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Old 24th August 2008, 14:09   #1  |  Link
SeeMoreDigital
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AAC LATM/LOAS Audio Streams

I'm interested to understand what the fundamental differences are between good 'ol fashioned AAC-LC/HE audio streams and these new fangled AAC LATM/LOAS audio streams, that can be found in european broadcasts.

Can anybody oblige?
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Old 24th August 2008, 17:25   #2  |  Link
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There are two extra levels of multiplexing in the stream:

"The MPEG-4 HE AAC or HE AAC v2 elementary stream data shall be first encapsulated in the LATM multiplex format according to ISO/IEC 14496-3 [17]. The AudioMuxElement() multiplex element format shall be used.

The LATM formatted MPEG-4 HE AAC or HE AAC v2 elementary stream data shall be encapsulated in the LOAS transmission format according to ISO/IEC 14496-3 [17]. The AudioSyncStream() version shall be used. AudioSyncStream() adds a sync word to the audio stream to allow for synchronization.

Semantics: The semantics of the AudioMuxElement() and AudioSyncStream() formatting are described in ISO/IEC 14496-3 [17].

Encoding: The MPEG-4 HE AAC and HE AAC v2 elementary streams shall be formatted with AudioMuxElement() LATM multiplex format, and AudioSyncStream() LOAS transmission format."

Somebody needs to make some open source LATM/LOAS demultiplexing code. I'm too busy on other things right now.

Last edited by Guest; 24th August 2008 at 17:29.
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Old 24th August 2008, 19:52   #3  |  Link
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Originally Posted by SeeMoreDigital View Post
these new fangled AAC LATM/LOAS audio streams, that can be found in european broadcasts.
Is there a chance you could upload such a sample?
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Old 24th August 2008, 20:33   #4  |  Link
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H.264 + AAC LATM/LOAS:

http://neuron2.net/misc/One News AAC_.edit.ts
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Old 24th August 2008, 22:11   #5  |  Link
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Thanks, that's helpful!
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Old 24th August 2008, 22:13   #6  |  Link
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I've got more if you need them.
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Old 25th August 2008, 07:42   #7  |  Link
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I've got more if you need them.
Not sure yet. I've had a quick look into the specification and there seem to be 3 different types of LATM/LOAS. I've identified the type that your sample has. But I'm not sure if all samples have the same type. So I'd say: If all your samples are from the same broadcaster I think I don't need more right now. But if you have samples from different broadcasters, that would be helpful, cause there'd be a chance that they differ in some ways.

Anyway, I'll add support for demuxing these audio tracks to eac3to. But it may take a while...

Thanks!
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Old 25th August 2008, 10:38   #8  |  Link
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This is all sounding (no pun intended) very positive . I'll see what samples I can get hold of too.

I wonder how many broadcasters have elected to use AAC LATM/LOAS audio. Given that AAC audio bit-steams can't be decoded by surround sound amplifiers, its use seems an odd choice
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Old 25th August 2008, 10:56   #9  |  Link
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FWIW, some Japan broadcasters use AAC, too, but they use ADTS instead of LATM/LOAS.
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Old 4th September 2008, 09:32   #10  |  Link
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Quote:
Somebody needs to make some open source LATM/LOAS demultiplexing code.
Delphi: http://www.dvbviewer.info/forum/inde...howtopic=23347

C++: http://blog.monogram.sk/janos/2008/0...decoder-v0900/
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Old 16th October 2008, 02:47   #11  |  Link
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New Zealand uses HE-AAC in it's freeview digital terrestrial broadcast's. Currently there's no directshow splitter/demuxer that supports it fully. The Cyberlink Demux (PDVD8) Filter can split the H.264 video and HE-AAC Audio and be played back successfully in graphedit but if you save the graph and relaod it the video decoder pin cant be reconnected the Cyberlink Demux (PDVD8) Filter. Graphedit just crashes if you try too.
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Old 29th October 2008, 00:47   #12  |  Link
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Would those code samples be good enough to build a DS filter to decoding the audio? If so many folks in our part of the world would be pretty happy since we could process it all using avisynth.

