[LAD] resampling with sp/dif

Joshua Boyd jdboyd at jdboyd.net
Mon Apr 19 15:07:35 UTC 2010

On Mon, Apr 19, 2010 at 12:38:35PM +0200, Conrad Berh?rster wrote:
> Hello, 
> i want to grab some external sources (cdplayer)  with jack. i run a jack 
> instance with 48000 Hz and it works perfectly with my cd player and ardour. 
> but if i take a cheaper cdplayer, the signal will only be came in with
> 44100,  because there seems no resample and sync between the master
> (soundcard) and  slave (cheapoCD) . so, the signal will always being
> played to slow. i found a  workaround with a small external box, which
> resamples the incoming signal  before it get into the soundcard. 

Your spdif sound card might have a sample rate converter.  If it did,
most likely the work flow would be to set jack at 48k and then just play
normally and the sound card would convert from the incoming 44.1k to the
48k it is set to.  The SRC chip in your sound card (again, presuming it
has one, which I think is likely) is probably similar to whatever is in
the external box you've tried.

> Since a while, i think about a software solution, which puzzles me a
> bit. if i  doing the process without the  external resampler, what
> must be done with the  signal. 
> If the original source is 44100 Hz, the incoming signal is 48000 (because of 
> jack). if i try to resample it, the current buffer get shorter, because the 
> signal need to be decreased. 
> How can a correct resampling process be done? 

Unless there is a real need to do this in realtime, say for a live
performance, I would suggest just ripping the CD and using a software
sample rate converter on the resulting files.  I'd be surprised if this
couldn't be done in better than realtime on todays systems, meaning you
might be able to rip and convert a 60 minute CD in under 30 minutes.

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