[mythtv-users] HDMI ac3 passthough audio

[Steven Adeff]

On Thu, Feb 12, 2009 at 10:36 AM, Joel Means <means.joel at gmail.com> wrote:
> On Thu, Feb 12, 2009 at 8:11 AM, Joel Means <means.joel at gmail.com> wrote:
>> Just one last update from me.  I updated to ALSA 1.0.19 with no luck.
>> I will also point out that I have tried all of this with and without
>> Pulseaudio installed.  I have a feeling that Abit just screwed up
>> their implementation of this chipset, since it seems to work fine for
>> other people on Asus motherboards.  The sad thing is that I bought
>> this motherboard specifically for the HDMI port and took a long while
>> to decide between Abit and Asus.  In the end, I let ~$20 decide for
>> me.  Oh, well.  Live and Learn.
>> Joel
>>
>
> One other note and questions on this.  I did note that with the
> installation of ALSA 1.0.19, I have an additional entry in the output
> of 'aplay -L' which wasn't there before.  Now, the last three entries
> are:
>
> iec958:CARD=NVidia,DEV=0
>    HDA NVidia, ALC888 Digital
>    IEC958 (S/PDIF) Digital Audio Output
> hdmi:CARD=NVidia,DEV=0
>    HDA NVidia, NVIDIA HDMI
>    HDMI Audio Output
> null
>    Discard all samples (playback) or generate zero samples (capture)
>
> Before, I didn't have the hdmi entry.  I could, however, use "hdmi" as
> an output device for speaker-test or mplayer and get sound over hdmi.
> It just wasn't listed here.  I am wondering if someone who has the
> same ALC888 controller (preferably with the GeForce 7050 chipset) and
> has AC3 passthrough over HDMI working can post their output of 'aplay
> -L'.  The reason I ask is that mine lists iec958 as being a Digital
> Audio Output, but hdmi is only listed as being Audio Output.  I don't
> know if that is relevant or not, but I would be interested in seeing
> what another mobo has to say about it.
> Thanks,
>
> Joel


pre-ALSA 1.0.19, HDMI was "supported" but indirectly, requiring
asound.conf hacking. 1.0.19 explicitly supports HDMI, as you see, by
listing it as a default device.

when you do an aplay -L what you get listed is what devices ALSA
creates for your sound card as a default.

the asound.conf file lets you 'override' those with your own devices.

plughw is a conversion layer for hw, depending on how you use it, can
allow conversions for rate, channels, etc. From what I understand,
used alone, will only perform rate conversion for PCM output but allow
for bitstream'ed content to pass to the hardware untouched.

hw is the direct channel to the device, unfettered by all the
conversion layers that ALSA can perform. as such its very powerful,
but requires specific intent and knowledge about your hardware and
what your trying to accomplish.

This is what I learned at least, and why I went with setting MythTV to
use plughw as it would allow for the required rate conversions when
playing back stereo audio as PCM but then also pass AC3/DTS untouched
when used for that (ie -ac hwac3,hwdts in mplayer).

Anyway, this is what I was able to gather when getting HDMI audio
working on my mom's computer, for which I was successful, so at least
some of it must be *true enough* to have gotten it to work =P

-- 
Steve
http://www.mythtv.org/wiki/index.php/User:Steveadeff
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