View Full Version : Firewire capture = bad audio sync?

06-09-2004, 01:52 PM
I'm trying to capture DV via firewire within VT. No matter what I do, I can't get audio to sync. Sync is off by a second even at the start of the capture, and off by at least 10 seconds after 10 minutes.

I've tried both Batch Capture, and the Capture Panel with DV selected on the Switcher. Batch Capture controls the camera perfectly (but no sync). One oddity is that firewire audio does NOT show up in either the batch capture or capture panel's meters. It DOES show up on the audio mixer, and in the resulting files (but no sync). Is this normal?

I'm not maxing out the CPU, disk, or as far as I can tell the PCI bus during capture. I AM running a VIA-based firewire card, and have heard here more than once that TI is the way to go (I don't think this spec is usually on the box when you're buying one). But it's "OCHI compliant" for whatever that's worth, and for a commodity interface I would hope this isn't the real problem...

I'm assuming this isn't normal. Any ideas on what I can try / do to get this working? My system is old (P4 2.4) but has been working great since T2. I'm upgrading my GF4 video drivers now (the old ones were only from last December). AFAICT there are no external drivers for the firewire card (all within Windows 2000). No packet writers or other bus-stealers. Is the VIA card really that bad?

I know I can capture analog using Y-C (and even reencode back into DV 1 or 2 on the fly), but this should (?) be working, and accurate batch capture would be extremely helpful for my current job...

Thanks for any advice! -MG. :confused:

Edit: forgot another symptom: while I capture, the image in a VT-Vision and the audio out of VT are in sync. It's only when I load the resulting clip into VT-Edit that you can tell the audio is off. Thanks!

06-09-2004, 06:24 PM

Went out and bought a TI-based card ("Office Essentials" by Belkin if anyone else is looking, don't tell them that I opened the package in the store). It did not solve my immediate problem. Seems to perform the same as the Via card.

Found the Default DV Type option in Preferences (thanks NT!), changed to Type 1. Now I can capture in sync! Although... now the audio has many nasty clicks and pops - presumably all the bad audio frames which were being discarded before (video looks fine. :confused: ) Feature request: is it possible to have capture repeat the last audio frame (or something equally benign) instead of dropping it? (if this is in NT's court, it may be Microsoft's problem).

I listened to the audio output from the camera, and it sounds more or less fine (seems to gloss over the dropouts better than the computer). So I decided to go back to analog. Figured out that you can configure batch capture to control the deck via DV, and capture via an analog input (thanks NT!). This should work nicely for my needs. Although... capturing analog and transcoding to DV type 1 apparently uses too much of my CPU - I get a "can't support this configuration" message after a minute or two (CPU shows 80% usage). Turned off all other VT widgets except Batch, same message. I'm trying again with type 2, seems to be working better (fingers crossed, way behind schedule on this project).

Seems like things are overly complicated for something as "easy" as DV. Advice still appreciated.

Thanks! -MG.

06-09-2004, 09:40 PM
Ah, the ol' Type-2 problem.

Unfortunately, there is a problem that some of us have with Type-2, but unfortunately Newtek has never been able to reproduce the problem and therefore they can't fix it. PLEASE tell Newtek about your problem so they see it isn't just my system that does this!

I just did some testing at Newtek's request, and now my Type-2 was working fine, so it's definately an intermittent problem. But I still believe it is without question something in the VT[3], since ALL other NLEs or DV apps I've tried on my system have NEVER shown the problem with Type-2.

Note the problem will ONLY show up when you are capturing DV as Type-2 via firewire, VT-Edit will PLAY Type-2 perfectly and render Type-2 perfectly, and I encourage you to use Type-2 for EVERYTHING except VT[3] capture.

But yes, the audio when capturing Type-2 live will be in sync, and only out of sync when you play it back.

The TI-chipset makes a much better card, but nothing to do with this A/V sync issue. It fixed other problems I had like External Control losing my DV deck, and therefore captures failing.

As for the "clicks and pops" you are hearing with Type-1, isn't this only during the capture, as in live? I have this happen while capturing, but I ignore it since it is NOT in my saved Type-1 DV clips. Are you getting "clicks and pops" in the recorded file?

06-10-2004, 09:21 AM
Hi Eugene, thanks for the reply.

I'm definitely getting bad audio in my recorded clips:

Type 1 captured via firewire: definitely clicks and pops and nastiness in the recorded file (we used a lav mic so one channel should be mute, but there are even big pops on the mute channel), but all in-sync. While recording I hear very minor noise, but the recorded file is quite bad.

Type 2 captured via firewire: NO clicks and pops in the recorded file, but quickly and increasingly out of sync - I assume it's dropping those bad audio frames and pulling the audio sync further and further back (at the end of the file, the audio waveform goes flat many seconds before the video ends)... Live monitoring is same as type 1, minor noise and in-sync.

Notes: 1. The problem seems worse at the begining of the tapes, which makes sense (I don't know about DV, but good analog practice is to run tape for a few minutes to get over the dropouts, we didn't have a chances to do that :rolleyes: ). 2. Also this may have been recorded as "16-bit" audio from a Canon XL1, next session I'll try changing to 12-bit and see if that works any better. 3. The audio is hot, and may have occasionally clipped. This shouldn't affect anything, but may be triggering a bug?

Frustration: even if you're capturing analog, it appears that if you use firewire to control the deck, it still needs to decode the DV, taking a good hunk of CPU time and making transcoding dicey on a (my ;) ) slow machine. (Normally, analog to DV takes 50% of my machine. Analog to DV plus firewire control takes 80% which is pushing it).

It's very interesting to me that your problem went away while testing (which seems to break my dropping-frames hypothesis). The effect seems quite consistent to me per capture tape...

One more question: when you capture via firewire, do you get an audio signal in the Capture or Batch capture VU meters? I get nothing, which seems odd, but may be by design...

I've been logging some random VT bugs, time to add this and send it in...

Thanks! :D -MG.

06-10-2004, 12:30 PM
Oh, as for not seeing the firewire transfer audio on the meters in capture, that is by design. If you could see it, it would make you think you could alter the level while you transfer, which you can't.

As for in my tests: I had a communion that the A/V was out of sync with for Type-2 (under patch 3), recaptured as Type-1 and did the editing. Then tested the same video under Type-2 again and it captured in sync.

Bizzare, but I still insist, VT[3] related.

06-10-2004, 02:49 PM
Originally posted by mgrusin
3. The audio is hot, and may have occasionally clipped. This shouldn't affect anything, but may be triggering a bug?

If the audio is hot to start with, it is possible you are overloading the output from the VT[3] card, check your mixer to make sure the edit audio is not turned up (passed the normal or 0 mark). You might also try adjusting the audio master level on the clip down -6db and see if that makes it better.

I have never had DV sync problems (since we went to VT[3] anyways), so I don't have any answers there.