In this mode, along with changes in the signalling of changes in quantization parameters, the quantization process is improved in several ways. In the baseline mode, both the luminanace and chrominance components in a block are quantized using the same quantizer. In the modified quantization mode, the quantizer used for the luminance coefficients is different from the quantizer used for the chrominance component. This allows the quantizers to be more closely matched to the statistics of the input.
The modified quantization mode also allows for the quantization of a wider range of coefficient values, preventing significant overload. If the coefficient exceeds the range of the baseline quantizer, the encoder sends an escape symbol followed by an 11-bit representation of the coefficient. This relaxation of the structured representation of the quantizer outputs makes it more likely that bit errors will be accepted as valid quantizer outputs. To reduce the chances of this happening, the mode prohibits “unreasonable” coefficient values.