Device for controlling the quantizer of a hybrid coder

Claim 1

In a hybrid coder for coding and quantizing a plurality of data blocks of pictures of a video signal, wherein said hybrid coder comprises a buffer memory and a quantizer and assumes a stable first state (Z1) wherein the quantization of said quantizer is at a first quantization level which depends on the fill level of said buffer memory, a device for further controlling the quantization of said quantizer by causing the coder to assume a second state (Z2) wherein said quantizer is subject to a second control, said device comprising: a) means coupled to the input of said hybrid coder, for sequentially determining the respective differences between consecutive pairs of data blocks of a sequence of equivalent data blocks and whereby an index L having an initial value is incremented by one, each time a respective difference between a pair of said consecutive pairs of data blocks of a sequence of equivalent data blocks is less than a predetermined value, and whereby said index L is set to said initial value each time the respective difference between a pair of said consecutive pairs of data blocks of a sequence of equivalent data blocks is more than said predetermined value; b) means coupled to said quantizer, for switching said coder into said second state and said quantizer from the first quantization level to a predetermined fine quantization level whenever the value of index L is greater than a threshold value L max and a further system parameter satisfies a further system condition; and c) means coupled to said quantizer for setting said quantizer from said fine quantization level to a predetermined coarse quantization level when both said coder is in the second state and the value of index L is greater than said threshold level L max .