Re: AppleColor Composite Monitors discussion

[glen herrmannsfeldt <gah@ugcs.caltech.edu>]

Michael J. Mahon wrote:

(snip)

> With a 3.58MHz chroma subcarrier and amplitude modulation, it is
> only a theoretical possibility to represent 3.5MHz information.
> The practical limit is much closer to 2.5MHz or less.  In the NTSC
> standard the limit is under 1.5MHz.

The idea behind NTSC is that the eye sees higher resolution
in luminance than chrominance.  The other is that the chosen subcarrier
is an odd half multiple of the line frequency such that in many cases
the spectra don't overlap.  (The other way to consider it is that the
color subcarrier phase shifts 180 degrees between successive lines,
which tends to cancel out the visual effect.  It also allows for a 
filter to separate them out using a delay mechanism.)

NTSC should allow close to 4MHz of luminance through.

NTSC also was designed to support most chrominance resolution for
colors where the eye is more resolution sensitive, but to allow
receivers to ignore the extra resolution for easier decoding.
Only recently were receivers designed to decode that extra resolution
in these last days of NTSC.

-- glen