This invention relates to an audio processing circuit and, in particular, to an echo cancelling circuit employing band pass filters and notch filters having substantially the same center frequency.
There are two kinds of echo in a telephone system, an acoustic echo between an earphone or a speaker and a microphone and electrical echo generated in the switched network for routing a call between stations. In a handset, acoustic echo is typically not much of a problem. In speaker phones, where several people huddle around a microphone and loudspeaker, acoustic feedback is much more of a problem. Hybrid circuits (two-wire to four-wire transformers) located at terminal exchanges or in remote subscriber stages of a fixed network are the principal sources of electrical echo.
Many techniques have been developed to improve the clarity of the sound in a telephone. One such technique uses what is known as a comb filter; i.e. a plurality of filters wherein band pass filters alternate with band stop filters. Comb filters with complementary pass and stop bands are coupled in the two audio channels connecting the two stations of a telephone call. That is, the pass bands in one channel are the stop bands in the other channel. As a result, a signal traveling in one direction will be slightly attenuated but a signal traveling in a loop, i.e. an echo, will encounter both sets of stop bands and be highly attenuated.
The use of the complementary comb filters reduces the acoustic coupling between the speaker and microphone at each station as well as inter-station echo. Echo canceling circuits, which try to recognize a delayed signal as an echo, are much more complicated than complementary comb filter circuits and the two are often used together to eliminate echoes and other noises.
Complementary comb filter circuits are disclosed in U.S. Pat. No. 5,386,465 (Addeo et al.). This patent includes complementary comb filters in combination with other apparatus for processing audio signals to reduce noise. U.S. Pat. No. 4,991,167 (Petri et al.) discloses a slightly different system. Signals in the sending and receiving directions are each separated into a set of bands, each including an attenuator. The signal in a band in one direction of transmission is controlled as a function of the amplitude of the signal in the same band in the other direction of transmission.
A problem with this approach is that, unlike the complementary comb filters, one can attenuate the signals in adjacent bands, thereby noticeably degrading the quality of the voice transmission. In the extreme, the telephone is reduced to xe2x80x9chalf duplexxe2x80x9d or simplex operation, i.e. single direction at a time because sounds from the other station are inaudible. The speaker at one station must stop and the circuits must re-settle before the person at the other station can be heard.
Another problem with complementary comb filters is the phase distortion introduced by each filter. Each band pass filter has a phase shift associated with it and even the desired signal passes through two such filters in the path from microphone to speaker, which affects the quality of the sound. Unwanted signals, such as echoes, travel through at least two more filters. The total phase shift introduced by the four filters can be enough to allow the undesired signal to pass through the system rather than being fully attenuated.
In view of the foregoing, it is therefore an object of the invention to provide a noise reducing circuit that retains the advantages of a complementary comb filter with less signal degradation.
Another object of the invention is to provide an improved technique for reducing acoustic and electronic noise in an audio communication system.
A further object of the invention is to provide a complementary comb filter having minimal phase distortion.
The foregoing objects are achieved in this invention in which a complementary comb filter includes a plurality of band pass filters and a plurality of notch filters, wherein the notch filters have the same center frequencies as the band pass filters or the same center frequencies as the dead bands defined by the band pass filters.
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