Telecommunications – Transmitter and receiver at separate stations – Distortion – noise – or other interference prevention,...
Patent
1997-09-19
2000-03-07
Eisenzopf, Reinhard J.
Telecommunications
Transmitter and receiver at separate stations
Distortion, noise, or other interference prevention,...
455296, H04B 110
Patent
active
060351799
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to transmitting voice-frequency signals, especially DTMF signals, over the radio interface of a digital mobile telephone system.
BACKGROUND INFORMATION
In a digital telephone system, a speech signal is coded in some manner before it is channel coded and transmitted to the radio path. In speech coding, digitalized speech is processed frame by frame in periods of about 20 ms by using different methods in such a manner that the result is a group of parameters representing speech for each frame. This information, i.e. the parameter group, is channel coded and transmitted to the transmission path. In channel coding, the information is protected by different error correcting codes.
The speech coding method used in the GSM cellular system is the so-called RPE-LTP (Regular Pulse Excitation LPC with Long Term Prediction). The basic parts of the algorithm are the linear predictive coding filter LPC and residual signal coding as the last stage of the pulse sequence. The operation is completed by tone pitch estimation LP. The coder thus produces short term filter parameters, long term prediction parameters LTP, and RPE parameters. In a decoder, the RPE parameters act as a filter excitation signal, and the received short and long term parameters act as filter parameters. The speech coding algorithm employed by the digital cellular system in the USA belongs to the category of code excited coders CELP (Code Excited Linear Prediction), and the coder is referred to by the term Vector-Sum Excited Linear Predictive Coding (VSELP). The result of the speech coding is a group of parameters, by means of which and also by means of code books having a predetermined structure a speech signal is synthesized in the decoder of the receiver. The speech signal residual is not transmitted at all, as is done in a RPE-LTP coder. The coders of both systems have in common the fact that the coder produces speech frames the duration of which is 20 ms and that a speech frame consists of subframes of 5 ms, each of which contains a speech parameter group.
In addition to actual coding, the following functions are also built in in digital speech processing: a) on the transmitter side, voice activity detection (VAD), by means of which the transmitter can be activated only when there is speech to be transmitted (Discontinuous Transmission, DTX), b) on the transmitter side, background noise evaluation and generation of parameters corresponding to the noise, and on the receiver side, comfort noise generation in the decoder from the parameters, this comfort noise making an interruption in the connection sound more comfortable than absolute silence, and c) acoustic echo cancellation.
As an example of speech processing, speech processing arrangement used in the known GSM mobile telephone system is described with reference to FIG. 1 showing a transmitter side. The input of the speech coder 1 is either a 13-bit PCM signal arriving from the network, obtained by sampling an audio signal at a frequency of 8000 samples per second, or A/D converted 13-bit PCM arriving from the audio part of the mobile station. The duration of the speech frame obtained from the output of the coder is 20 ms and it comprises 260 bits, which are generated by coding 160 PCM coded speech samples.
The speech coder 1 produces the parameters mentioned above for each 20 ms speech frame, and the voice activity detector (VAD) 2 determines on the basis of these parameters whether the frame contains speech or not. According to the information contents of the frame, the VAD detector sets an appropriate flag controlling the operation of a DTX control and operation block 4. Its value may be VAD=1, the frames applied to a channel coder 5 and therefrom further to the radio path as so-called traffic frames being thus speech frames produced by the speech coder. The DTX control and operation block 4 sets a flag SP controlling channel coding for each frame applied to the channel coder 5.
In transmitting speech, background noise is also included
REFERENCES:
patent: 4782485 (1988-11-01), Gollub
patent: 5018136 (1991-05-01), Gollub
patent: 5835486 (1998-11-01), Davis et al.
Multifrequency Signalling System to be Used for Push-Button Telephones, T/WG II "Switching and Signalling" (CS), T/CS 46-02 (Innsbruck 1981, revised at Nice 1985) Ed. May 15, 1986, pp. 1-16.
Banks-Harold Marsha D.
Eisenzopf Reinhard J.
Nokia Telecommunications Oy
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