Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Patent
1996-08-23
1998-12-29
Chin, Stephen
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
375342, 36472419, 704220, H03D 100
Patent
active
058548149
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a digital transmission system, more particularly a mobile radio system comprising at least a transmitter for transmitting codewords which can be derived from a digital signal, and at least a receiver for receiving codewords, the receiver including a decoder with a prediction filter which is excited by an input signal to produce an estimated digital signal.
The invention further relates to a transceiver arrangement, more particularly a base station or a mobile station for such a digital transmission system, as well as a receiver, for such a transmission system.
Such a digital transmission system operates, for example, according to the so-called LD-CELP (=Low-Delay Code Excited Linear Prediction) speech coding algorithm, as this is described, for example, in EP 379 296 A2. An analog speech signal is then digitized in a coder at the transmitting end. Subsequently, in the transmitter this digital signal is converted into codewords by a coder and a decoder. These codewords are transmitted by the transmitter over a transmission channel, for example, in a mobile radio transmission system by an air interface to the receiver. In the receiver the codewords are converted into an estimated digital signal by means of the decoder and subjected to a postprocessing. The decoder in the transmitter and the decoder in the receiver then have identical structures and, if there is no disturbance, are also supplied with the same codewords so that, ideally, identical digital signals are produced as output signals i.e. the digitized speech signal at the transmitting end corresponds to the estimated digital signal at the receiving end. However, if the codewords to be transmitted are disturbed on the transmission channel, in a mobile radio transmission system, for example, by radio fading or dead spots caused by high-rise buildings, the decoder of the receiver is supplied with other codewords which do not correspond to the transmitted codewords. In consequence, disturbances are developed which are also subjectively perceivable.
SUMMARY OF THE INVENTION
It is an object of the invention to improve the quality of the estimated digital signal in the case of disturbances of the transmission channel.
This object is achieved in that the decoder comprises a first switching means for switching off the input signal of the prediction filter when a disturbed codeword is received and a second switching means for switching substitute values formed from previous values of the estimated digital signal instead of the estimated digital signal when the input signal is switched off.
The invention is then based on the recognition that in the case of disturbance i.e. when a disturbed codeword is received, more particularly in the case of group errors (with several successive disturbed codewords), the faulty reconstruction of the estimated digital signal in the decoder of the receiver is to be interrupted, so that the subjectively perceivable speech quality in the receiver remains relatively unchanged. Together with the switching-off of the input signal of the prediction filter in the decoder of the receiver, it is further necessary that substitute values are rendered available for the estimated digital signal. Surprisingly, it has appeared that the subjectively perceivable speech quality is experienced as relatively unchanged when-disturbances i.e. disturbed codewords occur, if the most recent sample values of the synthesis filter are substituted with already determined sample values of the estimated digital signal from the past. This may be effected by extrapolation in the prediction filter.
When speech signals are transmitted, an improvement of the subjectively perceivable speech quality can particularly be established if similar sample values to the estimated digital signal are used as substitute values.
A very good speech quality and a slight signal delay may be ensured in that the transmission system is provided for transmitting codewords according to the LD-CELP (Low-Delay Code Excited Linear
REFERENCES:
patent: 4980916 (1990-12-01), Zinser
patent: 5127053 (1992-06-01), Koch
patent: 5299281 (1994-03-01), Coolegem
patent: 5359696 (1994-10-01), Gerson et al.
patent: 5602961 (1997-02-01), Kolnesnik et al.
Brox Wolfgang
Hofmann Rudolf
Chin Stephen
Halajian Dicran
Roundtree Joseph
U.S. Philips Corporation
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