A closer look at your MPEGs

Gatot Tio, Contributor, Jakarta

MPEG stands for Moving Picture Experts Group and is the acronym for the family of standards used for coding (in a digitally compressed format) audio-visual information such as movies, videos or music.

This group works on standards for the coding of moving pictures and associated audio formats. The MPEG family of standards includes MPEG-1, MPEG-2 and MPEG-4, formally known as ISO/IEC-11172, IO/IEC-13818 and ISO/IEC-14496.

MPEG-1 is for the coding of moving pictures and the associated audio format for digital storage media at up to 1.5 MBit/s. MPEG- 2 is for the generic coding of moving pictures and associated audio.

Unfortunately, MPEG-3 no longer exists and has been merged into MPEG-2. The other, MPEG-4, is used for very low bitrate audio-visual coding.

MPEGs approach the growing need for step-by-step multimedia standards and are different to JPEGs. They have different charters and requirements. JPEGs are used for the compression of still images.

The major advantage of MPEGs compared to other video and audio coding formats is that MPEG files are much smaller, yet are of the same quality. This is because MPEGs use very sophisticated compression techniques. MPEGs start with a relatively low resolution video sequence (possibly decimated from the original) of about 352 by 240 frames by 30 frames/s, but are of original high (CD) quality audio.

The images are in color, but are converted to YUV space, and the two chrominance channels (U and V) are decimated further to 176 by 120 pixels. They get 352 by 240 from the CCIR-601 digital television standard, which is used by professional digital video equipment.

In the U.S. it is 720 by 243 by 60 fields (not frames) per second, where the fields are interlaced when displayed. Remember that fields are actually acquired and displayed one 60th of a second apart.

The chrominance channels are 360 by 243 by 60 fields a second, and are again interlaced. This ratio of chrominance decimation (2:1 in the horizontal direction) is 4:2:2.

The source input format for MPEG I, called SIF, is CCIR-601 decimated by 2:1 in the horizontal direction, 2:1 in the time direction and an additional 2:1 in the chrominance vertical direction.

And some lines are cut off to make sure things divide by eight or 16 where needed.

Talking about MPEG audio coding, we hear a lot about "Layer 1, 2 and 3". For example, MPEG-1, IS 11172-3, describes the compression of audio signals using high performance perceptual coding schemes. It specifies a family of three audio coding schemes, simply called Layer 1, Layer 2, or Layer 3, with increasing encoder complexity and performance (sound quality per bitrate).

The three codecs are compatible in a hierarchical way, i.e. a Layer-N decoder is able to decode bitstream data encoded in Layer-N and all layers below N (e.g., a Layer 3 decoder may accept Layer 1, 2 and 3, whereas a Layer 2 decoder may accept only Layer 1 and 2.).