• Status Closed
  • Percent Complete
  • Task Type Bugs
  • Category Codecs
  • Assigned To No-one
  • Operating System iPod Nano
  • Severity Low
  • Priority Very Low
  • Reported Version Version 3.2
  • Due in Version Undecided
  • Due Date Undecided
  • Votes
  • Private
Attached to Project: Rockbox
Opened by ericw - 2009-04-28
Last edited by saratoga - 2009-04-29

FS#10174 - Distortion in AAC playback in Rockbox versions after 3.0

I think I’ve found a bug in Rockbox’s AAC decoding which causes the output to be slightly (but audibly) distorted. I started to notice this on all of my AAC files while using 3.1, and found that switching back to 3.0, the problem went away.

Here are some samples which make the problem easier to see/hear:
-sampleFLAC.flac is a flac encoding of a bell sound, which sounds correct on my PC and all versions of Rockbox.
-sampleAAC.aac is the same sample encoded with iTunes, using the iTunes Plus setting (256kbps VBR 44.1khz). It sounds identical to sampleFLAC on my PC and on Rockbox 3.0, but the sample is distorted on Rockbox 3.1 and 3.2.

I made recordings in Audacity by hooking my ipod up to my computer’s line in, and playing my two sample files on different versions of Rockbox. I used the default settings except changed the volume to 0db to make the recordings.

rockbox-3.0-sampleAAC.flac (sampleAAC played on rockbox 3.0 - not distorted)
rockbox-3.0-sampleFLAC.flac (sampleFLAC played on rockbox 3.0 - not distorted)
rockbox-3.1-sampleAAC.flac (sampleAAC played on rockbox 3.1 - distorted)
rockbox-3.1-sampleFLAC.flac (sampleFLAC played on rockbox 3.1 - not distorted)
rockbox-3.2-sampleAAC.flac (sampleAAC played on rockbox 3.2 - distorted)
rockbox-3.2-sampleFLAC.flac (sampleFLAC played on rockbox 3.2 - not distorted)

You can see the distortion quite easily in Audacity.

This is a guess, but I wonder if r18412 is responsible? It looks like one of the only changes to libfaad that made between 3.0 and 3.1.


Closed by  saratoga
2009-04-29 17:44
Reason for closing:  Fixed
Additional comments about closing:   Warning: Undefined array key "typography" in /home/rockbox/flyspray/plugins/dokuwiki/inc/parserutils.php on line 371 Warning: Undefined array key "camelcase" in /home/rockbox/flyspray/plugins/dokuwiki/inc/parserutils.php on line 407

Fixed in r20812.

Good catch. I had to use the test_codec plugin to dump wav’s of the rockbox output and then compare sample to sample to find anything wrong, but it does look like accuracy went down a bit when I merged the IMDCT from Vorbis. The actual problem is probably this:

I don’t think I should have hard coded which window types to use. I’ll try and fix that when i have more free time.


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