Output file with a specific bitrate

[osullic]

If I want AutoGK to output an AVI with (approximately) a given bitrate, eg. 2000 kbps, how could I go about this?

Selecting a target quality of 70% seems to produce quite different bitrates depending on the input file. I read in this thread (http://forum.doom9.org/showthread.php?t=121532) that target quality percentages refer to quantizer values, but unfortunately that doesn't mean anything to me.

Could someone shed some light, or point me to where I could find out more (without finding out too much!)?

Thanks

[unskinnyboy]

Please search before you post.

http://forum.doom9.org/showthread.php?p=1130641#post1130641

[osullic]

I did search.

So a bitrate calculator is the solution to obtaining output of a particular bitrate. Thanks.

Any pointers to explanations of quantizer values so I can intelligently use these to obtain my desired results?

[unskinnyboy]

It's obvious that you aren't searching hard enough. There are plenty of threads on this too (especially older ones, circa 2002-2003)! Even the thread you linked to in your first post talks about this. Also, make sure you read the Xvid FAQ.

If I could summarize,

lower the quantizer = bigger the file size = better the quality.

[osullic]

The thread I linked to above does indeed include discussion of quantizer settings, but basically the only thing it says, as you repeated, is that a lower quality setting means a higher quantizer value. It still leaves me none the wiser as to what information I can take from specific quantizer values.

In the Xvid configuration panel, when Single pass is selected, it's possible to toggle between Target quantizer and Target bitrate. The quantizer scale goes from 1 (maximum quality) to 31 (smallest file). The bitrate scale goes from 16kbps to 10000kbps. I wonder if these scales correlate, and if so, if they correlate linearly.

I found a good explanation of the Xvid configuration at http://www.gromkov.com/faq/conversion/xvid_options.html. But at the end of the article, in the "I'm a newbie. What settings do you recommend?" section, it says, "Only use target size, not target bitrate or quantizers" - not particularly helpful in this case.
The article does also link to further articles on quantization, but these look like in-depth scientific articles, and I doubt I have the pre-requisite knowledge level to justify going that far into it.

I also came across another post here at http://forum.doom9.org/showthread.php?t=74784 where quantizer settings are discussed. There are some helpful comments (for me) there like "anything over 67% (quant 3) for clean sources is pretty good, and reasonably close to DVD quality." I'm sure there's more to it than that though.

Of course with encoding settings, a lot depends on the source input and subjective opinions. But like people in the posts I linked to have said, I'm looking for reasonable quality but don't want to increase file sizes just for the sake of it. I don't really have the time or patience to do my own testing, on everything I have to encode - I'd rather hear the opinions of a few others about what I can expect from quality percentages/quantizer values based on their experience.

Thanks

[Dark Shikari]

The mapping of quantizer to bitrate is entirely dependent on the source and encoding settings. There is no formula.

[unskinnyboy]

osullic, for the same quantizer settings, you are getting different bitrates, because different sources compress differently! Lower the compressibility of the source is, the more bitrate and file size it would require to encode it, at the same quantizer. If a movie compresses very well, then even at a very low quantizer of 2, the bitrate and hence the file size needed would be very less (look here (http://forum.doom9.org/showthread.php?t=136726) to see what I mean by compressibility).

If you understand the above, then there should be no room for confusion. Best would be to do some testing on your own to understand the concept better. If you can't do that, then there's nothing much we can do.

[osullic]

So encoding using a particular quantization setting across different source files results in a more consistent (albeit subjective) "quality" level regardless of the compressibility of the input (and individual bitrate required).

I'm still curious though what quantizer values refer to. What does 2.5 or 2.67 or 3.0 actually mean? And if the quantizer scale in the Xvid configuration panel is a linear one.

Just for context, the whole reason for asking is so I can select encoding settings and just use the same ones for all my DVD backups. I'd rather make a good decision on this now and not have to go back and re-do anything. And as I said earlier, I'm looking for reasonable quality but don't want to increase file sizes just for the sake of it.

To make an analogy to digital audio, it's not unusual to always use say the same bitrate to encode CD audio to MP3. Of course here too "compressibility" comes into consideration when some bitrate will be fine for some songs or portions of songs and not for others. But it's still possible to make a good judgement on acceptable general encoding settings. Spending time investigating the optimal encode settings for each individual piece of media is, at least for me, not practical.

(For MP3 encoding, of course other factors such as the specific MP3 encoder come into consideration too, but I'm ignoring that for the purposes of my simplistic analogy.)

Thanks again for your feedback.

[unskinnyboy]

So encoding using a particular quantization setting across different source files results in a more consistent (albeit subjective) "quality" level regardless of the compressibility of the input (and individual bitrate required).Yes.

I'm still curious though what quantizer values refer to. What does 2.5 or 2.67 or 3.0 actually mean? And if the quantizer scale in the Xvid configuration panel is a linear one.I have been repeatedly asking you to SEARCH (http://forum.doom9.org/showthread.php?p=106605)! Why is that so difficult for you?!

Just for context, the whole reason for asking is so I can select encoding settings and just use the same ones for all my DVD backups. I'd rather make a good decision on this now and not have to go back and re-do anything. And as I said earlier, I'm looking for reasonable quality but don't want to increase file sizes just for the sake of it.Read the above link and understand the concept of quantization and quantizers.

[osullic]

That last link was a helpful explanation. I also found a similarly helpful explanation in the DivX user guide
http://www.divx.com/divx/windows/codec/guides/

Again, I did search. Hard too. As it happens, I didn't find the post you linked to.
But I do apologise because my apparent lack of searching and complete ignorance in having to ask a few questions seem to be a real thorn in your side.
Thanks for your help though

[laserfan]

Perhaps you are ready to graduate from AGK to a more flexible tool. I can recommend StaxRip, and MeGUI looks nice tho I haven't used it much. Fairuse Wizard is also excellent but it only works w/DVD discs or ISOs.