Not getting exactly 25fps when capturing video in pal

[cesm23]

This is definetly a quite bizarre problem i have... only now i noticed that every time i capture tv from analog channels, the average frames captured that displays in the program itself (i have tried a few capture programs that capture in avi) in some channels is betwen 24.995 and 25 (it's never 25) and in others is between 25 and 25.005, very strange indeed, i also notice that in those below 25 fps i get a slighter bigger audio (the more time i capture, the more it loses sync, it's always at the same rate) and in those that capture above 25 fps the audio gets smaller.

Now it's not that i get dropped frames at all, when that happens there is a bigger gap in the audio at the place that happened, so it's the tv capture card that seems to not be able to get exactly 25 frames each second and therefore the longer the videos are captured the more the way loses sync.

I discovered this because i also have a dvd recorder that records from tv as well and in there there isn't that problem anymore, this only happens when using tv capture cards.

I have a few friends in the same country i live, they all have different tv capture cards but they all use the same chipset and they have the same problem that i have, altough they get this problem in different channels. I am starting to wonder if this is some kind of problem in the clock on the capture card or the way that the card recognizes frames, i also noticed that in the composite input the problem is even worse...

The only solution i find to fix this is maybe trying another tv capture card ? I guess that when the tvcard has this problem there is nothing to do right ?

[GrofLuigi]

What hardware?

I was getting this problem a long time ago (BTXXX tuner cards) before I started using programs that resample audio during capture (VirtualDub, VirtualVCR).

It depends on the path of Audio/Video (whether the device has integrated audio or you are capturing it through sound card, most of which drift a little).

GL

[cesm23]

"What hardware?"

Asus My Cinema P7131TV Hybrid
Msi @nywhere plus
Avermedia hybrid super 007

It seems all these three cards use tha same chipset and drivers if you google to it, maybe this has something to do with that... I think it's the chipset Saa1731 but i am not sure, what i know is that all three cards have the same device name on their drivers! (in fact when i got a msi anywhere plus card and the avermedia card in the same pc, the pc mistaken the avermedia card for another msi anywhere plus card !! So you see how similar they are...)

"I was getting this problem a long time ago (BTXXX tuner cards) before I started using programs that resample audio during capture (VirtualDub, VirtualVCR)."

Hum... i see... well i am using iuvcs and the program does dupllicate frames in channels in which the audio gets bigger and skips frames to keep sync in audios that get smaller, but the program itself does NOT resample the audio, indeed i think it's time to try virtual dub and virtual vcr for this indeed.

"It depends on the path of Audio/Video (whether the device has integrated audio or you are capturing it through sound card, most of which drift a little)."

They all have integrated internal sound device, since they evn capture sound without having any soundcard installed ! (lool). By the way in a specific channel that use to have the sound bigger by 0.015s each 20 minutes, i actually did a SIX hours capture and at the end the difference was 0.180s but the program has duplicated 3 frames to keep sync and indeed it seemed to work since i haven't notice any lag at the end of the video.

By the way i also tryed changing all the capture settings i could, absolutly nothing had any effect in the audio length!

[GrofLuigi]

Ok, this seems to be a different problem than what I had.

I encountered Asus My Cinema XXXX (forgot the number) card when I installed Windows for a friend. The drivers were difficult to find on the net and I used Vista drivers, since they were much newer than XP and both worked. That card was really a cheap one, made to work only with the bundled program (Power Director or Power Cinema, again I forgot the name, I think it was from Cyberlink). VirtualDub and DScaler didn't work well with it. VirtualVcr worked (in regards of tuning channels), but I didn't have enough time to test it with audio.

From what I read on the Internet, that card has Philips based tuner, and I read there was an universal driver for that family of cards (that was my first instinct - to find something btwincap alike :) ), but sadly I had to return the computer soon and didn't have time to experiment.

That's all I know about Asus My Cinema, and it might not even be your exact card. :( Maybe someone else can help.

Another thing: if it's a constant drift, have you tried changing frame rate in Virtual Dub on the captured file (Video>Frame Rate>Change so video and audio durations match)? That will not give you exact 25 FPS, but A and V will hopefully be synced. You can then use other methods, like resampling audio in external program (stretching it a little) so it matches the duration of the 25 FPS video... Many things can be done in VirtualDub.

GL

[cesm23]

"Another thing: if it's a constant drift, have you tried changing frame rate in Virtual Dub on the captured file (Video>Frame Rate>Change so video and audio durations match)? That will not give you exact 25 FPS, but A and V will hopefully be synced."

Well. the problem here is that it's the audio that gets de-synced on the capture, not the video (which is indeed quite strange) and yes it's a constant drift but only if it's the same channel i capture, for example in one channel the audio always grows 0.015s each 20 minutes, so that in a 5 hours and 15 minutes the diference was already by 0.180s...

