Subject: Re: Recording to VHS
From: Ernie Wright <[email-masked]>
To: Multiple recipients of list <[email-masked]>
Date: Sat, 27 Apr 1996 00:40:01 -0700

Adrian Onsen wrote:

> Why is that? Why can't NTSC handle 24 bit color? Why are values over
> 200 RGB, considered illegal?

They're not.  A white of (255,255,255) is fine, for example.  But some
parts of the RGB cube are illegal.  Given the YIQ components of an NTSC
signal, the amplitude in IRE is

   a = 7.5 + 92.5 * ( y + sqrt( i * i + q * q ))

and the signal is illegal (overmodulated) when a > 110 or thereabouts.
This happens for fully saturated yellow and cyan.

> And since computers came after the development of TV, why did computers
> adopt a ystem (RGB) of encoding data which is not compatible with the
> current video signal?

They didn't.  What's inside both the camera and the TV is RGB.  NTSC
is just the lossy compression method used to transmit the signal.

> Why not make e computer have it's value for 255 rgb, be the actual
> limit of NTSC signals?

It is.  Most encoders set maximum white (255,255,255) at 100 IRE, the
maximum NTSC luminance level.

- Ernie

P.S.:  To convert from RGB to YIQ,

   #define GAMMA(x) (((x)<5) ? 4.5*(x)  \
                             : 1.099 * pow((x)/255.0, 1/2.2) - 0.099)
   r = GAMMA( red );
   g = GAMMA( green );
   b = GAMMA( blue );
   y = 0.2989 * r + 0.5866 * g + 0.1144 * b;
   i = 0.5959 * r - 0.2741 * g - 0.3218 * b;
   q = 0.2113 * r - 0.5227 * g + 0.3113 * b;