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Message
From: InvalidError at gmail.com<InvalidError@g...>
Date: Wed Jul 20 08:12:16 CEST 2005
Subject: [oc] NTSC Video Encoding/Generation
> From what I understand, you need quite a high clock to give you the
> resolution to generate the colour information for more than a few
> colours.
It does not need to be very high but best results and simplicity do
require that the reference clock be an exact multiple of the color
burst frequency.
Yes, color information is encoded as a phase shift between color burst
and chroma. For reasonably accurate phase generation, oversampling
needs to be at least 4X, preferably 6X.
With a color burst of 3.569545MHz, a clock rate of 21.41727MHz or
28.55636MHz would be ideal, extra temporal resolution should be
attainable by using the FPGA's DCM/PLL/etc. facilities but would most
likely turn out to be overkill.
Black&White is relatively simple, the only particularly touchy things
are horizontal and vertical blanking intervals. For color though, the
phase-amplitude coding using CrCb data is somewhat tricky. To generate
the chroma signal, the CrCb vector's angle and magnitude are needed,
both could be obtained from a lookup table, with the angle being
expressed as a sin table offset. To generate the chroma signal, all
that is needed then is to do Amplitude * sin[n + Offset], the rest
from here is mostly trivial.
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