AAC 5.1 sound through hdmi/optical splitter
#1
I don't know if this has been answered before but here's the situation:

I am trying to play H264/AAC 6 channel movies with openelec 4.0 (gotham) beta 3. I get only 2 channels through my chinese HDMI/optical splitter as the TV doesnot support passthrough.

Pi -> HDMI optical splitter -> TV (video) - Yamaha amp optical cable (audio)

This works fine for passthrough formats like AC3 and DTS but not for AAC as I get only 2 channels. I have tried every possible combination. The Yamaha amp is an old model and does not have HDMI inputs. If I set HDMI out at 2.0 channels in xmbc and try to transcode to 5.1, I still get 2 channels. Most probably because 6 channels LPCM is being output from the HDMI and the splitter does not know how to feed that into the optical link or the amp does not support multichannel PCM.

Is there any way I can transcode the AAC 5.1 into AC3 and then give it to the splitter? Maybe do an upmix of stereo? How?
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#2
Sorry, posted in the wrong thread.
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#3
Just a small note: I mean transcode on-the-fly so that the pi outputs AC3 instead of multichannel PCM.

>Milhouse: No prob! Your input however would be valuable here!
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#4
Sorry, the Pi doesn't have enough CPU to decode multichannel audio and re-encode it to AC3.

You can enable this feature when using dvdplayer or paplayer using the milhouse or miappa test builds,
but I suspect it will struggle to keep up.
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#5
Well I did enable "Enable Dolby Digital transcoding" under the "Enable passthrough" option hoping that I would get the transcoding working to test it but nothing happened. I got the usual downmixed output (I had "Number of channels" set to 2.0). I tested it using openelec 4.0 beta 3. Doesn't it work with it? Do I need a milhouse build for this to work?

Also doesn't it work with omxplayer? I have to use dvdplayer?
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#6
(2014-04-02, 12:28)vprasinos Wrote: Also doesn't it work with omxplayer? I have to use dvdplayer?

Yes. This is a feature of the ActiveAE engine, so only works with dvdplayer and paplayer.
omxplayer has it's own audio engine.
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#7
I tested it last night. No go with transcoding. Videos are unwatchable, they buffer every second. I guess I will have to re-download the AAC files with others having AC3.

But still I have a question. My 2011 Samsung can play fine those AAC 5.1 movies and gives the amp DD5.1 through optical. How is that? To my mind there is no chance that the TV does on-the-fly AAC->AC3 transcoding...Any thoughts?
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#8
(2014-04-04, 10:11)vprasinos Wrote: I tested it last night. No go with transcoding. Videos are unwatchable, they buffer every second. I guess I will have to re-download the AAC files with others having AC3.

But still I have a question. My 2011 Samsung can play fine those AAC 5.1 movies and gives the amp DD5.1 through optical. How is that? To my mind there is no chance that the TV does on-the-fly AAC->AC3 transcoding...Any thoughts?

You are quite sure the files with AAC don't have a second AC3 audio track?
There is no AAC passththough. You can't send multichannel PCM through optical.
The only remaining possibility is your TV transcodes the audio to AC3 (which I agree is surprising).
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#9
Definately sure. Check mediainfo on the file:

General
Complete name : H:\sample.mp4
Format : MPEG-4
Format profile : Base Media
Codec ID : isom
File size : 33.0 MiB
Duration : 1mn 11s
Overall bit rate : 3 868 Kbps
Writing application : Lavf54.29.104

Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : [email protected]
Format settings, CABAC : Yes
Format settings, ReFrames : 5 frames
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 1mn 11s
Bit rate : 3 470 Kbps
Width : 1 280 pixels
Height : 544 pixels
Display aspect ratio : 2.35:1
Frame rate mode : Variable
Frame rate : 23.976 fps
Minimum frame rate : 23.881 fps
Maximum frame rate : 24.121 fps
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.208
Stream size : 29.6 MiB (90%)

Audio
ID : 2
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 1mn 11s
Bit rate mode : Constant
Bit rate : 384 Kbps
Channel(s) : 2 channels
Channel(s)_Original : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 KHz
Compression mode : Lossy
Stream size : 3.31 MiB (10%)


I tested again today. I get a full Dolby Digital 5.1 from this movie when played through my Samsung PS51D6900. When I jump inside the movie I get a delay of 10-15 secs before sound appears. So it looks like the TV is actually transcoding! Very successfuly I must say. That got me into thoughts. The TV is a 2011 model with an MSTAR processor with details:
Type No: MSTAR-X6, SoC (System on a chip) -> [a.k.a. 3DHyperReal Engine + 400Hz]
CPU: MIPS@600MHz + Parma (GPU)

If the transcoding is done by Samsung on an mstar, why wouldn't this be possible on a pi? The pi's processor is actually better!
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#10
Who tells you that they use that weak cpu for doing the transcode ... there can even be a dedicated audio processor for doing this. Also this TV can decode ac3 and downmix to stereo iirc. I doubt this is done with the mips. Mostly on the TVs the cpu is only used for the management (everything that feels so slow on those tvs - like channel sorting or the overlay menu and such Big Grin ).
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#11
ShockedShocked

These features (transcoding, downmixing, etc) are not even documented for this TV!! Not even marketed! Dedicated audio processorHuh I am completely speechless!

Will try to have a look on the undocumented features on the net...
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#12
In Europe, where over-the-air broadcasts use a mix of MP2, AC3 and AAC depending on the country, it isn't unheard of for OTA set-top-boxes to have AAC 5.1 to AC3 5.1 transcoding, to allow you to watch 5.1 broadcasts on legacy DD/DTS amps which can't handle multichannel PCM audio (as older receivers often don't have HDMI inputs). I can see that this would also be useful in a TV with a Toslink output.

Similarly the UK OTA audio description standard (which provides additional audio for those with visual impairment) which sends an additional, mono, low-bitrate (MP2 for SD, AAC for HD) signal which the receiver (set top box or TV) needs to decode and then mix with the main programme audio (2.0 MP2 for SD, 2.0/5.1 AAC for HD) prior to output. Again because you are decoding and mixing you either need multichannel PCM or transcode capability.

AIUI this is handled by the standard hardware video/audio decoding systems in the TV - not by the CPU - which is more of a controller. TVs and Set top boxes use optimised SoCs with hardware decoding and encoding facilities. They aren't really sales-worthy because they are just inherent in TV/set-top-box technology.
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#13
That explains a lot! Thanks man, will switch to my TV when I want to watch an AAC5.1 file
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