Force 320x200 sync signal?

[olePigeon]

Quite a while ago I managed to find an Epiphan AV.io video capture device on clearance at B&H. It was ridiculously cheap, I think I paid under $100 for it. I intended to use it for pixel perfect capture of my vintage PC and Macs. It does allow for custom timings, but only if it matches the input signal.

When I first set it up, it seemed to work fine. It would detect the 320x200 signal coming from my DOS PC. However, at some point I changed something in the hardware either in or between my PC and the capture device, and now it's convinced it's seeing a 320x240 signal. So the captured image is wrong with the extra 40 pixels it interpolates vertically. If put in a custom 320x200 timing for the video capture, it just gets a black screen.

Does anyone know of an adapter or something that I can put between the VGA cable and the Epiphan that ... I don't know ... somehow cleans the signal so it's a clear 320x200? I know that 320x200 and 320x240 are very close. My old IBM LCD sees it perfectly, as expected. My modern LCD does not. And now, for some reason, neither does my video capture gizmo.

 

[Trixter]

They're actually not that close; 320x200 runs at 70Hz and 320x240 at 60 Hz, so different timings. You can hack together a 320x200 60 Hz mode but it changes both the horizontal and vertical timings, is a PITA to maintain (and some games will override it anyway), and will crash some software.

The Epiphan cards are great at framegrabbing, but terrible at autoswitching. It's possible you had it working in text mode or 320x200 (both 400-line modes) and then switched to a 480-line mode like 640x480, and then back again, and it didn't pick up the switch. Or vice versa.

I only used my epiphan card to capture footage when I knew the source material wouldn't change modes. I don't use it any more, and instead use a startech VGA to HDMI converter box and a traditional HDMI capture solution. The box switches sufficiently quickly (less than a second) between mode changes.

 

[olePigeon]

Oh crud, I had it wrong. I found my original post on Epiphan website (sounds like an opioid drug.)

The device is incorrectly detecting it as scan-doubled text mode @ 720x400 instead of CGA. What I meant to say is that 360x200 text mode and CGA 320x200 appear to be similar. At least, that's what I had read. So it's misinterpreting CGA as Text Mode.

When I set a custom timing for 640x400, it's like it's taking a 720x400 and then attempting to remove the extra lines and interpolating back down to 640x400, creating a weird artifact every few pixels.

I'm wondering if it's my DOS PC's video card. I could be remembering wrong, but I might have swapped out the video card at some point for something else. I'll try a different video card, assuming I can find one.

 

[Trixter]

VGA text mode is 720x400 @ 70 Hz. When you display 200-line modes on VGA, they are scan-doubled by the VGA card to 400 lines. So the actual 400 or 480 lines is the correct number output, and the numbers that the Epiphan card should be seeing, regardless of what mode the card is in. If you're hooking it up to a VGA card, this is normal. (If you're hooking it up to a CGA card, then you need to be more specific and provide more details on the signal path.)

You cannot have only one setting on the Epiphan card because games and other software will switch between 400-line and 480-line (and sometimes others) modes during normal operation: Start in DOS text mode, then go into 640x480x16 for a title screen perhaps, then 320x200x256 for normal gameplay, etc. Your card needs multiple definitions (which are the default, I think?) and the driver needs to be set to autodetect and switch.

I never found the Epiphan cards to be reliable with switching. I used them for a time with a CGA2RGB converter to go from CGA -> RGB -> Epiphan for a while, but I've moved on to better solutions for CGA capture.

 

[olePigeon]

@Trixter Thanks for the description. That is in line what I've seen in the Epiphan (I have the external USB box, not the PCIe card.) It shows 720x400 @ 70 Hz when booted into DOS. Maybe if I power the Epiphan off, then turn it back on after the game is fully loaded, it'll detect the correct resolution. I'll give that a try.

Unfortunately my DOS PC is a bit wonky. Could be a bad solder joint or capacitor (I've never recapped it.) It'll fail to boot if I restart it after it's been running for a while. Another problem to add to the pile.