Telephonic communications – Echo cancellation or suppression
Patent
1993-06-23
1995-05-30
Dwyer, James L.
Telephonic communications
Echo cancellation or suppression
379406, 379410, 370 321, H04M 100
Patent
active
054209218
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method for the detection of a disable tone signal in an echo canceller, the disable tone signal being within a predetermined tolerance range from a nominal frequency and including phase inversions occurring at predetermined intervals.
BACKGROUND OF THE INVENTION
End-to-end connections of a data transmission system, such as a telephone network, often show long transit time delays, in consequence of which echo is observed for instance in the case of normal speech, when a signal is reflected from the far end of a connection back to the talker. An echo canceller is an analog or digital device for processing a signal, such as a speech signal, so as to reduce echo by subtracting estimated echo from the echo (signal).
Normally, an echo canceller is capable of detecting a so-called disable signal. Upon detecting the disable tone, the echo canceller is switched to a "transparent" state, in which the echo canceller does not process a signal passing through. The characteristics of the disable tone are defined accurately in CCITT recommendation V.25. A disable tone is a signal of about 2100 Hz with phase inversions at intervals of (450.+-.25). The disable tone detection of the echo canceller responds to this particular signal, but not, e.g., to speech or a 2100 Hz signal with no phase inversions. Disable tones are monitored both at the receiving and transmitting end of each telephone channel. Prior art echo cancellers with disable tone detection are disclosed e.g. in U.S. Pat. No. 5,029,204.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method for the detection of a disable tone signal in an echo canceller.
In a preferred embodiment, the method includes: band-pass filtering, sampling frequency being about half of a given nominal frequency, the resulting sample signal component being aliased to the vicinity of a zero frequency, component, is a positive integer, logical XOR operation executed on the aliased sample signal component and the delayed version thereof.
According to the invention, all frequencies falling outside a monitored frequency band are removed from a received signal by band-pass filtering. After this, the tendency of a signal of being aliased to lower frequencies, when the sampling frequency is lower than the Nyquist frequency, is utilized for the detection of a disable tone. To this end, the filtered signal is sampled at a sampling frequency which is only about half of the nominal frequency of the disable tone, resulting in an aliased sample signal component close to the zero frequency. When the sample rate of the original signal is for instance 8000 samples/second (PCM signal) and the sampling frequency is 1000 Hz, then only every eighth original sample signal transmitted in the telephone channel is further processed. Due to this solution, a signal processor or another device applying the method of the invention is capable of processing both signal directions in several telephone channels.
When a signal at the preset frequency and on a preset level is detected, the signal is tested in order to find a phase inversion of about 180 degrees. This takes place by means of an XOR operation executed between the aliased sample signal component and the version thereof delayed by two sample signal period. In-phase signals always have like signs (plus or minus), but if there exists a phase inversion in the disable tone, the original sample signal component and the delayed version thereof have temporarily unlike signs.
By means of the method of the invention, it is possible to provide a very simple disable tone detection requiring a short processing time.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in the following in greater detail by means of an illustrative embodiment referring to the enclosed drawings, in which:
FIG. 1 shows a block diagram of an echo canceller which is useful in practicing the method according to the invention,
FIG. 2 shows a graph illustrating frequency characteristics of a disable tone d
REFERENCES:
patent: 4658420 (1987-04-01), Fukushi et al.
patent: 4829566 (1989-05-01), Lassaux et al.
patent: 5014307 (1991-05-01), Joffe et al.
patent: 5029204 (1991-07-01), Shenoi et al.
patent: 5123009 (1992-06-01), Winter
Dwyer James L.
Nokia Telecommunications Oy
Shehata M.
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