[mythtv-users] Raspberry Pi audio centre and sub swapped

Andrew Wilkinson andrewjwilkinson at gmail.com
Sun Dec 3 08:08:43 UTC 2017


Switching to ALSA from OpenMax resolved this issue, but not in the way I
expected. Rather than needing to remap the channels for multi channel pcm
after switching to ALSA the audio is encoded into Dolby Digital, which has
the channels mapped correctly.

I had some trouble switching to ALSA because I only had OpenMax options. It
turns out my user wasn't in the audio group.

Hope this is useful to others in the future.

Andrew

On Mon, 27 Nov 2017 at 01:04, David Hampton <mythtv at love2code.net> wrote:

> On Sun, 2017-11-26 at 19:47 -0500, Peter Bennett wrote:
> >
> > On 11/26/2017 05:03 PM, Andrew Wilkinson wrote:
> > > Hi,
> > >
> > > I recently tried to start watching the BBC's Blue Planet 2, but
> > > unfortunately I have a problem with the audio. It's broadcast in 6
> > > channel AAC which my onkyo can't decode, so for the first time I'm
> > > using multichannel PCM.
> > >
> > > Front left and right, and rear left and right all work fine. The
> > > centre and sub woofer are swapped though, which doesn't do David
> > > Attenborough's voice any favours. I can confirm this using the
> > > speaker
> > > test screen, which also uses multichannel PCM.
> > >
> > > I'm using a Raspberry Pi and OpenMax audio. Is there any way to
> > > correct the channel mapping? I suspect I might be able to use the
> > > vcgencmd hdmi_channel_map command, but I've no idea how to
> > > generate
> > > the correct value.
> > >
> > > Any insights you can provide would be much appreciated.
> > >
> > > Thanks,
> > > Andrew
> > >
> > >
> >
> > Does the HDMI connection go direct to the Onkyo? If you are using a
> > toslink optical or coax connection, they don't support multi-channel
> > PCM.
> >
> > Make sure you have selected the correct number of speakers in the
> > "Speaker configuration". You can also try switching this between 5.1
> > and
> > 7.1. Different selections will change the way the channels are
> > mapped.
> >
> > Another thing you can try - there are two ways to get digital audio
> > from
> > Raspberry pi, using ALSA or OpenMAX audio (see
> > https://www.mythtv.org/wiki/Raspberry_Pi#Digital_Audio_with_ALSA and
> > https://www.mythtv.org/wiki/Raspberry_Pi#Digital_Audio). I believe
> > the
> > ALSA method is superior, so I would try that.
>
> I had the swapped center/subwoofer problem many years ago with a
> different receiver/amplifier.  IIRC, I fixed it by remapping the
> channels in the ALSA configuration.  See the "Zotac ION / NVIDIA ION"
> section of https://www.mythtv.org/wiki/Digital_Audio_Tutorial for an
> idea of how to do this.
>
> David
>
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