Frame sync vs. frame TBC analogy?

[ThumperStrauss]

1. Putting aside the technical explanation, it is accurate that as a practical matter a frame TBC (eg, Datavideo TBC-1000) will ensure that the resulting AVI capture video file has audio and video in sync? Is that the main reason people want one?

2. As for the technical explanation, the way I have come to visualize it is that each individual frame of video is like a person in a line. Let's focus in on one of these people. We'll call him Charlie. As the line moves forward, there is a greater or lesser gap between Charlie and the person in front and behind. Charlie does a kind of Charlie Chaplin-esque movement as he moves forward, occasionally bumping into the guy in front, occasionally being too far from the guy in front. But as Charlie approaches the front of the line, he notices an archway, like a metal detector at the airport. When he finally reaches the front of the line, he notices an agent taking his photo with a digital camera. How nice. Charlie then notices that the other agent has photos of the last few people in line and then like clockwork, the other agent taps the SEND button—always at a constant rate—and sends each photo, one at a time, to his boss, Mr. Capture Card. Before Charlie could think about what all this means, the first agent said Goodbye, Charlie, and he disappears into a puff of analog chemicals.

Okay, that's my dramatization. I understand that analog tapes on mechanical devices (VCRs) are imprecise, and so the frame TBC gets these frames in shape, like a drill sergeant getting new recruits to march in step at an even rate, before delivering each soldier (frame) to the capture card.

But here is the part I still don't understand. In either of my analogies, what is the frame sync and what is the time base corrector?

[lordsmurf]

Sure, that can work.

But let's back it up a bit...

TBC = time base corrector. It literally corrects the timing, the time base.

You have X, Y, and Z axis. The flat image you see (like a photo) is X * Y, and the Z axis is temporal (the movement through time, the frames per second).

Line TBCs operate in the X * Y axis. Each line of analog video gets corrected, either one by one, or. (Imagine a bookshelf of 240 books, which is sort of like VHS. Then squeeze those together, and add almost twice as many books, to get S-VHS resolution. The vertical axis, the bookshelf, is fixed to the standard, either 525/480 NTSC or 625/576 PAL, where the second number is the visible signal.) These give lots of visual improvements, but does essentially zilch for temporals.

Frame TBCs operate in the Z axis, and it perfectly re-times to a clock. One in, one out, repeat. This is mostly non-visual, and yet as necessary to video as oxygen is to us. It has limited affect on the X or Y axis.

Frame syncs are essentially primitive frame pre-TBCs, and not really correcting timing (and mostly just monitoring it, like a stop watch). Consider round holes, with pegs getting thrown at them. The round pegs fit, but the square pegs should get instant correction. But since not a TBC (where C = correction), there is no correction. So the non-TBC frame sync takes a sledge hammer, and just whacks it as hard is possible to go through the hole. It's not pretty, not corrected, but it gets pushed through. This is obviously not smoothly timed, but it tries to fumble through. It may work, it may not. It may keep audio in sync, it may not.

VCRs have lots of slop in the heads and transports. The signal is chaos on mylar. Both quality VCRs and line+frame TBCs tame it for the digital world.

[latreche34]

Frame TBC digitizes the frames and release them in a timed fashion, Frame sync works after TBC, after the frames being digitized and timed, it can be synced to an internal clock, the TBC clock, or to an external video source. Frame sync is important when there is audio to lock to. A frame TBC like the TBC-1000 does not deal with audio, so it just times the video frames.

[ThumperStrauss]

Quote:

Originally Posted by latreche34

Frame TBC digitizes the frames and release them in a timed fashion, Frame sync works after TBC, after the frames being digitized and timed, it can be synced to an internal clock, the TBC clock, or to an external video source. Frame sync is important when there is audio to lock to. A frame TBC like the TBC-1000 does not deal with audio, so it just times the video frames.

But as a practical matter, is the primary benefit of a frame TBC that it keep the audio and video in sync when you capture?

[latreche34]

Again, not all frame TBCs deal with audio, The idea is that if you have stable stream of frames, audio will eventually evens out, but there are other factors that can contribute to audio out of sync even if you have a frame TBC in the mix, Factors such as slow computer that make the capture card drops frames, slow hard drive, programs running in the background eating up resources, pretty much anything causes frame drop at the computer level will cause audio sync issue. It is not the case if you have a TBC/frame sync with locked audio with digital output.