Re: USB card @48kHz only: at what layer to resample?

[Sergei Steshenko <steshenko_sergei@list.ru>]

On Sun, 24 Sep 2006 08:12:09 -0700 (PDT)
Bill Unruh <unruh@physics.ubc.ca> wrote:

> On Sun, 24 Sep 2006, enrico_groups@libero.it wrote:
> 
> > Dear All,
> >
> > Apologies in advance if I am asking a stupid question, I am quite new with alsa.  I posting to both lists because I am not sure about handling this issue at the application development level or at the alsa driver configuration level.
> >
> > I am developing an audio application using the CCRMA synthesis tool-kit (STK) in Fedora Core4 using alsa.  I recently bought a Terratec Aureon 5.1 USB MKII card, because I needed to have better sound quality than what offered by my laptop.
> >
> > My understanding is that the usb card only works at 48kHz -- is this correct?
> > I manage to get aplay to work using the following plugin in /etc/asound.conf
> > pcm.!default {
> >    type plug
> >    slave {
> >        pcm "hw:2,0"
> >        rate 48000
> >    }
> > }
> >
> > However, in SKT I can enumerate the audio cards on my machine, but this does not give me alsa plugins, only the audio cards.  If I try to open the Aureon at 44.1kHz, I get an error saying rate not supported.
> >
> > Is there any way to have alsa handle the rate conversion for me?  Shouldn't or couldn't the driver take care of this?  (Was it my mistake in installing the driver? I can post my /etc/modprobe.conf if it helps)
> > Or is there any way to get STK to see the plugin?
> 
> This is audio card insanity. All CDs are at 44100 but for some reason many
> card manufacturers designed their cards to run at 48000. This is a real
> problem since this requires rate conversion. Rate conversion is in general
> a very very noisy operation. In particular simple linear interpolation,
> which is quick, is also really really terrible for sound quality.
> 
> If you want to see some tests using the sox rate conversion facilities, see
> www.theory.physics.ubc.ca/soundcard/resample.html
> 
> Ie, you would be far far better off, rather than using an alsa rate
> conversion, to use say sox to do the rate conversion for you  (using either
> resample or polyphase) and then play that through your 48000 card. Ie, why
> in the world would you buy a really good sound card and then feed it crap.
>   As far as I know, the alsa conversion facilities are still primative
> although discussions are there to improve it. 
> (I suspect the rate conversion in Windows is also crap, but do not know.)
> 
> 
> See the discussion in
> http://www.hydrogenaudio.org/forums/index.php?showtopic=47591
> The problem with using good samplerate conversion is that it is expensive
> in processor time.
> 
> 
> 
> >
> > Thank you in advance,
> > Enrico
> >
> >
> >
> >
> >
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> 

Takashi Iwai meant to include quality resampler into ALSA - I do not know
what the status is.

A LADSPA plugin can be created for the purpose.

Anyway, there are good quality resamplers, and they consume less than 5% CPU
even on old CPUs.

--Sergei.

-- 
Visit my http://appsfromscratch.berlios.de/ open source project.

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