A rough guide to vbr encoding?

[0815]

Hello,

i own a rather slow computer, thus want to avoid a second pass. A 100% predictable size isn't important to me, so i think single pass vbr (target quality) is what i want. Before starting the encoding process, i need to know what kind of average bitrate/filesize i can expect for the different target size settings for an average 90-120 minute movie though.

My first encoding at a target size of 75 (default) came out way too large (about 1400 MB, the size of 2 CDs) with 448kbps for audio alone.

Because of my slow computer, experimenting with different settings would be very time consuming, thus i'm looking for experiences/recommendations. Target size should be approx. 500-1000 MB per 90-120 minute movie (no fast action movies), two channel vbr mp3 will be sufficient, no multi channel ac3 necessary.

So basically i'm looking for a rough guideline what kind of file size i can expect for different quality settings, something like this table (http://www.hydrogenaudio.org/forums/index.php?showtopic=28124) (scroll down a few lines), but for xvid and divx in autoGK.

Also, is there a way to use real vbr settings for the mp3 audio instead of having to specify a target bitrate (abr or cbr)?

How does the automatic output audio type (under advanced settings work)? Will it choose ac3 for higher target quality and mp3 with different average bitrates for lower target quality settings? If so, where's the threshold? Do i need to override the auto setting to get mp3 audio instead of ac3?

Thanks for guiding this stupid newbie.

[CWR03]

Before starting the encoding process, i need to know what kind of average bitrate/filesize i can expect for the different target size settings for an average 90-120 minute movie though.
It depends on the selected output resolution, among other things. You will most definitely have to experiment some to find a target bitrate that suits the input media, your selected resolution and your perception of acceptable compression. There are far too many variables to even hazard a guess at an approximate filesize you might get.
Also, is there a way to use real vbr settings for the mp3 audio instead of having to specify a target bitrate (abr or cbr)?
Not within AutoGK. Since you're not concerned with filesize, you could let AutoGK encode the video, then convert the audio yourself and mux them together. The ABR is similar to VBR, only without the ability to set the high and low parameters.
How does the automatic output audio type (under advanced settings work)? Will it choose ac3 for higher target quality and mp3 with different average bitrates for lower target quality settings? If so, where's the threshold? Do i need to override the auto setting to get mp3 audio instead of ac3?
I don't know how it chooses between AC3 or MP3, especially when using target quality, nor do I know the threshold which causes it to select one or the other using 2-pass encode. I do know that to ensure that you get MP3 audio you just need to select it under Advanced options.

[0815]

Thanks,
after some more experimenting i think i'll stick to xvid quality 60 (all auto settings) with 128kbps abr mp3 audio for a while. Being new to this scene, i want to keep the procedure as simple as possible for the beginning and these settings seem to give a reasonable tradeoff between quality and size (for me). Got approx. 700MB for a two hour movie with still pretty good video and audio quality, in fact much better than a lot of comparable encodings i've seen so far. I'm impressed and might finally consider upgrading to a faster cpu. Thanks again. Other experiences/suggestions are still welcomed.

[BigDid]

... How does the automatic output audio type (under advanced settings work)? Will it choose ac3 for higher target quality and mp3 with different average bitrates for lower target quality settings? If so, where's the threshold? Do i need to override the auto setting to get mp3 audio instead of ac3?
Hi,

From the FAQ: http://forum.doom9.org/showthread.php?t=72679
2.3 How is first audio encoded in Auto mode?
- it's VBR 128Kbps if target size is 700Mb or less, in all others cases it's original AC3/DTS/MPA track. LPCM tracks are always converted to VBR MP3: 128kbps for 1CD or less, 192kbps for more that 1CD.

... after some more experimenting i think i'll stick to xvid quality 60 (all auto settings)
For one pass quality mode, roughly use 60 to 70%.
Be aware that, for a given size, 2 pass mode should get better quality because the bits are better distributed along the whole movie, specially for low bitrate, less for medium to high bitrates (but in quality mode the size will climb quick).

Thanks for guiding this stupid newbie.
Newbie and stupid doesn't necessarily stick together :)

Did