Television – Bandwidth reduction system – Data rate reduction
Patent
1996-05-16
2000-02-08
Le, Vu
Television
Bandwidth reduction system
Data rate reduction
348384, 348410, 382246, 375240, 375241, 375246, 375253, H04N 724
Patent
active
060232943
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for estimating a bit budget for encoders with variable word length, as well as to an image compression device implementing this process.
In the field of image compression, especially in the field of high-definition television, the images to be compressed are subjected to various processing operations (motion estimation, transformation to the frequency domain, for example by DCT ("Discrete Cosine Transformation")), before the data thus obtained are coded for transmission. To eliminate the redundant data whilst preserving the integrity of the information, use may be made of coding processes with variable word length ("Variable length coding" or VLC). These processes take into account the probability with which the data appear among the set of words which are possible for these data. The words which appear often will then be coded by short binary words and the words which appear less often will be coded by longer binary words. The consequence of this will be a reduction in the mean word length, that is to say a reduction in the number of bits to be transmitted. The document EP-A-0 419 141 describes a variable length coding method.
An example of a device using such variable-length coding is described below.
According to a simplified example of a compression circuit represented in FIG. 1, an RGB video signal is digitized by an analog/digital converter 1, feeding an RGB/YUV conversion matrix 2, whose output is connected to a picture memory 3. An input buffer 4 forms the interface between the picture memory 3 and the motion estimator(s) 5. The results of this or these estimation(s) are processed by circuits which are not represented in FIG. 1. The signal is furthermore subjected to a discrete cosine transformation combinations of non-zero coefficients and of lengths of series of non-zero coefficients are finally coded by a variable-length coding process of known type, for example coding by the Huffman method (circuit 8). A buffer 9 gathers the compressed signal leaving the coder, outputting it as a function of the capacity of the transmission channel, this channel not being represented in FIG. 1. A problem arises when the buffer 9 is filled more quickly than it is emptied. In certain cases, this may lead to an overflow of the buffer and hence to a loss of data. To avoid this problem, a throughput adjustment feedback loop 10 controls the quantizing of the coefficients of the DCT, and consequently the throughput of data to the buffer 9. It therefore seems to be necessary to carry out VLC coding of the data before being able to ascertain the corresponding bit budget, that is to say the number of bits required to code the signal or the signal portion in compressed form. This solution is unsatisfactory since it is necessary to perform a number of purposeless calculations, if the coding is performed in two passes, the first intended for a first estimate of the bit budget, the second for the coding per se, possibly with a correction depending on the results of the first pass. This results in a loss of time and energy, as well as increased complexity of the circuits.
The purpose of the invention is to avoid these drawbacks.
The purpose of the invention is moreover to allow estimation of the data throughput leaving the coder, and to do so before the variable word length coding.
The subject of the invention is a process for estimating a bit budget in a circuit for compressing digital data, especially digitized television signals, characterized in that, the signals being represented by N-bit samples, the estimate is based on the probability of a bit of given order having a given value out of M binary words representing M samples of signal, an entropy calculation being performed on the basis of the probabilities corresponding to the N bits, the sum of all the entropies corresponding to the desired estimate.
The performance of certain coding methods is known relative to the lower limit constituted by the entropy. In the case of the Huffman coding method, the mean number of bits requi
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Herrmann Eric P.
Le Vu
Thomson Multimedia S.A.
Tripoli Joseph S.
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