Image Processing Reference
In-Depth Information
A Short History of Video Compression
Video compression has been used ever since television was first launched as a publicly
accessible service. The early compression was analog and was based on complex elec-
tronics inside the TV receiver. It was founded on the concept that interlacing the scanning
lines reduces the transmission bandwidth by taking advantage of our persistence of
vision, and the decay time of the phosphor in a CRT. Chapter 5 describes interlacing and
the reduced sampling of detail in the color compared with the full sampling of brightness.
Standards that describe how digital video-compression works actually only define
the decoder mechanism. If you produce a standards-compliant bit stream, then all past,
present, and future decoders should display the video that you intended. This is a very
important point. As encoding technology matures, better and better bit rates are accom-
plished without having to upgrade the installed base of decoders.
This has certainly been the case with the MPEG-2 standard, which is roughly twice
as efficient as it was when digital TV was first being transmitted. The H.264 standard is
easily twice as good as that and it is at the beginning of that maturity curve. The expecta-
tion is that H.264 encoders will improve to the point where high-quality video is delivered
at better than 1 Mbps bit rates.
Going Digital
The digital revolution happened in the TV studios long before it ever became a consumer
video format. In fact very different formats for TV production evolved.
Oddly enough, rather than store the unmixed red, green, and blue signals in a digi-
tal form, the engineers developed a way to digitally represent the luma plus the two dif-
ference components that comprise the chroma. This became known as digital component
video and is often referred to as 4:2:2. There are other related formats also described in
Chapter 5. These are essentially digital representations of an analog video format.
Nowadays the standards have evolved, and this digital video is stored using a vari-
ety of formats developed by SMPTE, MPEG, and other standards bodies. Most often it is
moved around the studio in a serial digital interface (SDI) format that has become the most
popular studio-quality interconnect standard for hardware.
The Consumer Digital Video Revolution
With the introduction of MPEG-2 video standards, products that were not feasible became
economically viable to produce. The two notable outcomes from MPEG-2 are DVB stan-
dard digital TV services and DVDs containing films and TV programs in a very conven-
ient portable format.
DVB services are delivered via satellite, cable and terrestrial transmission, and DVDs
are manufactured in the same pressing plants that produce CD audio disks.
The industry is poised on the brink of further performance enhancements with the
availability of the H.264 video codec, which improves the compression sufficiently to
make mobile devices such as phones and personal digital assistants with video playback
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