Video Compression

We have studied the theory of encoding now let us see how this is applied in practice.

We need to compress video (and audio) in practice since:

1.

Uncompressed video (and audio) data are huge. In HDTV, the bit rate easily exceeds 1 Gbps. -- big problems for storage and network communications. For example:

One of the formats defined for HDTV broadcasting within the United States is 1920 pixels horizontally by 1080 lines vertically, at 30 frames per second. If these numbers are all multiplied together, along with 8 bits for each of the three primary colors, the total data rate required would be approximately 1.5 Gb/sec. Because of the 6 MHz. channel bandwidth allocated, each channel will only support a data rate of 19.2 Mb/sec, which is further reduced to 18 Mb/sec by the fact that the channel must also support audio, transport, and ancillary data information. As can be seen, this restriction in data rate means that the original signal must be compressed by a figure of approximately 83:1. This number seems all the more impressive when it is realized that the intent is to deliver very high quality video to the end user, with as few visible artifacts as possible.

2.

Lossy methods have to employed since the compression ratio of lossless methods (e.g., Huffman, Arithmetic, LZW) is not high enough for image and video compression, especially when distribution of pixel values is relatively flat.

The following compression types are commonly used in Video compression:

• Spatial Redundancy Removal - Intraframe coding (JPEG)

• Spatial and Temporal Redundancy Removal - Intraframe and Interframe coding (H.261, MPEG)

These are discussed in the following sections.