Edit !
It is real 60hz !
I tried through the TV, that says its 640x480 60Hz :

and through a video/HDMI scaler that says exactly the same thing

Its great i think to have the chance to have this scaling engine inside ! I wish i have sound now 😉

Best regards
Philippe Dubois

Right, but that's not what it should be running at. Nearly all video modes used by DOS software were 70Hz, not 60. So you'll be missing one frame out of every seven.

jmarsh wrote on 2020-04-26, 11:26:

Right, but that's not what it should be running at. Nearly all video modes used by DOS software were 70Hz, not 60. So you'll be missing one frame out of every seven.

It would be nice if someone came up with a VGA to HDMI freesync capable adapter .

jmarsh wrote on 2020-04-26, 11:26:

Nearly all video modes used by DOS software were 70Hz, not 60.

They were?

I had a CRT for years that didn't seem capable of doing more than 60 Hz.

All modes with 400 lines or 200 scandoubled lines ran at 70 Hz, that includes 80x25 textmode. It flickers noticeably less on a real CRT than 640x480 or 320x240 modes.

It seems there's also a common misconception that 640x480 was always a 60Hz mode.

jmarsh wrote on 2020-04-27, 04:47:

It seems there's also a common misconception that 640x480 was always a 60Hz mode.

VGA mode 12h was 640x480 in 16 colors at 60Hz .

If you are talking about Super VGA, anything is possible ( within RAMDAC bandwidth limits).

I was wondering.
Producing sound with a SoundBlaster is just merely to send data/sounds to a specific adress port, 220H for example, with maybe some checkups somewhere.
Activating sound on a AC'97 chipset is just put a flag somewhere saying "work please".
Producing sound on a AC'97 should be easy, just as sending datas/sounds to another specific adress on the chipset (buffers), and voila.

So.
It should be easy to find/create a DOS program able to reproduce just a Soundblaster buffer at adress 220H and send these datas to the AC'97 chip buffer ? no ??

ps: Of course i'm not speaking about FM modulation.

There is the Virtual Sound Blaster software that emulates a Sound Blaster and uses a parallel port DAC or the PC Speaker as output devices and there is MPXPLAY, which can play audio files via AC'97.
Both are open source. So there has to be a way to get what you want by splicing them together, but don't expect it to be easy.
One potential problem that I can see is that IIRC the AC'97 standard does not require the DAC to support anything other than 48kHz, which means that you might have to do the resampling in software, as well.