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Re: Supercard

bieling@terra.es wrote:

Michael J. Mahon wrote:



I should have noted that for fidelity to a real composite display,
the luminance info must also be low-passed, but to, say, 2.5MHz - 3MHz.



DHGR/80 colums text is shifted out @14Mhz...
....why would you want to cutoff @3Mhz ?


Because the objective of this mode is to faithfully simulate
what a typical analog NTSC monitor would do--and that's what
they do.


You mean to cutoff only for color modes not monochrome ?


That would be an option, though AFAIK only the Applecolor Composite
monitor actually switched the luminance channel to higher bandwidth
when it was in monochrome mode.


Is luminance low-passed always in real life color monitors ?


In all the monitors of the day, yes, with the sole exception noted
above.

Modern digital NTSC monitors use comb filters to filter out the
chroma subcarrier, and so don't need to band limit so badly--but
they often still limit at around 4MHz since they expect the sound
signal to be in the 4.25-4.75MHz range.

If an S-Video input is used with a digital TV, then some monitors
will use the full bandwidth of the luminance signal, since no chroma
separation or filtering is required.

However, digital monitors are a different breed than the monitors that
the Apple II was designed for, and, more importantly, than all the
color graphics for the Apple II were designed for.


I think I read somewhere that chroma is extracted and substracted from
video in order to make it "invisible". Makes sense as chrominance is
added on top of luminance at the video sorce. Driving RGB @30,59,11%
straight from luma (=video-chroma) should give a full bandwidth sharp
B/W image almost free of chroma "noise" ?


That is the effect of a comb filter, though the subtraction is implicit,
and is done on the chroma subcarrier, not the demodulated chroma
signals.

-michael

Parallel computing for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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