Television – Bandwidth reduction system – Data rate reduction
Patent
1993-01-13
1995-01-10
Britton, Howard W.
Television
Bandwidth reduction system
Data rate reduction
348400, 348423, H04N 7133, H04N 7137
Patent
active
RE0348244
DESCRIPTION:
BRIEF SUMMARY
This application is related to copending, commonly assigned, U.S. patent application No. 07/326,663 filed Mar. 3, 1989 entitled "Data Encoding" and naming Messrs. Morrison and Heron as joint inventors.
This invention relates to a video coder, and in particular, though not exclusively, to a coder employing conditional replenishment techniques whereby data representing changes in an image between frames are sent to update a stored image at a decoder.
Transform coding may also be employed, where typcially each frame of an image to be coded comprises an array of picture elements (pels) which are divided into blocks of N.times.M pels. Each block of pels is transform coded, for instance by applying a discrete cosine transform (DCT). In general, a block of N.times.M pels will be transformed into an array of N.times.M coefficients. The resulting array of coefficients is then quantised by dividing each coefficient by a variable quantisation factor. The quantised coefficients may be coded in variable length code, for instance a Huffman code. Several variable length code books may be available, in which case a decision about which is chosen to be used for a particular coefficient is made depending on the statistics of the coefficients.
In such coding schemes, data is generated at an irregular rate at the transmitter, and an output buffer memory is generally used to ensure that transmission takes place at a uniform data rate.
The fullness of the output buffer may be used to determine the quantisation factor so that, when the buffer fills, the quantisation factor is increased. Conversely, when the buffer empties, the quantisation factor may be decreased.
It is also common practice not to send coefficients representing every block of every frame of the image, but to send them only for those blocks where changes have occurred, with respect to the preceding corresponding block, that exceed a threshold. The threshold may also be determined as a function of the output buffer memory's fullness.
Problems arise where overhead information is to be incorporated into the data stream prior to variable length coding and transmission.
According to one aspect of the present invention, there is provided a video coder comprising: image to be coded, image data indicative of the state of those elements; and/or in respect of groups of blocks of picture elements, the overhead data including information as to the nature of coding employed by the first coding means; for storing the overhead data; means for inserting flags into the first and second buffer memories; and second buffer memory, the reading means being responsive to the occurrence of a flag in the buffer memory being read to cease reading from that memory and to commence reading from the other memory, such that a desired sequence of image and overhead data is obtained.
In another aspect, the invention provides a video coder comprising: code; characterised in that the buffer precedes the variable length coding means, and that the control means 33, 34 is arranged to read from the buffer at a rate such that the rate of the encoded data is constant.
The invention will now be described by way of example with reference to the following figures in which:
FIG. 1 is a block diagram of the first stages of a video coder;
FIG. 2 shows a functional block diagram of a coder incorporating a preferred embodiment of the invention; and
FIG. 3 is a block diagram of a suitable decoder.
FIG. 1 illustrates the front end of a video coder. Digital video input signals are supplied via an input 1 and are differentially coded by subtraction in a subtractor 2 of a predicted picture element value from the actual picture element value received. The difference values are then coded by a transform coder 3 in which N.times.M (e.g. 8.times.8) blocks of elements are subjected to a transform, e.g. the discrete cosine transform (DCT), to produce a like number N.times.M of coefficients. The coefficients are quantised 4 and fed to an output 5. It is noted for future reference that the quantiser 4 may be arranged to out
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Beaumont David O.
Heron Andrew P.
Morrison David G.
British Telecommunications public limited company
Britton Howard W.
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