HDV proxy editing: ac3 audio for downconvert

mbryant wrote on 5/17/2006, 1:14 AM

Ok, I'm editing HDV using Gearshift in proxy editing mode (though this question would apply if manually managing proxies). After editing I am rendering down to SD DVD in Vegas.

For the video, I swap back to the m2t for the SD MPEG render, for best quality.

As it is for DVD of course I need to do a separate render of the audio to .ac3. I accidently did this using the DV proxy rather than the original m2t, and noticed that this is much faster (it takes about 6 times as long to render the audio from the m2t as from the DV proxy.)

The question is: does it matter in terms of the audio quality if I render from the DV proxy or the m2t file?

Mark

Comments

farss wrote on 5/17/2006, 2:04 AM

I cannot imagine that it would as the 16/48K audio in the DV proxy is ahigher quality format than the compressed audio in the original m2t files.

johnmeyer wrote on 5/17/2006, 6:22 AM

This may be a function of a bug I discovered in AC-3 rendering. It has nothing to do with HD, HDV, proxies, Gearshift, or anything else. Basically, if you change the AC-3 template, something gets glitched in Vegas, and AC-3 rendering suddenly takes approximately 8x longer than normal. This is a HUGE deal. The solution is to quit and then restart Vegas, and then immediately render with the new AC-3 template, without changing templates again.

My guess is that if you quit Vegas and restart, and then do your AC-3 render again, without changing any templates, it will render at its normal fast speed. You can easily test this -- you'll know after a minute or two whether Vegas is in slow motion or normal speed rendering.

I have reported the bug to Sony. It's been there since Vegas 4.

farss wrote on 5/17/2006, 2:12 PM

It gets wierder than that, if you render ac3 using the standard template first and then your custom template all is well. Makes batch rendering a pain, I always start with a 'junk' ac3 render and then run the ac3 renders with my templates.

Bob.

Spot|DSE wrote on 5/17/2006, 2:17 PM

Aside from the problems mentioned in the above post, you're better off using the PCM audio vs mpeg1/layerII audio of HDV, particularly if you're processing the audio at any extreme.

mbryant wrote on 5/18/2006, 5:27 AM

Thanks all. I didn't change the ac3 templates (used the standard stereo DVD one), not sure if I saw the "bug" or if rendering from HDV MPEG audio is just slower than from the proxy with PCM audio. But I'll stick with rendering the audio from the proxy (PCM).

Mark

JohnnyRoy wrote on 5/18/2006, 7:16 AM

It makes sense that it should take longer. PCM is uncompressed so there is no decode stage you just encode it as AC3. With M2T files you must first decode the mpeg audio and then encode it as AC3 so Vegas is doing twice as much work.

That means the best GearShift workflow is to encode your AC3 audio before you Shift Gears for the final render. I’ll have to remember to add that to the User’s Guide.

~jr