EEVblog Electronics Community Forum

Products => Computers => Vintage Computing => Topic started by: Wawavoun on December 21, 2021, 08:45:52 am

Title: VGA questions
Post by: Wawavoun on December 21, 2021, 08:45:52 am
Hello there,

I have a computer with analog interleaved video output (monochrome, two vertical frames to get full screen).

Also are available separately Vsync, Hsync and video signals. Hsync is ~15625 Hz and Vsync is ~50 Hz.

My purpose his to use a vga screen instead of a tv.
I have to invert sync signals and put video into 0 - 0,7 V range, that s easy.

My question is about the interleaved signal and the frequencies. Will this work on a vga screen ?

Thanks for help.
Title: Re: VGA questions
Post by: GromBeestje on December 21, 2021, 09:11:36 am
I recall from the old days having an option for 1024x768 interlaced on VGA. So interlaced signals on VGA are possible.
However, if this particular signal works, I'd suggest to look at the manual of the monitor what signals it accepts.
Title: Re: VGA questions
Post by: Ian.M on December 21, 2021, 11:08:50 am
You are S.O.L unless your 'VGA' screen is actually a multisync monitor supporting legacy CGA and EGA standards or RGB or component PAL/SECAM.   You'll find VGA timings here: (
Please note the lowest line frequency is 31.778 KHz.

Therefore you'll need a scan rate converter to have any chance of success.
Title: Re: VGA questions
Post by: m k on December 21, 2021, 07:05:09 pm
There were two kinds of multisyncs, VGA multisync and real multisync.
VGA multisync goes up but not down from standard resolution.

If memory serves.
NEC Multisync II is the real one.
NEC Multisync 2A is not.

Possibly all NECs with rounded corners were real and squared corners were not.
I'm quite sure I had a Multisync XL with BNC connectors and other end was DB9.
Title: Re: VGA questions
Post by: woodchips on April 13, 2022, 05:46:04 pm
I think the Multisync will work but the picture shifts across the screen, the 2 is much better.

Also the Sony 1402 will work, but been a long time since I tried it, still my DOS VGA monitor.
Title: Re: VGA questions
Post by: james_s on April 13, 2022, 07:19:59 pm
These scan frequencies sound like NTSC, the horizontal is about half that of VGA. Most VGA monitors will NOT support 15kHz although there are a few old ones as well as some newer LCD monitors that will.
Title: Re: VGA questions
Post by: Ian.M on April 13, 2022, 08:45:23 pm
Yes, as I mentioned above, the only hope is a vintage true mutisync monitor, (or a modern one that supports PAL component)  and not a SVGA multisync as 'm k' pointed out.   Its worth noting that the original IBM CGA card had a NTSC compatible composite video output, which would have been approx. 15.734 KHz line rate and 60 Hz frame rate. 

Getting a CGA compatible multisync that doesn't explicitly support PAL to lock to a PAL timing signal is a matter of luck - the Hsync is likely to be close enough to NTSC/CGA timings for the line oscillator to lock, but the 50Hz frame sync might well fall outside the lock range of both 60Hz CGA modes and the lower 43.5 Hz of XGA interlaced.