Confusion on AAC + MKV

[Sagekilla]

I'm currently demuxing all my mkv's that I've created over the past year to transcode my AC3 audio to 5.1 AAC's, and as I've begun doing so I've had some confusion on whether or not I'm getting the full frequency range.

Under media player classic, when I play back the HE-AAC .mp4's created by BeLight, the properties page says the audio track is a 48000 Hz 6 channel audio file, nothing unusual. But when I mux that audio track into a mkv container, mpc says it's playing a 24000 Hz 6 channel audio file. I believe I'm using CoreAAC to decode the audio right now, and would like to know if this is normal. I'd hate to have to go back and rip all my DVDs once more just so I can get my AC3 track to transcode it again.


Edit: Could it be that MPC is seeing the audio stream as it is? As a 24 kHz audio file while forgetting about the fact that it has SBR recreating the upper frequencies? I have this "problem" regardless of whether "AAC is HE-AAC" is checked in mkvtoolnix.

[NanoBot]

Hi,

your problem description makes me think that your he-aac is encoded as aacPlus with parametric stereo. Whenever the decoder does not know how to handle parametric stereo it will show only the half of the real frequency range and of course it will not play back correctly.

C.U. NanoBot

[Dark Shikari]

Hi,

your problem description makes me think that your he-aac is encoded as aacPlus with parametric stereo. Whenever the decoder does not know how to handle parametric stereo it will show only the half of the real frequency range and of course it will not play back correctly.

C.U. NanoBotParametric stereo has nothing to do with frequencies...

[KoD]

Use ffdshow to decode AAC. Set AAC to be handled by libfaad. Don't forget MPC has an internal AAC decoder, so disable it. Disable MPC's internal MKV and MP4 splitter filters as well, and use only Haali' s splitter. While playing the file in MPC, check what ffdshow audio reports in its Info panel - if it says SBR AAC then everything is fine.

[foxyshadis]

But when I mux that audio track into a mkv container, mpc says it's playing a 24000 Hz 6 channel audio file.

Did you check the "AAC is SBR" box in mmg when you muxed? It can't autodetect it, but you can remux checking that box and all should be good.

If you can't hear the difference (really pretty easy to tell for most people), load it into audacity and run a spectrograph.

[Brother John]

MKVMerge does autodetect HE if it’s in an MP4 container. That seems to be the case here, so it should have worked.

In its properties dialog CoreAAC should say this for proper HE-AAC playback:
Profile AAC+SBR
Sample rate 24000 -> 48000 Hz.

[KoD]

CoreAAC is based on an extremely old version of libfaad2. It should not be used by anyone these days. The only uptodate version of libfaad2 is the one ffdshow uses.

[blizard]

CoreAAC is based on an extremely old version of libfaad2. It should not be used by anyone these days. The only uptodate version of libfaad2 is the one ffdshow uses.

I did some transcoding with NeroAAC encoder to produce AAC of different quality with -q switch. To play those files I used MPC+FFDSHOW. FFDSHOW would show in tray info about audio file that it could identify LC AAC, HE AAC and HE AAC+SBR+PS, so it seem to work better now to get the full range of AAC decoded. That is new too me, so the latest update from FFDSHOW have brought more complex capabilities to libfaad2.

Correct me if I am wrong, but doesn't FFDSHOW make better use of different features from CPU then CoreAAC? CoreAAC is only limited to 16 bit decoding as far as I know. FFDSHOW can be set to process audio from 16 up to 32 bit floating point and resample sound up to 96k Hz and also mix sound to 5.1 with rather good quality in surround.

Forgot to add that MediaInfo also can do a good job to identify what type of AAC it is: MediaInfo (http://mediainfo.sourceforge.net/no)

MPlayer doesn't seem to distinguish between different type of AAC, at least it did only show FAAD as decoder and that's it. Celtic Druids compile: dev-SVN-r25279-4.2.2 for Athlon64 with date 3. dec. 2007.