openmsx video recording - not precisely 50 and 60 Hz when replayed?

Por Bengalack

Paladin (721)

Imagen del Bengalack

25-04-2022, 18:37

I'm using the record feature a lot these days. This, to make sure that some parts of Lilly's Saga is experienced identically in 50Hz and 60Hz. To verify, I've been using a video editor to check the different frequency-recordings against each other.

They are seemingly(?) hard to get in sync. Looking at the files, I find that the 50Hz recorded ZMBV-files are marked as: 50.16 and 60Hz as 59.92.

Is it so that, even if openmsx runs at 50Hz, and dumps every frame to file, it is played back at 50.16 Hz? And likewise for 60Hz?

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Por wouter_

Champion (508)

Imagen del wouter_

25-04-2022, 19:48

That's correct. This is because a real MSX does not run at 50Hz or 60Hz. But at 50.16Hz or 59.92Hz.

More specifically: the V99x8 is clocked at 6 x 3.579545MHz = 21.47727MHz.
One display line takes exactly 1368 clock ticks (both PAL and NTSC mode).
PAL has 313 display lines, so that results in 21.47MHz / 1368 / 313 = 50.159Hz.
NTSC has 262 display lines, so that results in 21.47Mhz / 1368 / 262 = 50.923Hz.

Por Bengalack

Paladin (721)

Imagen del Bengalack

25-04-2022, 19:59

Thanks a lot for the quick confirmation and explanation! This complicates things a bit, but I can work around it in my current case.