 MPEG-1 actually uses two separate quantization matrices, one for intra-blocks (I-blocks) and one for inter-block (P- and B- blocks) so quantization of different block types can be done independently, and so, more effectively.This quantization process usually reduces a significant number of the "AC coefficients" to zero, (known as sparse data) which can then be more efficiently compressed by entropy coding (lossless compression) in the next step.An example quantized DCT block