As television technology continues to improve, broadcasters
want to ensure that viewers are getting a crisp, clear image. This is where special
contribution encoders come into play, as distinct from the normal sort of
distribution encoder, used to render images for end-user viewing.
The main difference between the two is how color, the
essential attribute of a digital picture element or “pixel” is treated. There
are two separate issues here. One issue is the number of bits used to denote
the color of a pixel, generally referred to as “8-bit” color, “10-bit” color,
and so on.
The other issue is the number of pixels that are retained in
an encoded picture. The terminology used here is a bit more obscure. 4:4:4
encoding means that 4 out of every 4 (i.e., all) pixels are retained in the
encoded image. As a rule, encoders do
not support this mode, since the essential function of an encoder is to reduce
the number of pixels needed to reproduce an image. 4:2:2 means that 2 out of
every 4 pixels are retained in the encoded image. As a rule, this mode is only
supported by source encoders. Distribution encoders instead support 4:2:0 mode,
in which only 1 pixel is retained out of every 4.
Perhaps surprisingly, retaining just a quarter of the pixels
in an image is sufficient to allow the image to very accurately reconstructed
during the decoding process; indeed, few people can detect any difference
between a 4:4:4 image, a 4:2:2 image, and a 4:2:0 image. As a consequence, for
purposes of viewing, the extra pixels retained in 4:4:4 and 4:2:0 can be
regarded as simple overhead, better dispensed with in a distribution encoder,
since the fewer the bits that need to be transmitted or stored, the lower the
cost of digital video.
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