[mythtv-users] HDMI ac3 passthough audio

[Steven Adeff]

On Thu, Feb 12, 2009 at 12:45 PM, Joel Means <means.joel at gmail.com> wrote:
> On Thu, Feb 12, 2009 at 10:38 AM, Steven Adeff <adeffs.mythtv at gmail.com> wrote:
>> 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
>>
>
> Good info.  Interestingly, even when hdmi wasn't listed in 'aplay -L'
> output, I could still use it as a device to send audio through HDMI.
> I got to looking at the files in /usr/share/alsa/cards (specifically
> HDA-Intel.conf) and found that the hdmi device was listed in there.  I
> am not sure why 'aplay -L' didn't show it.  In the same way, using an
> older version of ALSA, 'aplay -L' didn't show the iec958 device, but I
> could always use it.
>
> Joel

right, the hardware is/was still supported, it just wasn't set up with
a device name, so you could use the asound.conf to give it a device
name, or use plughw/hw to tell your application to talk to it
directly.

as to whether the older ALSA versions fully supported HDMI for audio I
don't know, but 1.0.19 supposedly can (bugs not withstanding =P )


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