What Are We Trying to Compress? - A Practical Guide to Video and Audio Compression - page 36

systems. At the other end of the scale is the enthusiastic amateur videographer who wants
to create a DVD to record some family history. Compression systems are available for both
extremes as well as for those whose needs are somewhere in between.
We will consider the larger scale first. Then we will work our way down to some-
thing suitable for single users running a small operation.

3.4.1 Broadcasters

There are relatively few large broadcasters in the world compared with the number of
small- to medium-sized potential compressionists. By large broadcasters, we mean organ-
izations that are broadcasting multiple channels of content simultaneously. Examples
might be the BBC; MTV; or News Corporation, which is the parent company of all the Sky
brands and others such as Fox Television.
The BBC broadcasts more than 26 hours of content for every hour of the day. This
includes multiple national and international TV services and many audio services that are
going out in the form of AM, FM, and digital radio. Some of this content is also streamed
on the Internet to augment a large-scale Web site operation.
The production processes for that content are now almost completely digital, from
inputting the source material and passing it through the editing process, to putting it on
the air. Some content still arrives in analog form, and the U.K. still has some analog broad-
casting for the domestic user. That analog service is actually delivered using digital sys-
tems almost to the very last point in the chain before the transmitter.
Video compression is fundamental to the broadcasting workflow. Storing everything
in a totally uncompressed form is unrealistic for both technical and financial reasons. The
compression chosen must allow the footage to be edited and reused without a genera-
tional loss being introduced. High-quality master copies of content might be preserved
using a 50-Mbps storage format and routine content would use 25 Mbps. Other interme-
diate formats are used that have lower bit rates.
Archives require a lossless or nearly lossless storage format. By the time the video
reaches the TV set in your living room, it has been compressed by a factor of 10 to 1 and
is no longer anything like archival quality.
Long-term archival storage will almost certainly be in a variety of different formats.
Some of them are lower quality than current production and broadcast standards allow
because the older legacy systems could not achieve the quality levels that are now com-
monplace.
You cannot put back what is not there. Repurposing old archival content presents
some special problems, and finding ways to get the very best quality out of old
material is an ongoing challenge. The BBC Research Centre develops technology solu-
tions to this sort of problem from time to time. A recent innovation was a digital
processing tool that samples the PA L recorded color sub-carrier more accurately in
order to restore old PAL footage, eliminating a lot of analog noise and cross-color arti-
facts. This sort of research requires considerable resources and it benefits the entire
industry.