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Re: AppleColor Composite Monitors discussion

glen herrmannsfeldt wrote:

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.


That's why it doesn't support chroma bandwidth in excess of 1.5MHz--
it isn't needed for viewing normal TV subject matter.


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.) 

Right--and some expensive older monitors had comb filters, and now
most digital TVs implement them (using DSP technology).


NTSC should allow close to 4MHz of luminance through.


Actually, for non-broadcast use (like a monitor), there is no need
to limit the luminance bandwidth, though it would take chrominance
separation circuitry more sophisticated than the typical analog monitor
has to allow the higher luminance bandwidt to survive.  ;-)

Typical "more sophisticated" separation would be a comb filter, which
is also required to get much more than 3MHz luminance bandwidth from
an NTSC signal.  No Apple monitors contain comb filters.


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.


And the NTSC standard was effectively modified (first in practice,
then in the standard) to make the U and V quadrature channels
have the same bandwidth--1.3MHz--so that distinction no longer
exists.

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

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