6-channel PCM audio with Pi 3?

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TenMinJoe
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6-channel PCM audio with Pi 3?

Post by TenMinJoe » Thu Mar 15, 2018 9:08 am

Hello -

I have a Pi 3 set up as a frontend, connected with HDMI. I installed Raspbian and the mythtv-light package.

Some UK DVB-T2 broadcast TV is in 5.1 surround, in some crazy format (HE-AAC?). Is there a setting that would let me watch this in surround sound? With my old "normal" PC frontend, I have it sending multi-channel PCM out over HDMI. However, I can't seem to figure out what settings would make this work on the Pi. I can watch the surround-sound recordings but it seems like it's sending stereo.

Since I don't think my receiver will understand the original audio stream, I don't want any kind of "pass-through" setting - it seems like sending PCM would be best, but if there's a setting for converting to DD or DTS then that would be okay too.

Thanks in advance for any help with this.

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pgbennett
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Re: 6-channel PCM audio with Pi 3?

Post by pgbennett » Thu Mar 15, 2018 5:11 pm

Did you follow the procedure described here?
https://www.mythtv.org/wiki/Raspberry_P ... _with_ALSA
This should let you select the same settings as you use on a desktop system. It should be able to send multi channel pcm.

TenMinJoe
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Re: 6-channel PCM audio with Pi 3?

Post by TenMinJoe » Thu Mar 15, 2018 10:19 pm

Thanks for the reply, I appreciate it.

I tried those instructions - I added the bcm2835.conf file as described, and rebooted, but I still don't have an ALSA:IEC958 device showing up in the MythTV audio config. I tried typing it in manually, but it indicated that it was an invalid device.

I tried the following command to list ALSA devices:

Code: Select all

pi@raspberrypi:~ $ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: ALSA [bcm2835 ALSA], device 0: bcm2835 ALSA [bcm2835 ALSA]
  Subdevices: 7/7
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
  Subdevice #4: subdevice #4
  Subdevice #5: subdevice #5
  Subdevice #6: subdevice #6
card 0: ALSA [bcm2835 ALSA], device 1: bcm2835 ALSA [bcm2835 IEC958/HDMI]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
I'm not sure how to debug further. Grateful for any more suggestions.

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pgbennett
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Re: 6-channel PCM audio with Pi 3?

Post by pgbennett » Fri Mar 16, 2018 12:22 am

This is the list I get when I press enter on the audio output device selection. What do you see? If you do not have the conf file correctly installed you will see the list without the iec958 entry.
Screenshot_2018-03-15_20-14-13.png
Screenshot_2018-03-15_20-14-13.png (190.79 KiB) Viewed 216 times

TenMinJoe
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Re: 6-channel PCM audio with Pi 3?

Post by TenMinJoe » Sat Mar 17, 2018 12:44 pm

I don't get a pop-up list, is it possible you're using a later version of Myth? I'm on 0.28.1-59-g7c931f1752-0on. Maybe I need to look at Myth 29.

The contents of my list are the same as yours, with the exception that the iec958 entry is missing. I definitely put the /usr/share/alsa/cards/bcm2835.conf file in place, with the contents described.

All that aside - from my research, I'm increasingly convinced that the Pi version of ALSA won't transmit 6 channels of uncompressed PCM over HDMI. Everything I read about adding the bcm2835.conf file seems to be related to passing through AC3 or DTS, which isn't what I want at all.

The closest I've come is to use the OpenMAX:hdmi device, which DOES output 6 channels of PCM when I'm watching a 5.1 Myth recording, but the LFE and the centre channels are swapped! Definitely not a wiring problem, other things work okay. The "omxplayer" command-line app is able to correctly play a 6-channel FLAC file, so it seems like it's maybe a Myth bug that has those two channels swapped.

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Re: 6-channel PCM audio with Pi 3?

Post by TenMinJoe » Sat Mar 17, 2018 1:06 pm

Update - if I go into the "advanced" options, under "Separate digital output device", it DOES list iec958, but as follows:

ALSA:iec958:{ AES0 0X02 }

and if I select it, it tells me "Passthrough device is invalid/unusable. Check configuration in Advanced Settings:ALSA:iec958:{ AES0 0x02 }"

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