[mythtv-users] mythmusic iec958 help

Allen Edwards allen.edwards at oldpaloalto.com
Sun Jul 6 22:00:19 UTC 2008


>
> Well, I was a little tired this morning so I may not have been to
> coherent. The two things I really want is simultaneous analog and
> digital output. I'm really tired of leaving the receiver on all the
> time just to watch TV when the speakers built into the TV would be
> good enough. The other thing I want is bit perfect audio, and since I
> have no budget for new gadgets, I want to do everything I can to get
> it with my nForce chipset. I may be asking too much...
>
> Even though I had everything working (all digital out) using the
> asound.conf from the wiki one consequence was that alsa mixer had no
> effect, which I thought was strange and as an engineer I couldn't
> accept that :) It was really weird, I could mute/unmute the iec958 and
> it didn't mute the audio...
>
> A third thing I really want is consistency... mythmusic was MUCH
> louder than other audio and then there is mplayer in the mix for AVI
> files.
>
> RIchard

I also want bit perfect audio and analog sound for my TV.  I tried
another experiment this morning.  Probably spent 3 hours on it just
today.  I have spent many more than that working on this and
researching it.  As far as I can tell, it isn't possible without a
modification to the sound card driver.

To get bit perfect audio, you must bypass ALSA by specifying either
iec958 or ALSA:spdif.  If you don't, then ALSA will decode - process -
encode and resample if needed.  This is not bit perfect and apparently
the resampling isn't very good.  You can't get analog without ALSA
processing.  Taken together, these two statements mean we can't get
what we want.

Today, I had hoped to find that the headphone output was active when
spdif was.  I could not tell from reading the Realtek data sheet.  I
tried it and it is not.  That was my final hope, other than a new
driver.

My solution is a Technics sound processor.  The non DTS version shows
up on ebay for about $20.   The one I have does DTS and sells for
about $100.  For watching TV, I don't think you need DTS.

I also want to get rid of the Technics but as it is only about 8
watts, I just leave it on. I am sure the hardware can do it but we
will have to wait for the software to catch up to what we want.

Allen


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