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Codec performance comparison, ARM9E results from rockbox
post Jul 11 2010, 00:03
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I recently did some performance comparisons of various codecs in Rockbox on arm9e (specifically a Sansa Clipv2). For this test I clocked the CPU and memory at 40MHz, to mimic the typical low power settings an MP3 players CPU would run at while in low power mode with the screen off. The results are pretty interesting.


128k (give or take for AC3) lossy files:

a52_stereo_192.ac3 17.29MHz
mpc_128.mpc 18.01MHz
wma_128.wma 19.26MHz
vorbis_128.ogg 20.16MHz
nero_128.m4a 24.26MHz
lame_128.mp3 26.35MHz

Vorbis uses tremor, mp3 uses libmad, mpc uses peter's fixed point decoder, wma uses my fixed point port of ffmpeg's decoder, nero uses libfaad combined with a lot of code from ffmpeg and a52 uses liba52.

Some caveats:

AAC isn't as well optimized as the others. It uses the same transform, windowing code as Vorbis, but libfaad is kind of a mess.

Vorbis and WMA are pretty well optimized.

MP3 is extremely well optimized, and probably faster then a lot of commercially shipping decoders judging by the much better battery life in rockbox then the retail firmware when decoding mp3. All the pure MDCT codecs use the same MDCT, which is pretty close to the performance arm ltd claims for their optimized mdct. MP3 uses its own MDCT, which is within in assembly and within a few instructions of the best known algorithm.

All low power, low accuracy, etc modes are disabled. All codecs decode at full resolution, typically > 16 bit.

Some thoughts:

A52 is really fast because its really simple. It has two transform block sizes, both of which are shorter then most codecs. In stereo mode its basically nothing but huffman, inverse quant, and then mdct.

AAC is really complicated and the spec isn't freely available, which makes optimizing it difficult.

MP3 is really slow compared to pure MDCT codecs or pure subband codecs in spite of intense optimization. This is because it has to do both an MDCT (like Vorbis, AAC, WMA, etc) and a synthesis filterbank (like MPC). Neglecting the synthesis filterbank, it performs similarly to the other small block sized mdct codec, a52. This leads to decreased battery life using the format.

WMA uses the most complicated MDCT scheme of the codecs, but is otherwise extremely simple at mid to high bitrates (low bitrate coding is more complicated). Consequently, its very fast and very easy to optimize. In terms of quality per decode time, it and vorbis are probably towards the top.

People say vorbis is slow, but actually its one of the fastest formats tested. Its fairly complicated, but reasonably easy to optimize and in general efficiently designed. It does have a very annoying habit of using an unpredictable amount of memory though. Some files can use up to 500KB of RAM, but most modern ones only need < 250KB. Annoying.

I didn't post ATRAC3 results because I don't have an encoder, but its also really, really slow because its a hybrid format like MP3 with a lot of really weird stuff added on top that no other format uses.

ffmpeg codecs are typically the fastest and best written.

Lossless Formats

flac_5.flac           7.07MHz
flac_8.flac           7.67MHz
wv_fastx3.wv          24.11MHz
wv_normx4.wv          28.69MHz
true_audio.tta        36.62MHz
ape_c1000.ape         40.66MHz
wv_high.wv            45.07MHz
ape_c2000.ape         57.69MHz
ape_c3000.ape         86.34MHz
ape_c4000.ape         221.24MHz

Some caveats:

flac uses ffmpeg, ape uses rockbox (which was since ported to ffmpeg), wavpack and TTA are the official decoders.

Lossless formats are more boring to me, so I can't say as much about them.

All the formats are really well optimized, though flac, and ape are probably the best optimized.

Some thoughts:

FLAC is amazingly efficient. Its probably the fastest compressed format in remotely widespread use. All formats seem to compress about the same, so if you want to use lossless on batteries, you should probably look at flac first.

APE is really, really slow. I didn't benchmark c5000 at low clock speed since I didn't have all night, but it nees around 900MHz. I would not use APE on a portable device.

Wavpack is probably fast enough that you wouldn't notice much difference in battery life verses flac. Most devices can't lower the clock below 20MHz anyway.

More results if anyone is interested:

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post Jul 11 2010, 09:48
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Wow, that's blown all my preconceptions out of the water!!

I've been using Lame V5 on my clip+ (rockboxed), because I thought it maximised battery life. Now I'm ready to chuck that in and move to ogg. I much prefer the artifacts introduced by ogg at low bitrate than those created by mp3.

I'll stay away from wma, and it looks like mpc wasn't optimised for low bitrates.

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post Aug 5 2010, 07:23
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QUOTE (edwardar @ Jul 11 2010, 08:48) *
Wow, that's blown all my preconceptions out of the water!!

I've been using Lame V5 on my clip+ (rockboxed), because I thought it maximised battery life. Now I'm ready to chuck that in and move to ogg. I much prefer the artifacts introduced by ogg at low bitrate than those created by mp3.

Correct me if I'm wrong, but AFAIK rockbox will run in boosted mode (240Mhz) on the Clip+ anyway, because changing clock speed introduced all sorts of crashes. So for now it shouldn't make any difference which codec you use...
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