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Topic: AAC making holes in audio (Read 1139 times) previous topic - next topic
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AAC making holes in audio

Encoded this way:

Code: [Select]
qaac.exe -v 256 -q 2 *wav

Encoder tag: qaac 2.67, CoreAudioToolbox 7.9.8.3, AAC-LC Encoder, CVBR 256kbps, Quality 96

Apple's own Music store doesn't fare better. Two holes for frequencies between 5 and 8KHz at 1:48 and 2:09 marks.

Ogg Encoder at quality 8.5 (roughly the same bitrate) doesn't do this but has a certain cut off for high frequencies.

I wonder if people can hear the difference. Files can be downloaded here.

Edit: updated to CoreAudioToolbox 7.10.9.0, there's barely any difference. In some ways it looks worse than 7.9.8.3 (leans towards higher inaudible frequencies at the expense of lower frequencies).

Re: AAC making holes in audio

Reply #1 –
(Perceptual) lossy compression mostly works by throwing-away sounds that are masked (drowned-out) by other sounds.

Different encoders may be "tuned" differently.

It's possible to have a better looking graph with worse sound.   It's easier to make a pretty graph than to get good sound.

That's the idea behind TOS 8:
Quote
TOS 8.   All members that put forth a statement concerning subjective sound quality, must - to the best of their ability - provide objective support for their claims.  Acceptable means of support are double blind listening tests (ABX or ABC/HR) demonstrating that the member can discern a difference perceptually, together with a test sample to allow others to reproduce their findings.  Graphs, non-blind listening tests, waveform difference comparisons, and so on, are not acceptable means of providing support.

Re: AAC making holes in audio

Reply #2 –
Quote from: birdie on
Encoded this way:

Code: [Select]
qaac.exe -v 256 -q 2 *wav

Encoder tag: qaac 2.67, CoreAudioToolbox 7.9.8.3, AAC-LC Encoder, CVBR 256kbps, Quality 96

Apple's own Music store doesn't fare better. Two holes for frequencies between 5 and 8KHz at 1:48 and 2:09 marks.

Scale bar says -70 to -90 dB for those frequencies, so they're so weak they do not matter and likely cannot even be reproduced by your stereo/headphones.  The encoder is recognizing that and removing them.

If you need the file to look exactly the same (rather than merely sound the same), you should be using FLAC. 
 

Re: AAC making holes in audio

Reply #3 –
Your screenshots were a bit misleading, but indeed it looks and sounds like there is a problem with that sample for this encoder, but it should not affect normal usage.

Why I say it is misleading? Because the problem is caused when downmixing the signal to mono. Spek downmixes the signal to mono and then analyzes it to show the spectogram.
 (ok, it's true that nowadays, downmixing to mono is done on smartphones, usb speakers...)

This sample has some hard panned sounds, and Apple's codec is being cheated when creating the stereo image, and somehow it generates inverted frequencies which, when downmixed, cancel each other.

If you listen to the file in stereo, the sound is perfect. If you downmix it with, for example, audacity, the artifact is not bad, but it is there as warbling (which is what one would expect with this kind of spectogram).