Re rendering and it's hit on video and/or audio quality?

Sticky Fingaz wrote on 4/12/2004, 9:40 PM

Here's a quickie that I bet has been asked 1,000,000 times. If I have a video, and re-render it say 20 times, all in vegas and everything originated in DV format, can the video, or more importantly in my case, the audio, take any hit at all? Keep in mind these are music videos with CD audio spliced in.

Thanks in advance.

Comments

pb wrote on 4/12/2004, 10:00 PM

I don't think it makes a bit of difference. Seems there was a link to a SOny test wherein they took a clip to the 50th generation without any measurable degradation. I do it all the time but then my s**t ends up as Windows Media most of the time so quality is not as important as it is for you.

Peter

chaboud wrote on 4/12/2004, 10:03 PM

The audio will not take a hit from multiple re-renders provided that the sample-rate of the audio remains untouched. Audio is uncompressed PCM in DV.

The video may suffer some degredation, but Vegas' codec is very stable. The Microsoft codec, by contrast, can show severe degredation over just five re-renders, and noticable degredation after only one. A simple way to test this would be to re-render multiple times and put the original and derived renders on separate tracks with the upper track in "Difference" compositing mode. To amplify the difference, apply "Levels" to the project and set "Input end" to something rather low (0.02-0.05 should work nicely).

For audio, this can be tested by putting the original and derived audio on separate tracks and inverting the phase of either audio event. This can be found in "Switches" on the audio event's context menu.

Of course, with DV specifically, we pass unchanged frames through. No change.

Chienworks wrote on 4/13/2004, 3:13 AM

I'm not sure why you would be re-rendering 20 times. My first question would be, "do you mean you repeatedly render the same material from the same timeline again and again as your project progresses and you make output files of new versions as you go?". If so then the answer is no, nothing is happening to your material (unlike editing from tape where continual playing can slowly degrade the signal). In a practical sense this seems to me to be the only way you could be "re-rendering" that many times.

On the other hand, if what you are really asking is what happens when you bring the rendered output file back onto the timeline and use it as a source for a new rendered file then there may be some cause for concern. I can't imagine why you would do this 20 times so that's why i had to check about the possibility in the first paragraph. But, pressing forward ... if you aren't making any changes at all to the material when you re-render then Vegas will simply copy the frames bit for bit with no rendering going on, so you can do this billions and trillions and quadrillions of times with no degradation to the video whatsoever.

If you are making some changes to the video each time then the major quality hit occurred when the camera recorded the original DV signal. After that, Vegas' DV codec holds up very nicely even past 99 generations of re-rendering. You can see some samples here: http://www.vegasusers.com/testbench/files/generations/.

Sticky Fingaz wrote on 4/13/2004, 11:03 AM

I simply have to mess with the audio a little bit over and over to get the volume just right. This requires rerendering everything but I am not doing anything to the video like effects or anything. Just wanted to make sure this wouldn't mess with video quality is all.

Chienworks wrote on 4/13/2004, 11:47 AM

In that case, you really shouldn't be starting over with the previous rendered output file each time. You really should be working from the same source and adjusting your tweaking there, then do another original render from the source material. Each time you tweak and render the previous file to a new output file you'll build up artifacts in the audio and can quickly end up with a garbled mess.