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Can audio encoders target quality w/o caring about bit rate/file size?, [OP = softrunner / split from “IETF Opus codec now ready for testing”]
softrunner
post Feb 14 2013, 02:33
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QUOTE (Martel @ Jan 1 2013, 14:46) *
QUOTE (softrunner @ Dec 29 2012, 04:50) *
I don't know weather it is possible for encoder to do such an analysis of a source audio, but it would be great it yes.
It's only a matter of finding the right formula/algorithm.

x264 video encoder has encoding mode called Constant Rate Factor. In this mode number (16, 17, etc) is used to define desired quality (lower - better quality and higher bitrate), and encoder does not care about bitrate, only about keeping rate factor constant. It is a question, why nobody has invented something similar for audio encoding (except lossyWAV, which needs too much bitrate for acceptable quality)?
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Opus 1.1 Alpha has some bugs, which can be found using samples from thread High Frequency Listening Test Samples. For example, at 16-24 kbps Opus gives this:

and for 32-40 kbps it gives this:

For samples 1_12kHz, 1_20kHz, 2_8kHz, 2_12kHz and 2_20kHz Opus sounds wrongly even at 512 kbps.
Full set of files is here (problematic sampes are marked with exclamation mark). Hope, developers will use this samples in their work.

This post has been edited by softrunner: Feb 14 2013, 02:34
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Big_Berny
post Feb 14 2013, 12:11
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QUOTE (softrunner @ Feb 14 2013, 02:33) *
x264 video encoder has encoding mode called Constant Rate Factor. In this mode number (16, 17, etc) is used to define desired quality (lower - better quality and higher bitrate), and encoder does not care about bitrate, only about keeping rate factor constant. It is a question, why nobody has invented something similar for audio encoding (except lossyWAV, which needs too much bitrate for acceptable quality)?

I think every encoder with real vbr (not abr) does that? Lame has V(0-9), QT AAC has --tvbr (0-127), Vorbis has -q((-2)-10). The bitrate may vary a lot with these settings between different songs/genres.
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db1989
post Feb 16 2013, 03:45
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Look, I don’t know whether you’ve actually read any of the other posts in this thread. Existing codecs with true VBR modes already target quality without caring about meeting a specific bitrate. You tell the encoder to use a level of quality that is defined by a number on a sliding scale, and then it can vary the bitrate as much as it wants based upon the signal that you supply to it. Have you done any real testing of this?

DonP had some samples of speech that did not encode to a much lower bitrate than samples of music, but this in no way proves that quality-based algorithms are unique to H.264. Please, try some more tests. Read some documentation on encoders such as LAME, oggenc, VBR AAC, etc. Then see whether you can continue to claim that no audio encoder offers a mode that targets only quality without worrying about bitrate.
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Nessuno
post Feb 16 2013, 11:02
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Just to add up my two pence to an already worked out question by other respectable fellows: my music collection, about 15000 tracks encoded with QT AAC at a target quality of 110, ranges from 74kbps (Edwin Fischer's Bach WTC, mono piano from early thirties) to 314kbps (Henk Van Twiller's transcription for solo saxophone of Bach's cello sonatas: BTW this's quite surprising to me!). The average bitrate is around 256kbps (exactly reached by about 300 tracks). About 5000 tracks are < 240kbps, and about 2500 > 270kbps.
Of course we are speaking of VBR, so those values for a single track are still average bitrate, as shown in iTunes column browser.

This poor man's statistical analysis demonstrates first of how targeting quality the encoder doesn't care the less about bitrate, then that to reach quality 110 the Apple AAC encoder uses on average about 256kpbs, but that if I had chosen to target bitrate instead of quality, say 256kbps CBR instead of quality 110 which is a thing some people consider practically equivalent, it would have been overkill (largely sometime!) for the first tracks and inadequate for the seconds.

On second thought: if I'm not wrong, iTunes store sells everything at 256 CBR. Maybe this is "transparent enough" for everything, but defeats the proven efficiency of their own very good encoder.

On third thought: one of these days I'm going to try to encode those two extremes at 256 CBR and try to ABX them from both the q110 and the lossless ones (though I'm rather sure I will fail all of the times... wink.gif).

This post has been edited by Nessuno: Feb 16 2013, 11:25


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db1989
post Feb 16 2013, 17:27
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Thanks for providing data! smile.gif

QUOTE (Nessuno @ Feb 16 2013, 10:02) *
if I'm not wrong, iTunes store sells everything at 256 CBR. Maybe this is "transparent enough" for everything, but defeats the proven efficiency of their own very good encoder.
A previous analysis of files from the iTunes Store suggests that iTunes Plus is ‘constrained VBR’, which I interpret to mean ABR:
QUOTE (rpp3po @ Mar 15 2009, 02:47) *
iTunes' standard setting is identical to Quicktime's ABR setting at medium encoding quality.
iTunes' VBR setting is identical to Quicktime's VBR constrained setting at medium encoding quality.
iTunes Plus is identical to Quicktime's VBR constrained 256kbit/s setting at maximum encoding quality.
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