Dropped frames observation

Lili wrote on 10/31/2005, 8:30 AM
I watched several captures very closely to try and determine if there was a pattern or particular place in the timeline where dropped frames most happen.
We know that dropped frames are more likely to occur at the beginning and/or end of a tape - especially if you don't let the tape roll for about a minute at each end before start/end of film - but I noticed something besides .

Without exception, any frames that were dropped in my last half-dozen captures all happened BETWEEN certain clips. That is, wherever I had turned off , or sometimes even just paused the camera and then turned it on again, between takes.
For example, in a capture I did this morning, there were 84 dropped frames, but they all dropped in one spot, between an outdoor and an indoor shot - (indicating that my camera had been turned off and on).
This may not be a great revelation to the experienced editors in the forum, and it certainly doesn't apply to every case of dropped frames, but I hope it might save a few people some time in needlessly looking into all the other reasons for dropped frames first. I for one am relieved to know that it's the cause ,in my case, even though I'm not sure as to the "why" it happens.

Has anyone who is experiencing dropped frames made the same observation?

Comments

Former user wrote on 10/31/2005, 11:11 AM
Which camera are you using?

Does it make a difference if you just stop and start as opposed to stopping, playing back and then starting again while shooting?

Dave T2
Lili wrote on 10/31/2005, 11:45 AM
1. Sony PD170.
2. Good point . Generally, I don't do a lot of rewinding during videotaping, other than to check once or twice to make sure everything is recording as it should. Next time I rewind and restart I'll make a mental note to see if that's the point where frames are dropping.
thanks.

3. Just realized it also happens when I use my Sony Handycam.
farss wrote on 10/31/2005, 2:54 PM
The cause of the problem is that when you power down the camera the tape is unthreaded from the heads. Next time when you power the camera up the tape can end up positioned a few frames past the last recording resulting in a portion of blank tape between the two clips.
Now silly VidCap doesn't handle that very well and reports dropped frames.
Bu there's a much bigger problem. If you start the capture on that blank section of tape using any Sony VCR or camera capable of 32K audio then the device sends 32K audio initially and VidCap locks the audio stream to 32K. Result is the entire audio stream is captured at 32K.
I've had a trouble ticket with Sony over this for months, still they don't even seem to comprehend that this is an issue. I wonder just how many captures are being compromised by this problem, after all who thinks to check the sample rate of there captured video!

Worse still, I recall many posts with users asking 'why does my audio sound so bad during capture'. Replies typically were "don't worry, VidCap's audio isn't too good, it'll be fine when you put it on the T/L".
WRONG ANSWER!
The audio you're hearing during capture sounds crappy because VidCap is resampling the incoming audio from 48K to 32K but your sound card thinks it's getting 48K audio, what you're hearing are sample rate errors. Once dropped onto the T/L of a 16/48K project the audio sounds OK because Vegas then correctly resamples the 32K audio to 48K to feed the sound card.
Unless you bothered to check the clips properties you'd never know what was going on of course. But it gets worse, not only have you lost a bit off the top end of your audio but during playback CPU power is being wasted reampling the audio, this no doubt impacts the achievable frame rate and adds a little to render times.
Bob.
johnmeyer wrote on 10/31/2005, 8:42 PM
As I said in the dropped frame FAQ I wrote for VASST, the only number of acceptable dropped frames is zero. Anything other than that indicates a problem. When capturing from DV tape you should never have a single dropped frame, much less dozens. Something bad is happening and needs to be fixed.

Capturing from analog tape, or through some sort of analog connection is a different animal, and dropped frames at edit points are not uncommon.
Steve Mann wrote on 10/31/2005, 9:23 PM
"The cause of the problem is that when you power down the camera the tape is unthreaded from the heads. Next time when you power the camera up the tape can end up positioned a few frames past the last recording resulting in a portion of blank tape between the two clips."

This is atypical of every DV camera that I've used: VX2000, PD-150 and PD-170. I stop and start my camera frequently, and the timecode is unbroken. If I remove and reinsert a tape, the camera backs up a bit to find the last frame and the timecode is unbroken.

I don't know about vidcap - I use Scenalyzer and it doesn't care a bit about breaks in the timecode. So if your scenario were happening, I wouldn't notice.

Come to think of it, I've never heard of anyone getting dropped frames from timecode breaks.

As John said - anything greater than zero (dropped frames) is indicative of a problem in your capture path.

farss wrote on 10/31/2005, 9:40 PM
Actually I think I wrongly described the scenario. The problem occurs if you leave the tape in the camera and change batteries. We mostly see it only on DVCAM cameras, with MiniDV cameras users tend to swap batteries at a tape change but with guys shooting sports footage all day with 3 hour tapes it's a pretty common occurance.
Bob.
Lili wrote on 11/1/2005, 8:30 AM
Pretty interesting info:

Bob - I will be sure to shoot in DVCAM format whenever audio is a critical part of the video (mostly throw audio out and sub with music track) because the sampling frequencies of audio and video are synchronized and sampled at 48kHz. Not so with DV format, where the two sampling frequencies are independent (pretty well quoting from the manual here). It's good to have another reason to shoot in DVCAM format because the 2/3 less recording time of mini DV cassettes makes it tempting not to.
We have had threads here on the pros and cons of shooting in DVCAM format and this was one I was not even aware of until quite recently.

John: I agree that anything more than zero is too much, but is there any solution to dropped frames occuring between clips after shutting off and restarting camera??

If not, then it seems the best I can do is continue to shoot in DVCAM format so that at least the audio is not messed up, and be glad that the dropped frames are not affecting the video portion.

johnmeyer wrote on 11/1/2005, 9:28 AM
ohn: I agree that anything more than zero is too much, but is there any solution to dropped frames occuring between clips after shutting off and restarting camera??

Well, as both "n19093" and I mentioned, we use Scenalyzer. It doesn't care about interruptions in timecode, and the author actually enhances his code when people report problems. By contrast, I don't think Sony has touched the capture code in years, and there are lots of known problems. When they decided to add HDV capture, they didn't even know how to get inside the existing code to add this capability, so they created a completely separate application. Doesn't bode well for getting fixes to the SD capture.

So, my answer is this: Download Scenalyzer. It is free to try, although the demo version will put a watermark on your video. I'll bet dollars to doughnuts that it will solve your problem. Here's the link:

Scenalyzer
Lili wrote on 11/2/2005, 7:37 AM
Thanks a lot for the Scenalyzer recommendation John.

I admit, I wasn't sure of everything it does, but after doing a search in the forum, I can see that for some people, it has lots of advantages over capturing in Vegas. However, for the type of work I do, I'm quite content with Vegas 5 capture capabilities for the time being.
Perhaps I'm missing something here, but i don't see what difference it makes (again, I'm only referrring to my own work) if frames are dropping only BETWEEN takes - and even then only when I shut off camera and start up again (probably, gathering from what's been discussed). It is not having a negative impact on my editing at all, which is why I posted the "observation", so that others who are experiencing dropped frames won't panic and jump to the conclusion that it's necessarily something more serious than it is.

I WOULD be concerned if frames were dropping in mid-scene, but that is not the case this time. When I first got into editing- I DID have dropped frames for other reasons, and they were affecting my videos, so finding the fix was all important. If I should indeed be concerned about dropped frames between scenes, I would like to know why. As I said, perhaps there's something I'm not understanding here. thanks again.