8629 Views 4 Replies Latest reply: Jun 26, 2008 5:37 AM by mattlewis
Yes, but the effect can be less pronounced on low-quality source footage, and also can be much less pronounced on footage that is largely unchanging, for example a long locked-off shot of a still-life type object with little or no movement.
One of the primary reasons for making multiple passes during compression is to better analyze the media and to make further optimizations to the compressed data. This can lead to smaller files and better overall quality. VBR encoding in general benefits from its ability to "borrow bits" from scenes that have less motion or detail, in order to spend them on scenes with high motion or detail. When a compressor makes 2 passes (commonly one internal CBR pass from which analysis data is gathered, and then a second pass to generate the actual VBR compressed media) it should generate a higher quality output from most typical video media.
Your mileage will vary based on your source content, compression settings, and so on. But generally speaking, you should be getting higher quality footage (when comparing with 1 pass of an identical bitrate) out of a 2 pass compression.
Here's a great article at IBM that discusses multi-pass encoding for MPEG-2 content intended for DVDs:
Notably, you can see the chart at the bottom which shows a flat-out SNR (signal-to-noise ratio) comparison between both CBR and VBR 2-pass compression.
With 2 pass VBR your movie is first analyzed so that in the second pass bit rate is increased where need (fast actions, transitions etc). That is rather important for quality. DVD studio can encode the video to MPEG2 but have less customizable controls available and more importantly will not encode audio as ac3 dolby stereo leaving it uncompressed (this will take more space and may not work perfectly in some players).
At least let Compressor encode audio to ac3 and more generally speaking keep using Compressor and learn the many features that it offers.
Most helpful Matt.
So would you think I'm being too conservative with my 5.2 and 6.1 min and max bit rates?
A bit of reseach makes me think now that perhaps compressor does a better job when I export directly from FCP to compressor - rather than making a self contained movie and dragging that into DVD Studio Pro. (Ignoring the multitude of extra options that Compressor affords me.)
I would not say that you're being too conservative, that is surely your decision based on your needs and your experience.
I tend to personally aim for slightly higher bitrates, but I typically work with contrasty content originated on film and with moderate motion. Your needs may be completely different.
For what it's worth, I always use Compressor to generate M2Vs for DVD Studio Pro, as opposed to DVDSP directly. I prefer the granular control.
It sounds like you're experimenting, which is certainly the right thing to do. I think we all optimize our workflows more and more every day, that's a battle that never ends. I'm sure you'll shortly find the settings you're happy with, then next month find even better ones.