Television – Bandwidth reduction system – Data rate reduction
Patent
1993-11-08
1995-10-17
Chin, Tommy P.
Television
Bandwidth reduction system
Data rate reduction
348419, H04N 732
Patent
active
054595150
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for dual-standard coding of very low data-rate images and a coder-decoder for implementing this method.
It applies particularly to the production of differential coding devices or "CODECs" at data rates lower than 64 Kbits.
DISCUSSION OF BACKGROUND
In conventional "CODECs" the transmission of moving images at very low data rates requires, in addition to employing a device for compression of the transmitted data, developing techniques for limiting the spatial and time resolution of the image signal. The limitation of the spatial resolution is obtained by sub-sampling the image in the vertical and horizontal directions. In this field it is known to use the CIF and QCIF formats, which are respectively the abbreviations of 37 Common Intermediate Format" and "Quarter of Common Intermediate Format". In the CIF format the number of points per line in luminance is equal to 352, the number of points per line in chrominance is respectively 176 for the Dr and Db components and the number of lines per image is 288 for the luminance signals and 144 respectively for the Dr and Db chrominance signals. In the QCIF format the number of points per line in luminance is 176, the number of points per line in chrominance is respectively 88 for the Dr and Db signals and the line number per image is 144 for the luminance signals and 72 respectively for the Dr and Db chrominance signals.
The limitation of the time resolution takes place quite simply by reducing the number of images transmitted with respect to the number of images delivered by the source. In practice, in the presence of an image source to the CIF standard at 25 Hz, ie. 25 images per second, the reduction may go as far as making it possible to code and transmit images at a frequency of 1.25 Hz.
The currently known coders and decoders generally conform to recommendation H261 version 11 1989 of the "CCITT specialist group on coding for visual telephony". In this context they are very well suited for data-rate applications which may vary by integer multiples of 64 Kbit/s. Below that it is difficult to produce coders which have both good spatial resolution and good time resolution. However, the transmission of images at very low data rate (32 Kbit/s, 16 Kbit/s, etc.) compels sub-sampling of the image signal to be carried out both in the spatial domain and in the time domain. Thus, the choice may be made to transmit images at 5 Hz in order to obtain spatial resolutions reduced to the QCIF format and at 1.25 Hz to obtain spatial resolution reduced to the CIF format, but in these conditions the CIF 1.25 Hz standard is rather intended for slowly moving images, while the other standard is more appropriate for scenes with a great deal of movement.
SUMMARY OF THE INVENTION
The object of the invention is to implement a method for dual-standard coding such that, in the presence of very slowly moving scenes, the spatial resolution is favoured by using the CIF format at 1.25 periods per second, while, for moving scenes, the time resolution is enhanced up to 5 periods per second to the detriment of the spatial resolution reduced to the QCIF format.
To this end, the subject of the invention is a method for differential coding of images at very low data rate, for transmitting images with low spatial QCIF resolution and with high time resolution according to a first standard, and images with high spatial CIF resolution and low time resolution according to a second standard, consisting in coding the images to be transmitted by means of a coding device including at least one quantifier circuit for a prediction error signal, characterized in that it consists: quantifier circuit, value and with a second predetermined threshold value higher than the first, the mean value of the quantification step is lower than the first predetermined threshold value, of the quantification step is lower than the second predetermined threshold value, quantification step is higher than the second predetermined threshold value, of the qua
REFERENCES:
patent: 4965667 (1990-10-01), Trew et al.
patent: 5150432 (1992-09-01), Ueno et al.
patent: 5193002 (1993-03-01), Guichard et al.
patent: 5214506 (1993-05-01), Lin et al.
patent: 5253058 (1993-10-01), Gharavi
IEEE Transactions on Consumer Electronics, vol. 36, No. 3, Aug. 1990, New York, USA, A. Artieri et al: "A Chip Set Core for Image Compression", pp. 395-402.
International Broadcasting Convention (IBC), Brighton, UK, Sep. 23-27, 1988, London, T. Trew et al: "Spatially Adaptive Sub-Branches for HD-MAC", pp. 66-69.
Chevance Christophe
Thoreau Dominique
"Thomson-CSF"
Chin Tommy P.
Lee Richard
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