What happens to the dropped frames?

[kb1985]

I'm wondering what happens with the dropped frames when i capture analog wideo wityh my ati radeon vivo card. Virtual Dub shows me the amount of dropped frames. What happens when the frame is dropped? Is the previous frame being duplicated and put in the place of the "Dropped frame"? Or maybe there is nothing in the place of the frame and the video and audio don't match and after several minutes of dropping frames the audio becomes out of sync?

greetings
k.

[hexxisoft]

the frame disappears never to be seen again. gradually, i think if there's many dropped frames audio and video go out of synch.

[erratic]

This is what the Virtual VCR programmer writes about dropped frames: "In all situations a blank frame is inserted into the AVI index thus when you go back over the video you just captured you will see dropped frames as one or more frames of the same image etc. This also stops the video from playing faster or slower you just get duplicated frames in your AVI file.

If you capture directly to MPEG-2 with ATI MMC dropped frames will also be replaced with the previous frame.

[lordsmurf]

Nothing. They are missing. No replacement. I can assure you ATI MMC does not insert black or next/previous frames. It just skips that data. Easy as that.

[erratic]

Then why did I find several duplicated frames in my ATI MMC MPEG-2 captures whenever there were dropped frames?

And when you search for dropped frames in AVI files with VDub, you will see the previous frame repeated wherever a dropped frame was reported.

[kb1985]

Nothing. They are missing. No replacement. I can assure you ATI MMC does not insert black or next/previous frames. It just skips that data. Easy as that.

But I capture with Virtual Dub without any compression. I've tried to look at the video frame by frame but still can't answer the question by myself. Does Virtual Dub duplicate the previous frame?

[erratic]

If there are dropped frames in a captured (unedited/unprocessed) AVI file VirtualDub can find them with Edit -> Next drop frame. VirtualDub will automatically jump to the location of the dropped frame. You can easily verify that the previous frame is repeated there.

[kb1985]

Thanx!

[lordsmurf]

Then why did I find several duplicated frames in my ATI MMC MPEG-2 captures whenever there were dropped frames?

And when you search for dropped frames in AVI files with VDub, you will see the previous frame repeated wherever a dropped frame was reported.

It's a glitch. But you still lose quite a bit of data.

[vcd4ever]

If there are dropped frames in a captured (unedited/unprocessed) AVI file VirtualDub can find them with Edit -> Next drop frame. VirtualDub will automatically jump to the location of the dropped frame. You can easily verify that the previous frame is repeated there.

Virtualdub will not find any dropped frames in a avi file is the capture program has replaced the dropped frame with the last captured frame before the dropped frame ocurred. A dropped frame is a empty frame because it doesn't contain any videodata that can be decoded and therefore virtualdub will show the previous frame until it finds the next frame that can be decoded.

vcd4ever.

[erratic]

@vcd4ever

Yes, I know the dropped frames in an AVI file are empty/blank, but the software that processes the AVI file (in this case VirtualDub) usually replaces them with the previous frame while recompressing or frameserving, to avoid losing sync. AVI_IO can insert drops as full frames while capturing, which can be useful when the software that opens the avi files afterwards simply skips the dropped frames.
http://www.nct.ch/multimedia/avi_io/avi_io_settings.htm
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.


@lordsmurf

I haven't done a lot of testing with ATI MMC 9.xx. When I first bought my AIW 8500 back in 2002 I noticed the duplicated frames problem. I don't remember what version of MMC was available then. I used to capture CNBC's stock ticker or CNN's news bar to test this. Those tickers are perfect for noticing dropped or duplicated frames, because they stutter slightly whenever a frame is dropped or duplicated.

I just captured a few seconds of CNBC with ATI MMC 9.03 and I ran a few other programs at the same time to cause dropped frames. Here's what I noticed when I watched the file frame by frame in VirtualDubMod:

- Obviously the stock ticker is interlaced, but in the MPEG-2 file I can find a few instances where there are two consecutive non-interlaced frames, as if only one field was captured and then resized to a full frame.

