Video signal coder using variance controlled quantization

In a video signal encoding system including a transform coder for encoding a video signal comprised of a plurality of complete video frames on a block-by-block basis to generate an array of transform coefficient blocks, a controllable quantizer, having a quantizer step size matrix, for sequentially quantizing the array of transform coefficient blocks to generate transform coded and quantized data, and an apparatus for generating a quantization parameter for use in controlling the quantizer step size matrix, characterized in that said apparatus comprises:

• first calculation means for receiving the video signal and calculating the respective variance of each complete video frame;
• means for receiving the video signal and dividing each complete video frame into a predetermined array of video slices;
• second calculation means for receiving the predetermined array of video slices and sequentially calculating the respective variance of each of the video slices;
• third calculation means for receiving said respective variance of each complete video frame from said first calculation means and said respective variance of each of the video slices from said second calculation means and calculating the respective spatial activity of each of the video slices;
• and
• means, in response to said respective spatial activity of each of the video slices, for generating a respective adjusted quantization parameter for each of the video slices.