[mythtv] Fwd: iec958 output (not passthrough) with Alsa rate pluginproblem

Lawrence Freil lef at freil.com
Tue Jan 16 00:30:35 UTC 2007


On Jan 15 2007, John P Poet wrote:

>On 1/15/07, Mark Spieth <mark at dclabs.com.au> wrote:
>>
>> > Hello,
>> >
>> > I have discovered a little problem with MythTV and audio output.  For
>> ac3
>> > output I'm using the passthrough feature to the iec958 device to a
>> > surround
>> > sound reciever.  This works fine, however when the audio is not ac3 I
>> also
>> > want the output to go to iec958.  I have created a rate plugin pcm
>> (hq48k)
>> > for alsa to change the rate from 44.1 to the 48Kz required by the
>> digital
>> > output device and then set the non-passthrough device to ALSA:hq48k.
>> Using
>>
>> is this feature desirable? if so Ill add it to the multichannel audio
>> patch.
>> cheers
>> mark
>>
>
>Why not do the rate conversion in your .asoundrc file?
>
> Not all sound cards require 48KHz. For example, sound cards based on the 
> Envy24 chip will pass through 44.1KHz just fine. The reason I have a 
> Envy24 card in my frontend, instead of using the onboard sound, is to get 
> 44.1KHzwithout any kind of conversion.
>
>MythMusic sounds *MUCH* better (to my ears) with my Envy24 based card
>compared to my old SB Live!  I attribute this to it's ability to pass
>through 44.1KHz without having to resample.
>
>I believe I read that Intel's new HD audio spec also will pass through
>44.1KHz without needing it converted to 48KHz.
>
>
>John
>

Not having the rate conversion would certianly be desirable, however it is 
not possible for everyone. In my case I am using the .asoundrc and the alsa 
rate plugin to create a high quality rate conversion and the sound is very 
good. i.e.

pcm.hq48k {
     type rate
     slave.pcm "iec958"
     slave.rate 48000
     converter "samplerate_best"
}

This is easy enough to do, that providing instructions on how to setup in 
the wiki is probably sufficient. If there were a way to determine by 
probing the hardware that a rate conversion was required then I would 
recommend it being automatic and in myth, however there are also other 
reasons beyond rate to setup local pcm's. I wouldn't mind updating the 
wiking on this as soon as he flow control issue is solved. Now if I could 
just figure out how to get alsa to map a OSS emulated /dev/pcm to an 
arbrary plugin pcm instead of just mapping to hardware pcms. Some of the 
software still likes to use oss devices.
-- 
Lawrence Freil
lef at freil.com


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