More About MPEG 1

The MPEG-1 Audio and video compression format was developed by MPEG group back in 1993. The Official description for it is "Coding of moving pictures and associated audio for digital storage media at up to about 1,5 Mbit/s".

MPEG-1 is the video format that has had some extremely popular spin-offs and sideproducts, most notably MP3 and VideoCD.

MPEG-1's compression method is based on re-using the existing frame material and using psychological and physical limitations of human senses. MPEG-1 video compression method tries to use previous frame's information in order to reduce the amount of
information the current frame requires. Also, the audio encoding uses something that's called psychoacoustics -- basically compression removes the high and low frequencies a normal human ear cannot hear.

Resolutions that video streams can use are:

352x480 (NTSC, MPEG-2 & MPEG-1)
352x576 (PAL, MPEG-2 & MPEG-1)
352x240 (NTSC, MPEG-2 & MPEG-1)
352x288 (PAL, MPEG-2 & MPEG-1)

MPEG-1 standard consists of five parts: named as Part 1 to Part 5.

Part 1 addresses the problem of combining one or more data streams
from the video and audio parts of the MPEG-1 standard with timing
information to form a single stream as in figure.
This is an important fuction because, once combined
into a single stream, the data are in a form well
suited to digital
storage or transmission.


Part 2 specifies a coded representation that can be used for compressing video sequences - both 625-line and 525-lines - to bitrates around 1,5 Mbit/s. Part 2 was developed to operate principally from storage media offering a continuous transfer rate of about 1,5 Mbit/s.

A number of techniques are used to achieve a high compression ratio. The first is to select an appropriate spatial resolution for the signal. The algorithm then uses block-based motion compensation to reduce the temporal redundancy. Motion compensation is used for causal prediction of the current picture from a previous picture, for non-causal prediction of the current picture from a future picture, or for interpolative prediction from past and future pictures. The difference signal, the prediction error, is further compressed using the discrete cosine transform (DCT) to remove spatial correlation and is then quantised. Finally, the motion vectors are combined with the DCT information, and coded using variable length codes.

The figure below illustrates a possible combination of the three
main types of pictures that are used in the standard.

Part 3 specifies a coded representation that can be used for compressing audio sequences - both mono and stereo. The algorithm is illustrated in Figure 3 below. Input audio samples are fed into
the encoder. The mapping creates a filtered and subsampled representation of the input audio stream. A psychoacoustic model creates a set of data to control the quantiser and coding. The
quantiser and coding block creates a set of coding symbols from the mapped input samples. The block 'frame packing' assembles the actual bitstream from the output data of the other blocks, and adds other information (e.g. error correction) if necessary.

Part 4 specifies how tests can be designed to verify whether bit streams and decoders meet the requirements as specified in parts 1, 2 and 3 of the MPEG-1 standard. These tests can be used by:

manufacturers of encoders, and their customers, to verify whether
the encoder produces valid bit streams.

manufacturers of decoders and their customers to verify whether
the decoder meets the requirements specified in parts 1,2
and 3 of the standard for the claimed decoder capabilities.

applications to verify whether the characteristics of a given
bit stream meet the application requirements, for example
whether the size of the coded picture does not exceed the
maximum value allowed for the application.



Part 5, technically not a standard, but a technical report, gives a full software implementation of the first three parts of the MPEG-1 standard.


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