[mythtv] [mythtv-commits] Ticket #5900: AO: Generalise upmix and AC-3 encoding

foo bar foobum at gmail.com
Wed Nov 12 21:52:06 UTC 2008


On 12/11/2008, John P Poet <jppoet at gmail.com> wrote:
> With modern ALSA, my ice1712 based sound card is just plug-n-play.  No
> .asoundrc file necessary at all.  Foo managed to make me paranoid that
> ALSA might actually be re-sampling 44.1KHz into 48KHz, and maybe I had
> not noticed the drop in quality - since I have not used my audiotron
> in so long.

The test you outlined just shows that the card and receiver support
spdif clocked at 44.1k. It doesn't tell you anything about ALSA's
resampling or lack thereof.

ALSA will, by default, resample pcm (i.e. samples that you send to the
main device rather than AC3/DTS that you send to the iec958/spdif
device) that's not 48k. If we request 44.1k it presumably resamples
again after mixing, etc - ugh. With no asoundrc, this is probably
happening to you. I guess using a hw device (hw0:0 etc) prevents this.

The SRC we use honestly makes no audible difference (and my ears
aren't very old :). But if you really want "bit perfect" (digitally
sampled, probably lossy compressed) audio and don't mind no sw mixing
make sure that you are using a hw device for your main device.


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