So far all I have been able to do is demux the audio using dgavcindex, play the AAC file in Winamp to produce a wav file and use besweet to encode the wav file as AC-3. It works but not exactly automatic

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Old 29th October 2008, 00:50   #13  |  Link
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Sure, it's not rocket science. But someone with the skills has to be motivated.
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Old 29th October 2008, 01:51   #14  |  Link
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I would do it in a heartbeat but the learning curve for me would be too steep I think, my last programming having been some dabbling in C++. I don't think it's the programming - it's understanding the formats of the data needed to be decoded and I guess, the way to write DS filters. I guess will just have to wait for the motivated person to do it!
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Old 29th October 2008, 01:57   #15  |  Link
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Don't sell yourself short. The format specification is readily available and quite clear. And there are good books on DirectShow programming. It's all a matter of how bad you want it. I don't have any need for it, so it's hard to get motivated.
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Old 29th October 2008, 10:13   #16  |  Link
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Don't sell yourself short. The format specification is readily available and quite clear. And there are good books on DirectShow programming. It's all a matter of how bad you want it. I don't have any need for it, so it's hard to get motivated.
Well I still think the learning curve is non trivial. As for motivation, I think there might be a few folks who live in territories with this kind of broadcast standard who might find suitable ways to get you motivated.

[edit]

Actually on reflection we don't need a DS filter for decoding the AAC files since that exists already. What we need is a DS filter for splitting the audio from ts file. Currently the Haali filter only does AC3 and doesn't appear to handle the AAC audio. When that splitter is done we ought to be able to get a graph that has the splitter feeding the AAC decoder, feeding a program that could reencode the file to more friendly format.

Last edited by lchiu7; 30th October 2008 at 05:04.
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Old 6th November 2008, 23:27   #17  |  Link
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Mplayer is now able to play the AAC LATM/LOAS Audio now directly from .ts; I'm testing using SMPlayer 0.6.4 SVN r2102

It reports the demuxer as: mpegts - MPEG-TS demuxer
the video decoder: FFMpeg h.264
the audio decoder: libfaad2

I don't know enough as to how this could work in graphedit/graphstudio or if there is a corresponding directshow splitter /decoder, but perhaps a dump could be used through ffmpeg ?

I briefly tried FFMpegSource and FFMpegSource2 but couldn't get either to work, hopefully someone more saavy can use this to figure something out?
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Old 6th November 2008, 23:56   #18  |  Link
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Mplayer is now able to play the AAC LATM/LOAS Audio now directly from .ts; I'm testing using SMPlayer 0.6.4 SVN r2102

It reports the demuxer as: mpegts - MPEG-TS demuxer
the video decoder: FFMpeg h.264
the audio decoder: libfaad2

I don't know enough as to how this could work in graphedit/graphstudio or if there is a corresponding directshow splitter /decoder, but perhaps a dump could be used through ffmpeg ?

I briefly tried FFMpegSource and FFMpegSource2 but couldn't get either to work, hopefully someone more saavy can use this to figure something out?
I briefly tried out smplayer (latest SVN) but couldn't get it to run since it wanted mingwm10.dll and I have no idea what that is or where to get it (safely).

Assuming you got smplayer to play the streams, we would need a way to capture those streams as perhaps wav files? That would solve the problems. If an AAC audio stream can be extracted from the ts files, then the rest is easier since the AAC audio can be played with winamp to get wav. That is why I used dgavcindex to demux the file but as noted in the other stream, it is failing on some streams I have captured.

There is also a DS filter for decoding the AAC files Monogram DS filter but needs a splitter to feed it and the most common open source filter for ts files (Haali) doesn't understand AAC-HE streams, yet.
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Old 6th November 2008, 23:57   #19  |  Link
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Here is a DirectShow filter:
http://blog.monogram.sk/janos/2008/0...-decoder-0950/
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Old 7th November 2008, 00:32   #20  |  Link
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Thx clsid, I'll test it out, but I can't find a splitter that works to feed it.

Ichui7 - The build of SMplayer I'm using is from Lord_MuldeR's homepage
http://mulder.dummwiedeutsch.de/home
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