What happens when frames are dropped?

[DavidHM]

I'm just wondering what happens when frames are dropped? I have a Matrox Marvel G200 PCI.

Does a marker get placed into the .AVI indicating when frames are dropped so that on playback it compensates accordingly, or does it just playback as if captured frames are the only frames causing audio synchronization to be lost?

The reason I ask is because I'm capturing a dnace routine, but in order to get the video quality I want, I lose about 6 frames over the 4 minutes. The audio is pretty bad so I'm removing the original audio and replacing it with the original song. When I do this, the A/V synchronization becomes off about midway. Do the dropped frames cause synchronization to be off? I'm capturing from a Hi8 tape. Could it be playing back at a slightly different speed than it was recorded at?

I'm now using MSP6. Should I capture my video in 29.97 fps or 30fps? What is 29.97fps drop-frame? When I look at the .AVI properties, it says 29.971fps even though I specify 29.97 during the capture. Could the .001 be causing the loss of synchronization?

Any help would be appreciated. Thanks.


David

[fluggo99]

Well, I'm curious about the dropped-frame thing myself. Either the previous frame is repeated, or it's left blank, I'll bet.

I can answer your question about 29.97 vs. 30 fps. Television video hangs around 29.97 fps, known as drop-frame to TV engineers because if you're counting at 30 frames per second, you'll have to skip counting a few numbers every minute or so. What is it, two frames for every minute except for every ten minutes? I forget what the formula is exactly, but you can probably find it by searching the 'net. Or look at your timeline near the one-minute mark and see for yourself.

30 fps is typically used for computer video, since it's a nice, round number, and counting the time is easy.

When you're done capturing, for some reason or another, your card will tell you that the video has been captured at 29.971 fps. It's always done this to me. There's two ways to get it perfectly at 29.97:

1) When you're done capturing, in the Save As, Options box, change the frame rate to 29.97. You'll have to do this for every capture.

2) In Video Editor, do File->Convert->Frame Rate and change it to 29.97 fps.

I have no idea if it even makes a difference at all, but I still do it. I also have no experience with sync problems; hopefully someone else will come along and help you with that.

[Haig]

Hi David,

when we drop a frame, we actually do not capture it. The audio track stays as is since during dropped frames, the only frame that is dropped is the video.

Haig

[Dayspring]

Hi David!

I'm no technical expert, but I have a thought as to what might be causing your audio/video sync problem. From what I've seen, it doesn't seem to me that lost frames should cause any video/audio sync problems. This seems to be supported by what Haig is saying. Even when a frame is dropped, the computer is still capturing what is currently coming into the computer, which is the current frames, and the current audio. I'm not certain completely how the technical side of the interlacing of the audio occurs, but when the card captures a piece of video, it should at the exact same moment be capturing the audio that is occuring, if you catch my meaning...?

Anyway, something that was brought up in a conversation today with a friend who has perfect-pitch (he hears musical sounds the same way as the rest of us see colors, if that makes any sense) is that when he listens to something on tape, for instance, it can really bother him. This is because when he hears notes that were perfect in the studio, they can be slowed down or sped up just slightly because of the movement of the tape, and it drives him crazy, because of course that makes notes sharp or flat from what they should be.

Anyway, I say all that to suggest that it might well be the same with video tape. I should imagine that if you made two captures of the same material and played them exactly side by side, I wouldn't be surprised if one got off sync with the other even slightly because of even the stretching of the tape, or whatever. Meaning that if you play it back next to a CD of the soundtrack, it would not surprise me one bit of the music that was included on the tape's soundtrack wasn't a bit off from the perfect playback of a CD.

Just a thought, albeit a rather long winded one.
Matt

[salacious]

I think this is a problem with PC-VCR. You don't get the problem with AVI_IO due to the way it handles dropped frames. To get round it with you current clips try rendering them out to new AVI files and then play back the new files. This will render the dropped frames as blank frames and the audio and picture stay in sync. What happens with MSP is when it generates the audio preview file it does it all. Then the plays back the video only from the AVI and when it gets to the dropped video frames it misses them out and therefore the sound and picture go out of sync since the picture is now ahead of the audio.
From now on use AVI_IO.

