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prediction is generally not as good and therefore the assumptions under which the variable-
length codes for the inter frames were created are violated. In these situations it is likely that
the variable-length codes designed for the intra frames are a better match. The alternative
inter VLC mode allows for the use of the intra codes in these sitations, improving compression
performance. Note that the codewords used in intra and inter frame coding are the same.
What is different is the interpretation. To detect the proper interpretation, the decoder first
decodes the block assuming an inter frame codebook. If the decoding results in more than 64
coefficients it switches its interpretation.
19.10.12 Modified Quantization Mode
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 luminance 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.
19.10.13 Enhanced Reference Picture Selection Mode
Motion-compensated prediction is accomplished by searching the previous picture for a block
similar to the block being encoded. The enhanced reference picture selection mode allows
the encoder to search more than one picture to find the best match and then use the best-
suited picture to perform motion-compensated prediction. Reference picture selection can
be accomplished on a macroblock level. The selection of the pictures to be used for motion
compensation can be performed in one of two ways. A sliding window of
M
pictures can
be used and the last
M
decoded, with reconstructed pictures stored in a multipicture buffer.
A more complex adaptive memory (not specified by the standard) can also be used in place
of the simple sliding window. This mode significantly enhances the prediction, resulting in a
reduction in the rate for equivalent quality. However, it also increases the computational and
memory requirements on the encoder. This memory burden can be mitigated to some extent
by assigning an unused label to pictures or portions of pictures. These pictures, or portions of
pictures, then do not need to be stored in the buffer. This unused label can also be used as part
of the adaptive memory control to manage the pictures that are stored in the buffer.
