Re: Chroma Decimation in Digital Video Signals

Throwing away every other sample is often done, but is not the best
idea. The media processors I know best (Philips TriMedia/Nexperia)
apply a 5-tap filter when changing from eg YUV444 to YUV422 (and from
YUV422 to YUV420 etc). The shape of filter is important, too, and may
be changed for different types of image. In some cases this os done by
digitizing or reconstruction hardware, in some by dedicated
co-processors.

Chroma decimation by throwing away is often acceptable, but it depends
what you mean by 'acceptable' - it does not give you such a good
picture. In fact, the new Nexperia processors enhance the chroma by
some clever processing that tries to put back what was left out, and
this is beginning to become perceived by viewers. I thik the reason for
the less-demanding processing being so common is, that viewers have
been trained to accept fairly poor picture quality.

Horizontal filtering is really a quite simple 1-D case, albeit one that
requires polyphase filter, and so can be done 'on the fly' as a signal
is captured or rconstructed - to do a 2D filter requires line or frame
buffering, but this is also often done.

One problem in this area is, lots of people 'know' how to process video
- but fewer seem able to produce pictures that look good - that
requires a lot of domain knowledge. When I was involved with
demonstrating video to prospective customners (ie TV manufacturers) I
noticed that engineers would often be proud of their video processing
but the intended customers would write it off very quickly because he
picture quality was not good enough. To be able to look for video
quality is not all that easy, as we get distracted by the content and
by being so used to watching awful quality.

Chris
===================
Chris Bore
BORES Signal Processing
chris@xxxxxxxxx

.

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