Introduction to compression - The MPEG - page 18

In general, compression should not be used for its own sake, but only where a genuine bandwidth or cost
bottleneck exists. Even then the mildest compression possible should be used. Whilst high compression factors are
permissible for final delivery of material to the consumer, they are not advisable prior to any post-production
stages. For contribution material, lower compression factors are essential and this is sometimes referred to as
mezzanine level compression.

One practical drawback of compression systems is that they are largely generic in structure and the same
hardware can be operated at a variety of compression factors. Clearly the higher the compression factor, the
cheaper the system will be to operate so there will be economic pressure to use high compression factors.
Naturally the risk of artifacts is increased and so there is (or should be) counterpressure from those with
engineering skills to moderate the compression. The way of the world at the time of writing is that the accountants
have the upper hand. This was not a problem when there were fixed standards such as PAL and NTSC, as there
was no alternative but to adhere to them. Today there is plenty of evidence that the variable compression factor
control is being turned too far in the direction of economy.

It has been seen above that concatenation of compression systems should be avoided as this causes generation
loss. Generation loss is worse if the codecs are different. Interlace is a legacy compression technique and if
concatenated with MPEG, generation loss will be exaggerated. In theory and in practice better results are obtained
in MPEG for the same bit rate if the input is progressively scanned. Consequently the use of interlace with MPEG
coders cannot be recommended for new systems. Chapter 5 explores this theme in greater detail.

1.14 Compression pre-processing

Compression relies completely on identifying redundancy in the source material. Consequently anything which
reduces that redundancy will have a damaging effect. Noise is particularly undesirable as it creates additional
spatial frequencies in individual pictures as well as spurious differences between pictures. Where noisy source
material is anticipated some form of noise reduction will be essential.

When high compression factors must be used to achieve a low bit rate, it is inevitable that the level of artifacts will
rise. In order to contain the artifact level, it is necessary to restrict the source entropy prior to the coder. This may
be done by spatial low-pass filtering to reduce the picture resolution, and may be combined with downsampling to
reduce the number of pixels per picture. In some cases, such as teleconferencing, it will also be necessary to
reduce the picture rate. At very low bit rates the use of interlace becomes acceptable as a pre-processing stage
providing downsampling prior to the MPEG compression.

A compression pre-processor will combine various types of noise reduction (see Chapter 3 ) with spatial and
temporal downsampling.

1.15 Some guidelines

Although compression techniques themselves are complex, there are some simple rules which can be used to
avoid disappointment. Used wisely, MPEG compression has a number of advantages. Used in an inappropriate
manner, disappointment is almost inevitable and the technology could get a bad name. The next few points are
worth remembering.

Compression technology may be exciting, but if it is not necessary it should not be used.

If compression is to be used, the degree of compression should be as small as possible; i.e. use the
highest practical bit rate.

Cascaded compression systems cause loss of quality and the lower the bit rates, the worse this gets.
Quality loss increases if any post- production steps are performed between compressions.

Avoid using interlaced video with MPEG.

Compression systems cause delay.

Compression systems work best with clean source material. Noisy signals or poorly decoded composite
video give poor results.

Compressed data are generally more prone to transmission errors than non-compressed data. The choice
of a compression scheme must consider the error characteristics of the channel.

Audio codecs need to be level calibrated so that when sound pressure level-dependent decisions are made
in the coder those levels actually exist at the microphone.

Low bit rate coders should only be used for the final delivery of post- produced signals to the end-user.

Don't believe statements comparing codec performance to 'VHS quality' or similar. Compression artifacts
are quite different from the artifacts of consumer VCRs.

Quality varies wildly with source material. Beware of 'convincing' demonstrations which may use selected
material to achieve low bit rates. Use your own test material, selected for a balance of difficulty.

Don't be browbeaten by the technology. You don't have to understand it to assess the results. Your eyes
and ears are as good as anyone's so don't be afraid to criticize artifacts. In the case of video, use still frames
to distinguish spatial artifacts from temporal ones.