Converting MSX from PAL to NTSC

Paragon (1570)

24-06-2008, 20:19

Hi guys,

I am thinking of converting a PAL MSX to output color information in NTSC. I located the xtal that is 5 x of PAL color carrier frequency (22.16809 ~= 5*4.43361875). To get VDP color output at NTSC frequency, I am supposed to replace it with 5 x NTSC xtal (17.89773 ~= 5*3.579545).

The problem is, no one sells such xtals. The closest things online retailers have is 18 MHz.

18/5 = 3.6 MHz, which is 2% faster then needed.

Do you think TV will still manage to decode color into with 2% error in color carrier frequency?

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Por Edwin

Paragon (1182)

24-06-2008, 22:45

From my research into PAL/NTSC signals, I must conclude that a 2% error is *a lot*. IIRC, the specs say something like the clock needing to be within 25 Hz of the target. In practice, devices are a bit more forgiving, but I don't expect a 2% error will get you anywhere.

On the other hand, the V9938 gives incorrect frame timings for both PAL and NTSC by default and it still seems to work. But that's still only a line length error of approximately 0.2%.

Por erikmaas

Expert (70)

24-06-2008, 23:58

Although you might be able to change the color frequency, this does not change the signal from PAL to NTSC. For PAL each line the color is encoded with a different phase (that is why it is called Phase Alternated Line) which is not like that for NTSC.
Suppose that the NTSC TV would be able to get 'some' color, your blue MSX screen will have a different color each line, and I wonder if any of those lines will be blue.

If you would have for example a Philips NMS8250 PAL machine, you can see in the service manual that there is a MC1377P near the VDP, which is a PAL/NTSC color encoder. That one can probably be converted by replacing the crystal and pulling one pin of the MC1377P to ground.

Since you are talking about 22.16809 MHz it is another machine.

Which model is it you try to convert?

Por dhau

Paragon (1570)

25-06-2008, 20:08

Thanks, Erik. It's a Sanyo with Toshiba T7937A chip. Alexandro from Brazil provided pinout for it, so I'll probably end up building RGB amp per Hans Otten recomendation and use it instead of converting to NTSC. 5x is a weird divider for clock. I mostly see 4x on various pcbs.

Por hap

Paragon (2042)

25-06-2008, 22:22

dhau: If you're familiar with IRC and would like to do a few VDP tests for us, could you join #openmsx on irc.freenode.net?

Por dhau

Paragon (1570)

26-06-2008, 01:27

Ok, sounds good. I was also supposed to do some tests for Manuel on AX350 that I was lucky to buy locally in very good condition...

Por hap

Paragon (2042)

29-06-2008, 19:17

You're welcome to stop by any time If we're away, just hang around for a bit. My nickname's "enen" over there, and manuel is "Quibus".

Por doba

Resident (39)

30-06-2008, 00:30

Well, I visited, and I still don't understand what program can dump system roms from my computers.

Por hap

Paragon (2042)

30-06-2008, 01:07

doba==dhau? It seems manuel was a bit busy with other things too on IRC so he couldn't devote all his time to you, that's perfectly normal but may seem awkward to someone new to IRC. Try another time

Por Manuel

Ascended (19462)

30-06-2008, 13:12

I sent dhau a disk image with dump tools. I wasn't at my keyboard when he asked how it worked and left 15 minutes later...

Check back and we'll continue!

In the mean time: try BLOAD"GETROM.BIN",R

Por dhau

Paragon (1570)

01-07-2008, 15:55

Oh thanks! I forgot about your file! Shame on me, going senile at moderate 2^5 years...