Re: [stella] Re: [Fpga2600] Where am I going wrong?

Subject: Re: [stella] Re: [Fpga2600] Where am I going wrong?
From: Chris Wilkson <ecwilkso@xxxxxxx>
Date: Fri, 3 Jun 2005 01:09:46 -0400
> Note: with Japan NTSC blank = black, which may be some of the confusion.

This is also the case in many North American systems...they run out of spec,
but "it's only a little'll be ok".  Standard broadcast doesn't do
this of course, but many game systems, pirate broadcasters, and VCRs do it.
Curse them all!  Oh...wait...that's what we're doing too.  Hmm.  :P

> sync  0 IRE = 0.000V
> blank 40 IRE = 0.286V
> black 47.5 IRE = 0.339V
> max white 140 IRE = 1.000V
> colorburst 40 IRE peak to peak (0.143 - 0.429V)
> max white with colorburst 120-160 IRE = 0.857-1.143V
> You may want to make COLU=0x?E 0.857V so you don't overdrive the video.
> Most TVs will do some AGC based on the sync pulse and colorburst, so if your
> voltages are off a little (but your ratios are correct) you should end up
> with a decent signal.

Just to clarify things a bit. Having to reduce the white level is a result of making
all the saturation levels the same.  In a "real" color system, the brighter colors
are also less saturated to avoid overdriving the video system.  (this is why they
look washed out.  They don't really have more white...they have less color!)  Because
the 2600 has simplified color generation where all colors have the same saturation
amplitude, and every color can be combined with any luma level, you have to make
sure that the positive peaks of the color signal when added to the highest luma
level (white) remains under 1V.  Also, in a real color system, people sometimes
cheat and transmit a little over 1V....but it can lead to problems, especially
with VCRs and cheap TVs (without the AGC that Eric mentions).  So you really
shouldn't do it.  You also must never transmit a signal that's less than 20IRE
(~143mV) as it can lead to sync problems.  So if you make your saturation ratio
more than 1:1, you'll have to increase the black level to compensate.  :(


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