9 Dec
1999
9 Dec
'99
9:40 a.m.
PDP-8 and other rare mini computers
http://www.pdp8.com
-----Original Message-----
From: Eric Smith <eric(a)brouhaha.com>
To: Discussion re-collecting of classic computers
<classiccmp(a)u.washington.edu>
Date: Thursday, December 09, 1999 7:35 PM
Subject: NTSC sync rate (was Re: Hewlett Packard A2094 Monitor (Standard
RGB?))
"John B" <dylanb(a)sympatico.ca> wrote:
Tony wrote:
I know.. I didn't want to get *picky*.. the vertical rate is 59.94...
US TV is 15.570kHz, VGA is 31.25kHz or
thereabouts.
John B wrote:
No, the US TV (NTSC) Scan Rate is 15.750Khz
Well, if you want to get nitpicky, the NTSC horizontal sync
rate is 15.73425 KHz.
The old EIA RS-170 monochrome standard was 15.750 KHz, but that
is not suitable for NTSC color use. The color burst frequency
is 3.579545 MHz +/- 10 Hz, and the the horizontal rate is required
to be exactly 2/455 of that frequency. Otherwise the color information
will produce luma artifacts and/or interfere with the sound carrier.
Note that they could have chosen to make the color carrier 3.583125 MHz,
and kept all the video timing exactly the same as for RS-170. Then they
would have had to raise the center frequency of the audio carrier by
0.1%. The FCC apparently would not let them make that change, in the
interest of compatability with existing monochrome receivers (which is
incredibly stupid, since the receivers wouldn't have even noticed), so
they had to lower everything else by 0.1% instead. Thus the need for
disgusting hacks like drop-frame time code. Sigh.
Eric
Reference: _A Technical Introduction to Digital Video_ by Charles A.
Poynton, John Wiley & Sons, 1996