28 replies to this topic

[EricBall]

doppel, on Thu Jul 29, 2010 11:42 AM, said:

It was you as I recall that mentioned there being 227 PAL color clocks per scanline (which, as I stated in the previous post, corresponds precisely to 454 MARIA clocks). How the external PAL clock of 4.43 MHz affects things, again, I have no clue, since I don't have any PAL hardware or any way of measuring any of this stuff.

On an NTSC 7800 a single 14.3181818MHz (4x NTSC colorburst) crystal is used to generate both the MARIA (and thereby the pixel and CPU/TIA) clocks. The color output is this clock divided by 4 and hue/phase via a tapped delay line (same as the TIA in the 2600). However, the PAL 7800 uses a separate 4.433MHz PAL colorburst crystal to generate the color output. TVs are relatively tolerant of variations in signal timing, but not in colorburst frequency or phase (especially PAL). Curiously the schematics for both the NTSC and PAL 7800s have a 14.31818MHz crystal, which is different than the 14.1875MHz measured by nichtsnutz. It would be interesting if someone could open a PAL 7800 and see if they can read the frequency off the crystal.

[Rybags]

PAL have the 14.187 Mhz crystal, same speed as that in the XE computers.

I used the one from one of mine for my VBXE machine... then replaced the 7800s one with an NTSC one.

[GroovyBee]

EricBall, on Wed Sep 22, 2010 9:50 AM, said:

It would be interesting if someone could open a PAL 7800 and see if they can read the frequency off the crystal.

On my rev C PAL its 14.187576 MHz.

[Mitch]

14.187576 on the couple of PAL 7800s that I checked.

Mitch