Old NTSC tricks: 240p?
hilpert at cs.ubc.ca
Tue Mar 17 14:06:18 CDT 2015
On 2015-Mar-16, at 7:20 AM, John Foust wrote:
> I'm trying to understand at a low level how some early computers
> and game consoles generated a non-standard form of NTSC.
> The Wikipedia http://en.wikipedia.org/wiki/Low-definition_television says:
> "Older video game consoles and home computers generated a nonstandard NTSC
> or PAL signal which sent a single field type which prevented fields from
> interlacing. This is equivalent to 240p and 288p respectively, and was
> used due to requiring less resources and producing a progressive
> and stable signal."
> Another source says this was true for the "NTSC Atari 2600, Apple II
> family, Commodore 64, Nintendo Entertainment System, Sega Master
> System, and the vast majority of games for NTSC Genesis, Super NES,
> PlayStation, and Nintendo 64."
> This page http://www.hdretrovision.com/240p/ calls it a "special timing
> signal" and gives examples of how contemporary flat-panel TVs can
> misinterpret the old signal. The issue has spawned the creation of
> dozen of devices to give the retro look on new TVs.
If you're just referring to the interlaced vs. 'progressive' scan issue, it comes down to the relationship between the vertical and horizontal scan rates.
The standard RS-170/NTSC sync-pulse frequencies of V=60Hz and H=15,750Hz [*], give an H/V ratio of 262.5, which of course is the number of lines which will be scanned during the vertical scan period. The half fraction results in the vertical sync ' interrupting' the last line of half the scan fields half way through the line and the first line of the other half of the fields starting halfway through the line so the fields are vertically offset slightly and thus interlaced.
(Multiplying 262.5 by 2 gives the proper 525 NTSC lines per frame (and of course notice it's an odd number)).
When ones looks at the V and H sync pulses in relation to each other, they are alternating/oscillating in relative phase to each other.
In the properly-implemented standard, that alternating phase relationship necessitated equalizing pulses in the signal at twice the H rate around the V sync/retrace period to keep the old analog-implemented sync-separators happy.
If the H/V ratio is adjusted slightly off the standard to an integral relation, for example by setting the H scan rate to 15,720Hz to give 262 (lines), the H & V sync pulses are always in the same phase relationship, and there's no interlacing.
The altered scan rates were still within the sync-lock range of the analog TVs of the day, perhaps needing a slight tweaking of the horizontal-hold control.
I was trying to find what the -exact- frequencies are for some of those early computer/game systems but nobody seems to want to readily present them on the interwebs.
[*] In NTSC, the rates were adjusted slightly from the original B&W spec with the introduction of colour, to, 59.94 & 15,734; IIRC, to deal with the colour information in the signal displaying artifacts when displayed on existing B&W TVs.
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