Rendering MPEG2 to MPEG2 - time & size compare

fultro wrote on 5/3/2004, 1:12 AM

For a test I converted a VOB from a DVD to MPEG2. I then rendered it in Vegas to the DVD NTSC template for MPEG2 . Render time was 50% longer than the file playback time and the file size was reduced 15%. Does this make sense? - more compression when the MPEG2 properties seem to be identical?
I need these files to be as close to the original DVD files as possible, and I wonder if I need to be doing something different or if what I experenced is normal.

Comments

Chienworks wrote on 5/3/2004, 3:23 AM

Do you know the properties for the original file? It may have been encoded with a different bit rate or by a different encoder.

Instead of doing all that conversion, can't you simply copy the files to a new DVD blank?

fultro wrote on 5/3/2004, 9:45 AM

Thanks - I know I could copy the VOBs to a new DVD project but I need to edit them and mix with new material - hence the need to break them out of the VOBs.
You have made me realize that there are variables at the original DVD encoding phase (done by someone else) that could have been different than how I encoded in Vegas.
But both were NTSC, variable bit rate - and audio has the same sample rate and number of audio channels - so I still don't see why there is a 15% difference.
A further note - I rendered the rendered MPEG again - resulting in an insignificant file size dif and 10% shorter time - in other words what I render with Vegas will not be recompressed with a second render

TheHappyFriar wrote on 5/3/2004, 2:11 PM

There is a noticable difference. I did that once to a part of the Simpsons to see how it looked. I noticed quality loss.

Here's what I do when I need to do what you want to do (peter piper picked a peck... :) )

Use TMPGenc & convert the VOB (mpeg2) to a DV AVI file (I use Matrox DV or DVCPro codec). Then edit that in Vegas.

And Vegas ALWAYS recompresses mpeg-2. Always. It might seem like it doesn't but it does.

fultro wrote on 5/3/2004, 2:31 PM

Well I was thinking it would be nice to keep the file size down to what the VOB to MPEG2 extraction provides as that is considerably smaller. I don't seem to have any DV AVI codecs installed so in the past I used uncompressed AVI which is rediculous - but now it seems that VEGAS 5 opens DivX AVIs so I'll try that - though I am curious about the effect of all this recompression on high quality video. Thanks for the info

Liam_Vegas wrote on 5/3/2004, 2:38 PM

I get the impression you are expecting to be able to edit MPEG files without any quality loss when you re-render.

By rendering (encoding actually) from an MPEG to an MPEG you are <always> re-compressing. Even if you have done nothing at all to the frames. That is the nature of the MPEG codec.

fultro wrote on 5/3/2004, 4:45 PM

So then what is Vegas doing to "pre-rendered" files? - I notice that I get a bunch of .AVIs that don't open in other programs

Is there any advantage to "pre-rendering" in terms of total time of final render ?

OR - in a nutshell - what is the best way to render parts of the project as the project develops so as to save time on the final render AND maintain quality?

thanks everyone - I could "trial and error" this myself but am in mid project with something else and am happy to get all your advice as I will need it immediately when I start the next one...

TheHappyFriar wrote on 5/3/2004, 8:23 PM

For me I find I normaly render a finished part of my video to a new track & continue to work on the rest in another instance of Vegas. Then, when I do my final render (well, i want it to b my final one!) it takes less time because the FX, pan, etc. are already done.

Here's the Matrox codec's. They're free.
ftp://redirect:r45j99t3@video.matrox.com/private/rt2500/utilities/VFWSoftwareCodec.zip

Converting the mpeg's to DVCPro shouldn' take up much space (no more then if it was normal DV). Just remember, these are DV codec's, so all the same DV size's, ratios, etc. apply to them.

fultro wrote on 5/3/2004, 9:37 PM

thanks for the matrox tip....
Regarding "rendering to a new track" -- if as Liam Vegas says MPEGs always get recompressed when Vegas renders than what happens to this already rendered tracks quality when doing a final render? -- sounds like generation loss as when bouncing audio tracks on a tape recorder

Liam_Vegas wrote on 5/3/2004, 11:58 PM

what happens to this already rendered tracks quality when doing a final render?

Yes.. you are getting it... you will be re-compressing every time you render to MPEG. So if you render to a new track using the MPEG2 template then you will get a generation loss and then again when you do your final render.

Which is why I usually work only in DV until the final render to MPEG2. All those DV AVI files you have created with your "render to new track" will not be re-rendered when you do your final render to AVI. If you have DV AVI files that have no effects applied to them when you render the entire timeline to DV AVI the previously rendered clips will be copied bit-for-bit.

Also although there is <some> generation loss when you really do neeed to re-render DV clips (added FX etc), the "Vegas" codec is known to be very very good and generation loss is considered very low (tests from other forum members have shown it to hold out well even after re-rendering up to 30 times or more).