Beware of video processing delays

[JVRaines]

I like to run audio straight into my capture card to keep the signal as pristine as possible. When using outboard video processors, this will put audio ahead of video by a small amount. How small? Will it make a noticeable difference? I recorded a U-matic videotape with 1-frame bursts of video and audio, then captured playback through a couple of common processing devices using my StarTech SVID2USB2 converter. Here's what I found.

These are the audio tracks of three different captures aligned at the same video burst frame (the marker). The waveforms are the corresponding 400 Hz tone bursts. There is a negligible video delay with no processing besides my analog transcoder. The delay with the TBC-1000 is about 1 frame, which is expected from a frame synchronizer. However, the delay with the DMR-ES15 DVD recorder is a whopping 2 frames!

As you can see from these study results, advancing the audio by 32 ms will not be noticed by most viewers, but will still have a subtly disconcerting effect. A 66 ms advance, however, is well into the range of annoying desynchronization. Therefore, captures must be remuxed to delay the audio and restore proper sync. This is easy to do with, for example, VirtualDub's Audio Interleave dialog.

[jagabo]

No surprises there. The ES15 has 3d noise reduction capability. Even if you're not using it the processing path probably remains the same. The same thing happens in TVs. That's whey external amps/receivers have the ability to adjust the audio delay.