TBC's & Audio Synch

[cbturn]

Hi -- If a TBC is put in a video chain and audio bypasses the chain, how is audio kept in synch with the video? I am looking at the Hotronic AR-31 to stabilize some 3/4 U-matic analog prior to a conversion to DV for editing. (Color correction, etc will be done in software once captured.) But I am concerned about the potential for audio synch problems resulting from any delay in the video processing circuit. Some boxes (the Canopus ADVC 300, for example which I know is just an LTBC) have audio circuitry and the ability to "lock" audio to video. Is this a concern with a stand alone TBC like the Hotronic? Any experience would be appreciated. And thanks to all for making this Forum a really useful source of good information.

[davideck]

The delay through a TBC/Frame Synchronizer can be anything from 0 to 1 Frame (about 16ms). It is constantly changing because the VCR timing drifts with respect to the TBC output timing. Attempting to compensate for this would be problematic, but a delay of 16ms or less is typically not enough to notice.

[cbturn]

Thanks for your quick response. I have lurked here for weeks and appreciate the way you and others share your knowledge. It is useful to more people than you might think.

Regarding your answer, I assume there is no chance of this 0-16ms delay building up over time on long captures. Makes me wonder why audio lock is even an issue on some units. Is that because throwing a proc amp into the process does inject more substantial video delays?

Also, do you or any others on the forum have any experience with the Hotronic AR-31 for straight TBC function without any proc amp signal processing.

Thanks again.

[lordsmurf]

Usually audio sync errors are a problem of the capture device, not external filters.

[davideck]

I assume there is no chance of this 0-16ms delay building up over time on long captures.

No chance. The TBC will repeat Frames or drop Frames in order to stay within 16ms.

Makes me wonder why audio lock is even an issue on some units.

Many A/V sync issues are the result of corrupted video frame timing. This can cause the capture device to lose its video timing reference.

A TBC/Frame Synchronizer guarantees valid video frame timing at all times. The capture device doesn't get lost, and the timing relationship between audio and video is accurately maintained.

[cbturn]

Thanks again. You've made the decision simple.