MPEG explained

Explaining MPEG ... that's easy No? MPEG is the video compression thingie that makes files smaller so you can put them on CD's or DVD's. Close, but no cigar..

Most people that have ever worked with a computer and video files get this far. But what is it really, what does it do, and how does it work ?

In order to be able to exploit the capabilities of MPEG video compression we need to have a basic understanding of what MPEG video compression is, how it works, and why it works the way it works. There is much more involved than simple compression.

In this article I will try to give an overview of the MPEG compression systems and a basic working. As you read on I will go deeper in detail on how the compressor, and decompressor works. You have to understand that MPEG video compression is an extremely complex system based on difficult mathematical algorithms and models.

The additional fact that most of the compression is done in the time domain adds to the complexity of the system, and makes understanding its inner workings more difficult.

What is MPEG?
MPEG 1
basics of MPEG-1
MPEG-2
MPEG-4 and newer standards
MPEg myths and nonsense
Quality comparision
MPEG encoding
MPEG compression algorithms

What is MPEG?

The easiest way to explain MPEG is to start by explaining what it is not! Contrary to what most people think, MPEG is NOT a video format, nor is it a compression system. It's also not an audio file format. But then what is it?

MPEG is an acronym for Moving Picture Experts Group. In other words: a committee of a bunch of people that are studying motion pictures and their encoding and delivery systems.

The MPEG was established in January 1988 with the mandate to develop standards for coded representation of moving pictures, audio and their combination. It works as a part of the Joint ISO/IEC Technical Committee (JTC 1) on Information Technology and is formally Work Group 11 of Sub Committee 29.

Starting from its first meeting in May 1988, with 25 experts participating, MPEG has grown to an unusually large committee. Typically about 350 experts from some 200 companies and organizations, hosted in more then 20 different countries take part in MPEG meetings. As a rule, MPEG meets three times a year (in March, July and November) but meets more frequently when the workload so demands. A large part of the MPEG membership is made of individuals operating in research and academia.

MPEG standards

Even though the MPEG environment looks rather informal, it has to be borne in mind that standards can be of high strategic relevance. The MPEG exists to produce standards. Those currently produced by ISO are indicated by 5 digits (the ISO number for MPEG-1 is 11172 and for MPEG-2 is 13818).

Published standards are the last stage of a long process that starts with the proposal of new work within a committee. When the scope of new work has been sufficiently clarified, MPEG usually makes open requests for proposals. So far proposals have been requested for:

MPEG-1 Audio and Video (July 1989) and formally approved as a standard in November 1992
MPEG-2 Audio and Video (July 1991), formally approved as standard in November 1994
MPEG-4 Audio and Video (July 1995), approved in October 1998 (version 1) and December 1999 (version 2)

Currently the MPEG committee is working on MPEG4 version 3, 4 and 5 and MPEG-7.

The work done by this group has also sprouted 2 other standards currently in use today: H.261 which is aimed towards telecommunications and ISO10918 or .. JPEG, used for encoding still images. Elements of these standards are included in the MPEG standard, and MPEG builds on them and goes beyond what these standards implement.

MPEG-1 or ISO/IEC 11172

MPEG-1 was the first format released in 1992 by the MPEG committee. MPEG-1 is an extremely popular standard and there are players for almost every computing platform available today. It is widely used in many multimedia and gaming applications to deliver complex video sequences while using minimal storage space. Originally developed in response to industry needs for storing and retrieving video information from digital media, it quickly found its way into all sorts of applications requiring motion images.

MPEG-1 addresses the problem of combining one or more data streams from the video and audio parts with timing information to form a single stream. Once in a single stream the data can easily be stored as a sequential file on any media conceivable. MPEG-1 has provisions to handle both 625-line and 525-line video. It was developed to operate principally from storage media offering a continuous transfer rate of about 1,5 Mbit/s. Nevertheless it can be used more widely than this because the approach taken is generic.

The MPEG-1 format is an asymmetrical codec. This means that the effort required to encode is not the same as the effort needed for decoding. Encoding can be slow but the decoding needs to be fast enough so it can deliver its 25 or 30 frames a second to show on a TV. A typical real-time MPEG-1 encoder needs about 250 MIPS (million instructions per second) and requires a lot of CPU power. Where as the decoder can be made using a 5 $ component that runs as slow as 20 MHz.

Basics of MPEG-1

MPEG-1 will start by scaling the image to about 352 by 240 pixels at either 25 or 30 frames/s (depending on whether you are in Europe or the US) and original high quality audio. The images are but converted to YUV space to make the subsequent calculation easier and faster. (See my article on selecting color for TV output to learn about YUV color space and why it is preferable over any other video format).

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