NLMS adaptive filtering is employed, tone signals, such as those employed in telephony tone dialing systems, may have a detrimental effect on other components, such as noise level calculators, Echo Return Loss (ERL) calculator, and Echo Return Loss Enhancement (ERLE) calculator, etc.
U.S. Pat. No. 4,363,100 discloses a tone detector in the form of a digital correlator, in which PCM signal samples are delayed in a RAM for the correlation period. Although U.S. Pat. No. 4,363,100 addresses tone detection which could be used in an echo cancelling system, it does not use the infrastructure of echo cancelling system to detect the tones.
U.S. Pat. No. 4,669,114 discloses a digital progress tone detector, for connection to a telephone line and a PABX. Again, this disclosure could be used in an echo cancelling system but it does not use the infrastructure of echo cancelling system to detect the tones.
What is needed is a method of detecting tones in a NLMS adaptive filter echo canceling system, and alerting this system to the presence of such tones.
It is an object of an aspect of this invention to detect tones in an echo canceling system.
In an aspect of the invention, there is provided a method for using the adaptive filter of an echo canceling system for detecting tones. In this method, a determination is made as to whether coefficients of the adaptive filter oscillate or do not oscillate. If the coefficients oscillate, the echo canceling system is notified that a tone is present.
An embodiment of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:
Echo Return Loss (ERL) is a measure of the actual amount of signal reflected back. The higher the ERL, the lower the reflected signal back to the talker, and vice versa.
For each discrete time k, signal x(k) is input to un-adapted signal path 210 and adaptive filter 220. The path 210 outputs the signal d(k) which is a combination of the echo from original signal x(k), noise, and near end speech. The adaptive filter 220 output is y(k) for each discrete time k. The subtractor 230 generates an error e(k) as follows:
The adaptive filter output y(k) is based on a number, N, of previous inputs x(k) according to the formula
There is a drawback to employing adaptive filtering in telephony echo canceling systems. Highly correlated inputs, such as the continuous tones generated in Dual Tone Multifrequency (DTMF) tone dialing subsystems, can have a negative effect. In the presence of highly correlated signals, the NLMS adaptive filter does not reflect the transfer function of the hybrid; the reflection position cannot be exactly located.
When all coefficients have been compared, i.e. the coefficient counter equals the number of coefficients, branch 430 is taken to branch 475, where the counter (CoefsAboveMaxRplRatio) of all coefficients above the threshold is compared to a predetermined ToneTriggered value. At the same time, the ERLE (Echo Return Loss Enhancement) is monitored to ensure that it is above a minimumERLE threshold. This condition is used to prevent false tone triggers, when the coefficients have diverged or just starting to converge, in which case the current ERLE will be smaller than the minimumERLE. If there are more coefficients above the MaxRplRatio than the predetermined ToneTriggered value, and the Echo Return Loss Enhancement is higher than the minimumERLE threshold, then the narrow band detector reports a tone detected to the echo canceller system (step 480), and further execution (step 490) is in accordance with the method 300.
It will be appreciated that, although embodiments of the invention have been described and illustrated in detail, various modifications and changes may be made. Different implementations may be made by those familiar with the art, without departing from the scope of the invention.