[mythtv-users] .24 Audio results: mixed

Jean-Yves Avenard jyavenard at gmail.com
Fri Jun 25 06:08:46 UTC 2010


Hi

On 25 June 2010 15:09, Douglas Peale <Douglas_Peale at comcast.net> wrote:

>> Nothing was changed here... did selecting iec958 ever worked ?
>
> No.

probably an ALSA bug then ...

>> Do you have to do it all the time, or just after a restart?
>
> Almost but not quite always after a restart
> Almost never, but sometimes after I have exited Myth and restarted Myth

your answer is a bit confusing.

If it's mainly after a restart, you could use alsactl and mute /
unmute by software and it will fix the issue...

>
> You read more into that suggestion than I intended. I only meant change the text "5.1" to "Digital Pass through" in the drop
> down menu when an HDMI or SPDIF output was selected. In those cases, usually you could send the max available channels and let
> the receiver downmix to the number of speakers actually available.

I think you haven't read the explanation I gave earlier carefully..

What you're suggesting wouldn't cover all the situations. What about
people using analog audio?

If you have a motherboard with a 7.1 analog output, it appears the
same as a an hdmi port which is digital.
The only information myth get is that it supports up to 8 discrete
channels. That's it. there's no distinction between digital and analog
; as in both case you would send a PCM stream anyway.

As I explained earlier, Myth reports 5.1 for an audio card reporting
as supporting stereo only, but the user checked AC3 and/or DTS.
You can not reliably check with ALSA if a device is a digital or an
analog one. This is why you have the AC3 and DTS check-box: for the
user to tell mythtv what it is

While an hdmi port may support up to 8 discrete channels, not all
audio equipment supports 7.1 LPCM in ; only hdmi 1.3...
I have a Yamaha system, if I feed it 7.1 LPCM, all it plays is the
front and right speaker. It will only play 5.1 audio if encoded as AC3
or DTS.

That's what the LPCM flag is for..
It tells myth if the audio can be sent as discrete channels, or needs
to be encoded (or re-encoded) as AC3.

Say the system is :
case a) hdmi -> amplifier , amp support 7.1. LPCM, AC3 and DTS box are checked,
case b) hdmi -> amplifier , amp support 5.1, only AC3 and DTS box are checked,

You are playing a stereo audio, with the default 5.1 upmixer enabled.

case a). stereo is upmixed in 5.1, send to the hdmi port as is (PCM)
case b) stereo is upmixed in 5.1, re-encoded in AC3, send to the hdmi
port (as AC3 stream).

If you are now playing a 44.1kHz 5.1 AC3 audio, but your audio card
only support 48kHz (like what my HDMI port reports).

case a) 5.1 44.1kHz AC3 is decoded, resampled to 48kHz and send to the
hdmi port as is (PCM)
case b) 5.1 44.1kHz AC3 is decoded, resampled to 48kHz, re-encoded as
48kHz AC3 and send to the hdmi port (as AC3)

If the amp support multi-channels LPCM, there is no need to re-encode
to AC3, it wouldn't give any advantage, quite the opposite as this
conversion is lossy.

>
> I just tried to use the analog inputs on my receiver. I don't like the hiss I get, I like the silence of SPDIF even with the
> bugs. I'm going back to the digital input.

if using analog ; select the audio card that supports 5.1 output (most
system it's called surround51)

uncheck the AC3 and DTS bit, and check the PCM flag

>
> Scratch that, I found I could kill the hiss by turning off all of the analog inputs. I'll give Analog a try for a while.

If you were to simply unmute via a little command (no need to do it
manually) it would probably make your life easier


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