Maybe this is something about the internal clock of the tv card capture device that has some rounding on the fps (you see, the main system in my country is pal-b, but the cards default values are 29.97fps... this probably has something to do, maybe the card wasn't well prepared to capture in 25fps, since it captures more exactly at 24.997, but that doesn't explain how the heck it has different values for each channel, maybe it's something to do with the stregnth of each channel signal or the amount of noice on it or even the frequency in which the channel is, unfortunately i haven't found yet a pattern on that.

"You can then use other methods, like resampling audio in external program (stretching it a little) so it matches the duration of the 25 FPS video... Many things can be done in VirtualDub."

Well yes but i am aware that without changing pitch too, we will always get a few artefacts, to date i am still to find a program that is able to stretch audio without changing pitch and without introducing artefacts... i found that out when i tried to stretch 25 fps audios to 23.976.

[GrofLuigi]

pal-b, but the cards default values are 29.97fps...
Yes, yes, this was the problem with the card when I tried it - the driver reverted to 29.97 FPS all the time (in all programs). Only VirtualVCR could keep it at 25 FPS, that's why I said it worked. But I didn't test it with audio so I don't know if it had audio at all... :(

"audio that gets de-synced on the capture, not the video"

I can't quite follow you on that - they are synced with each other. Either they are in sync or not. Or you mean "one is 25 FPS, other is not"?

GL

[cesm23]

"I can't quite follow you on that - they are synced with each other. Either they are in sync or not. Or you mean "one is 25 FPS, other is not"?"

No they are ONLY in sync with each other if the software does drop frames sometimes in the channels that the audio gets bigger, and if the software skips one frame sometimes in the channels that the audio gets smaller... If the software does not does this, even tough the video has all 25 frames per second, the audio is always smaller or bigger depending of the channel ! And i know this because i have compared with the same recording on a dvdr records records tv, in the dvd when i go compare it with the tv capture in the pc, when the software hasn't droped frames in the pc, the nr of frames is EXACTLY the same, and the the audio captured in the pc is already a bit smaller or bigger depending on the channel. Like i already said, i in a specific channel it gets 0.015s bigger in 20m capture and in another channel gets 0.065s smaller after 20 minutes (that means for example that if i compare the capture at 10:00, the audio is already smaller by 0.032s...) so you see this is probably a audio clock timing issue for each channel but only happens with the tv capture card.

Yes i know it's a big mystery but... at least i am getting a new tv capture card in a week and i will see if it also does this or not, since it's from a completly different chipset and brand (at tv wonder 650 with the ati theater 650 pro chip).

[Comatose]

Well. the problem here is that it's the audio that gets de-synced on the capture, not the video (which is indeed quite strange) and yes it's a constant drift but only if it's the same channel i capture, for example in one channel the audio always grows 0.015s each 20 minutes, so that in a 5 hours and 15 minutes the diference was already by 0.180s...
I'm thinking the FPS is bad, so you get a constant drift. The audio is getting desynced, but it's more likely to be the strange framerate. Try forcing it to be 25 FPS (don't change it, just assume).
In Avisynth: assumefps(25,sync_audio=false)

[cesm23]

Oh you mean doing that on the captured video instead of during the capture ? (since we can't capture video with avisynth) Ok i can try that... by the way i already changed the input fps in the capture program and there is absolutly no change in the audio, only in the video frames...

By the way when capturing video, it's also possible to assume fps instead of changing it ? If yes tell me how exactly.

[GrofLuigi]

By the way when capturing video, it's also possible to assume fps instead of changing it ? If yes tell me how exactly.

Read this. (http://virtualvcr.sourceforge.net/html/virtualvcr/sync.php) That's what these two programs I mentioned do. I doubt you'll notice the difference in resampled 48040 (for example) to 48000 Hz.

Virtual Dub also gained some audio/video resampling options since then, that's what I use. I don't know if iuvcr/iuvcs resamples.

But the question is whether any of these programs work with your drivers. If not, you can only correct audio drift post-capture.

GL

[cesm23]

WOW this is EXACTLY the problem i am getting indeed ! i never seen that before... ok i will explore this better later, at least virtual dub accepted my internal audio capture device a lot better than virtual vcr since virtual vcr didn't even reconized the existence of a internal audio device.

Now the real strange thing is that this should only happen when capturing audio by a sound card, but it appears that at least on SAA713x capture cards this happens too in the internal audio device!! This is a shame, i really hope this doesn't happen too in other capture cards which doesn't use that chipset, for example this should NOT happen in ati theater 650 pro chips, at least that's what i am hoping.

[DonQ]

I had somewhat similar problem with Philips SAA713x based card, capturing using Virtualdub (audio was internal (PCI)).
What helped to syncronize audio - Virtualdub menu Capture, Timing and choose "Do not resync between audio and video streams". I've unchecked "Correct video timing for fewer frame drops/inserts" either. And using big buffers (Capture, Disk IO) to avoid frame drops :)