Image Processing Reference
In-Depth Information
MPEG-2 has improved in efficiency by a factor of about 2:1 since the original coders
were introduced. Bit rates as low as 3.5 Mbps are routinely used for broadcast. There is still
some room for improvements to the efficiency, but this is bounded by the need to remain
100% compliant with all the decoders that have been deployed. New opportunities for
more radical improvement are presented when rolling MPEG-2 decoders out to “green-
field” situations where there is no prior deployment, although these are also candidates
for H.264 and VC-1 codec implementation.
By defining the characteristics of a compliant decoder, the encoder manufacturers are
free to innovate in any way they like, so as to generate the most efficiently coded repre-
sentation of the original video source material.
Profiles and Levels
Certain features might be allowed or disallowed by defining profiles. This helps to man-
age the complexity. Simple decoders check the profile of the incoming video and either
attempt to decode it or display a message explaining why they will not.
Having defined a profile, the encoder must honor that and not create a coded repre-
sentation that is any more complex than the profile allows.
Decoders are implemented up to a certain profile level and support content from the
lowest to the highest they are capable of and everything in between. A maximally imple-
mented decoder that supports everything MPEG-4 defines would be a very unwieldy and
large piece of software that would be inappropriate for a lot of applications. A complete
MPEG-4 encoder would be of no use in a portable device because you don't need the more
esoteric features. The portable device would probably not have the memory or compute
power to cope with it anyway. So profiles help to keep the software implementation under
control as well.
Better Compression Factors
In general, the more modern the codec design, the better the compression ratio com-
pared with the original source material. Longevity of the codec also comes into play. The
longer a codec has been around, the more tricks have been developed to exploit the
On the downside, newer codecs or upgrades to existing models will require greater
CPU power to leverage the improved algorithms. This is less of a problem for the decod-
ing in the client because the algorithms tend toward techniques that are very easy to
decode. So you may need to upgrade your processing hardware as you roll out new
encoders even though they target decoders with a longer lifespan.
My Codec Is Better Than Yours
This is a game people like to play with one another, usually when discussing commercial
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