Standard Codecs: Image Compression to Advanced Video Coding (IET Telecommunications Series)

6.1 Video format and structure

Figure 6.1 shows a block diagram of an H.261-based audio-visual system, where a preprocessor converts the CCIR-601 video (video at the output of a camera) to a new format. The coding parameters of the compressed video signal are multiplexed and then combined with the audio, data and end-to-end signalling for transmission. The transmission buffer controls the bit rate, either by changing the quantiser step size at the encoder, or in more severe cases by requesting reduction in frame rate, to be carried out at the preprocessor.

Figure 6.1: A block diagram of an H.261 audio-visual encoder

The H.261 standard also allows up to three pictures to be interpolated between transmitted pictures, so reducing the frame rates to 15, 10 and 7.5, respectively. The use of quarter-CIF, or QCIF, resolution will reduce the sample rate even further to suit low bit rate channels.

In CIF and QCIF, DCT blocks are grouped into macroblocks of four luminance and two corresponding C_b, and C_r chrominance blocks. The macroblocks (MB) are in turn grouped into layers termed groups of blocks (GOB). A CIF frame has 12 GOBs and QCIF has three, as illustrated in Figure 6.2.

Figure 6.2: Block, macroblock and GOB structure of CIF and QCIF formatted pictures

The objectives of structuring an image into macroblocks and layers are as follows:

• similar inter/intra coding mode for luminance and chrominance blocks at the same area

• the use of one motion vector for both luminance and chrominance blocks

• efficient coding of the large number of 8 × 8 DCT blocks that will be expected to be without coded information in interframe coding; this is implemented via the inclusion of VLC codes for coded block pattern (CBP) and macroblock addressing [1]

• to allow synchronisation to be reestablished when bits are corrupted by the insertion of start codes in the GOB headers; note that since DCT coefficients are VLC coded, then any error during the transmission renders the remaining VLC coded data undecodable, hence, with a GOB structure, only a portion of the picture is degraded

• to carry side information appropriate for GOB, macroblock or higher layers; this includes picture format, temporal references, macroblock type, quantiser index etc.