Salacious

[jeff b]

Another source of audio drift is capturing at a 'perfect' 29.97 frame rate for the file, while the camcorder is actually not that precise in its playback.

For instance, if your camcorder is running at 29.4 to 29.8fps, while you're capturing into a file tagged to play back at 29.97fps, then the audio (which will run according to the source file duration in MSP preview) will drift when the video is previewed at a 'perfect' 29.97fps.

If you capture a clip in MSP VidCap and DON'T check the box for 'exactly match specified frame rate', you can then find out what framerate your camcorder is really sending out, and what your system is really ending up with.

This situation can also result in the capture program reporting dropped frames, if you try to capture at an exact frame rate that the camcorder either can't keep up with, or which it exceeds.

[Meek]

Interesting question. I would not blame the frame rate stability of a camcorder... although that may be the problem as well. When you say "replace with original song" what do you mean exactly?

Do you ahve the song on a tape, on CD, as an MP3, or a .WAV file and how do you combine it with the video?

The problem may also be that some part of your equipment has it's 44 kHz slightly off... (sound card...)

M.

[Aryko]

Salacious has the right idea - if you're having problems with audio drift, AVI_IO is really the right way to go.
http://www.nct.ch/multimedia/avi_io/

Actually, I happened upon something on the avi_io home page that might be relevant, on an option called "insert drops as full frames":

"If a drop appears, Windows usually stores it as an instruction to repeat the last frame. However certain programs don't seem to support this correctly. This option was created to overcome this bug. It's use is strongly discouraged if not needed for this purpose because inserting drops as full frames may lead to even more drops if the HD performance was the reason for the drop."

So some programs might record dropped frames as an instruction to repeat the last frame, while others might just skip recording the frame altogether and go on to the next one.

From what I've heard about MSP6, and people can certainly feel free to correct me here, but if you're not using DV format it's not worth your while - might as well use MSP5.2 instead.

If you can kill all your background tasks, defrag your hard drive, etc., to get rid of the six frame loss, then of course this is a non-issue. I do wonder however if by setting your capture rate to 30fps and sending in a 29.97 fps signal, you create a situation where you will gradually drop frames. I don't know the answer to this one but in general I know for best results, make sure all your capture settings are identical to what you're sending to your Marvel... I guess Jeff B is addressing this point.

Good luck,
- Aryko

[hippie91]

fluggo: you were right about the drop frame... it's 2 per minute, except minutes that end in 0 (10, 20...)

[hippie91]

fluggo: you were right about the drop frame... it's 2 per minute, except minutes that end in 0 (10, 20...)

[hippie91]

fluggo: you were right about the drop frame... it's 2 per minute, except minutes that end in 0 (10, 20...)

[coolfish]

looks like your record is skipping there, eh bud? need a pat on the back?

[Walrus]

Actually, you (in theory) drop exactly 1.8 frames/minute when capturing in 29.97 as opposed to a 30fps.
30 - 29.97 = .03, .03/30 = .001, .001 / 1 = 1000, therefore you skip 1 out of every 1000 frames, and there are 30 * 60 = 1800 frames/minute @ 30fps, so you drop 1.8 frames in a minute
Sorry for the excessive math, and of course, this is under ideal circumstances.

[Aryko]

Of course, if anyone wants to venture some opinions on why NTSC operates at such a wacko number, I'd love to hear it... the best answer I've been able to find in my (limited) research was that "it was electrically convenient".

- Aryko

[hippie91]

yeah... it wasn't going to the next page so i clicked it again... sorry for the many posts.

walrus: you are right, but since it's hard to drop 80% of a frame every minute, they drop 2 and don't drop any in minutes that end in 0 so it averages to 1.