LT1256 crossfader for VGA

Hi everyone,

I’m new to video synthesis and have recently been investigating how to fade analog video signals into one another, specifically VGA or composite. I understand why non-genlocked signals give glitchy results (like the dirty mixer lesser and the Schele mixer), but am having trouble understanding how circuits using the LT1256, like in the Cadet 6, seem to avoid this issue.

Are the LZX video inputs going in to the crossfader already synced elsewhere? If so, could the sync signals from one VGA source be stripped / generated and applied to another source?


Yes. All video is synced in an LZX system. Or more accurately, all video should be synced. You could input unsynced video, but then you’ll get results kind of like the Schele mixer. Video VCOs and other video modules that need sync all sync up to the system’s Sync Generator (think of it like a master clock that everything follows, except at video rates). And any external sources (video inputs) either need to sync to the signal from the Sync Generator (some pro broadcast equipment has sync inputs), or they need to be time base corrected. A Time Base Corrector (TBC) takes at least one video input and a sync input and outputs the video synchronized to the sync signal. (LZX has TBC module (called TBC2) coming out hopefully real soon that many people have anxiously been waiting for, so they can input multiple unsynced video signals into their systems.)

The LT1256 won’t help you with syncing the two different VGA signals. In fact, if you look on the LT1256 datasheet, you’ll notice “sync” isn’t mentioned at all. All the LT1256 knows and cares about is voltages. Give it voltage input A (IN1) and voltage input B (IN2) and voltage input C (Vc), then it’ll take the mix (or is it the average?) of A and B, weighting it according to C. The fact that it’s a video IC is just because it can work well at the high speeds of video.

I know this probably isn’t what you were hoping to hear, but I hope this helps you understand it better at least.

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I’m personally not sure what kind of options there are for crossfading VGA out there. I think some old standalone video mixers could take VGA input and could probably do that, but I don’t know which ones. There are definitely some people around here who know a lot more about standalone video mixers who can hopefully suggest some models to look for.

I don’t know what other options there are, or if there are any VGA sources that can be synced. I imagine some dual-VGA graphics cards might have that ability, but they might be hard to find, since sync’ing video usually only matters for broadcast purposes. So maybe there are some old dual-VGA cards made for broadcasting?

Probably easier to find/use than that would be a standalone TBC that takes VGA. I imagine those exist, but again, like video mixers, I don’t really know what to recommend but other around here might. Then if you have your VGA signals TBC’d you could crossfade them pretty easily, though you’d need three LT1256’s if you went that route… one for R, one for G, and one for B.