- I can find a few spots where a frame appears to be missing completely, because the ticker moves too fast there.

- I can find a frame with incorrect interlacing, as if it contains two fields from different frames. For example: the top field from frame 30 and the bottom field from frame 31 combined into one frame (with the other two fields missing).

There's definitely some data missing, so what does this do to the playback if you capture a big MPEG-2 file with a lot of frames missing? If you just throw away dropped frames without replacing them won't the video play too fast? And what about the audio then?

[lordsmurf]

ATI MMC is one of the few softwares that will drop audio too to keep sync. It is smart in this regard.

The other thing, again, is a glitch. It's hard to explain. It's a file thing, computer thing.

[hech54]

I'm wondering what happens with the dropped frames

[kb1985]

I'm wondering what happens with the dropped frames

lol

[hech54]

Just trying to inject some humor...

[CaZeek]

lol.. I was reading the post and didn't expect to see that at all.

[lordsmurf]

I'm wondering what happens with the dropped frames

Brilliant!

[vcd4ever]

@vcd4ever

Yes, I know the dropped frames in an AVI file are empty/blank, but the software that processes the AVI file (in this case VirtualDub) usually replaces them with the previous frame while recompressing or frameserving, to avoid losing sync. AVI_IO can insert drops as full frames while capturing, which can be useful when the software that opens the avi files afterwards simply skips the dropped frames.
http://www.nct.ch/multimedia/avi_io/avi_io_settings.htm
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.

I was always having audio sync problems when encoding a captured avi file to mpeg when the captured avi file had dropped frames and the only solution that worked for me was to capture with avi_io and using insert drops as full frames. I have been using avi_io now for some years and after I read your post I decided to test again if avi_io still was the only solution that works for me.

The strange thing is that it now works with virtualdub and also without virtualdub and I have no idea why it didn't work some years ago when I tried with virtualdub and also with other programs. I'm now using Win98se because Win95b(osr2.1) doesn't work after I upgraded my computer and I wonder if it works now because I'm using Win98se and not Win95b(osr2.1).

vcd4ever.

[mpack]

Sorry about the late addition to this thread - which I wish I'd seen at the time! The reason I'm commenting is that I've suddenly started getting dropped frames during Huffyuv captures with ATI MMC 8.9, resulting in loss of audio sync during playback. Dropped frames used to be insigificant on my PC, and I can't think what has changed, except that I've applied a few XP "critical updates", none of which I would have expected to affect performance on my separate video capture drive...

I decided to investigate exactly what the software was doing with the dropped frames - this thread would have saved me some time. Ah well, at least I can give a more detailed description of what happens.

FYI (in case you don't already know) - an AVI includes a frame index, and each entry in that index gives the file position and length in bytes of one frame in the clip. A dropped frame is still allocated a slot in the index, but the length of the frame data is given as zero bytes. What the reading software does when it encounters one of these is up to that software: ATI seems to assume that readers will skip that frame and go on to the next, but I'm not aware that any software does that (even Media Player doesn't - but maybe ATI File Player does... I've not checked).

Now to the effect on the audio. For some reason ATI seems to have decided that, if it drops a frame then it should drop a frames worth of audio as well. This means that if the AVI reading software skips dropped frames then the audio will remain in sync.

But, if the reading software substitutes for dropped frames with copies of the previous good frame (IMHO, this is the most common recovery strategy) then the audio will go out of sync: it will trail the video.

If ATI were to capture all the audio then this would of course give you audio sync problems in AVI readers which skip dropped frames (in this case the video would trail the audio).

So, there are two ways that dropped frames can cause audio sync problems, and both depend on the exact behaviour of the reading software. In an ideal world the reading software would detect that the AVI includes dropped frames and treat the audio track appropriately: you can tell how the capture software treated the audio by checking whether the audio track is longer or shorter than the total duration of the non-dropped frames in the video clip.