Re: [xwax-devel] It works! USB soundcard American Audio Versaport

[Mark H. <ma...@xw...>]

On Sun, 23 Sep 2012, Olivier Gauthier wrote:

> Hi !
> 
> I finally came to get both decks working on my American Audio Versaport
> usb soundcard.
> 
> At first I could only use the first deck, but then I figured out that
> using a .asoundrc file similar to this one
> http://www.pogo.org.uk/~mark/linuxdj/u46mk2.asoundrc made it work really
> easy.
> 
> I now can mix using only one usb sound card, that's perfect.
> 
> I updated the wiki with informations I got using that usb sound card,
> along with the .asoundrc file I'm using.
> http://wiki.xwax.org/list_of_soundcards#working_usb_soundcards

Thanks for the contribution!

> It sadly doesn't have two preamps, only deck have a mechanical button to
> activate an (cheap) internal analog phono preamp. This interface is been
> used with multiple commercial DVS software as MixVibes and Reloop, they
> most probably use an software phono preamp from the documentation I have
> read. The internal preamp is so cheap that it's worthless for timecode
> application.

Actually, you probably only need the cheapest preamp for timecode, rather 
than a particularly good one. The signal is very easy to decode and robust 
to poor quality equipment (think how bad the signal gets during 
scratching)
 
> I managed to build a patch that provides a simple 2-pole biquad
> equalization filter to apply the RIAA de-emphasis curve. I then amplify
> the signal 40 times (~32db) to get the timecode displays nice and clear.
> I've been thinking about intergrating a software phono preamp in xwax
> since last year, but I just recently managed to succeed with the
> implementation. I tested it this weekend, the software preamp didn't
> crash xwax and the touch feeling is pretty much identical to my analog
> preamps.
> 
> Mark, do you think it would be a worthy feature to add to xwax mainline
> code base ?

I think it's a clever implementation, and would be interested to see it 
(as I'm sure others would be) but I don't think it justifies inclusion. 
Perhaps not for the reasons you're expecting, here's why.

For timecode, you don't need an RIAA preamp at all, just an amplification 
of the signal. Adjusting this way uses CPU time and could actually make 
the decoding harder.

And then in xwax's case, the timecode decoder is self calibrating. The 
only part of the timecode decoder which is not (and is absolute) is the 
ZERO_THRESHOLD. Easier than implementing a pre-process to amplify the 
audio (which can result in clipping) is to adjust this level.

So if you have a soundcard with a low enough noise that a 'software 
preamp' would hope to work, then all that should be needed is to lower the 
ZERO_THRESHOLD, but still keep it above the noise floor of the soundcard. 
Note that I haven't actually tried this, give it a try and let me know?

Hopefully you can see how this is used in detect_zero_crossing() -- to 
control what values are ignored as not an actual crossing.

So better would be some kind of control over this parameter, but even 
better to auto-calibrate this value or eliminate it entirely. It could 
then be fully automatic and would adapt to variable conditions, including 
the 'software preamp' case.

Hmm, that's got me thinking. Maybe this could be made more robust and 
better.

> I would polish my beta code then try to submit a patch file
> if you'd like to integrate this in a later release. I believe that this
> feature would simplify lots of user's DVS systems and open new
> possibilites (affordable ones).
> 
> 
> 
> I leave you with good live mixsets and mixtapes made solely with xwax.
> Some of those are live freeparties recordings, don't expect studio
> quality mixing techniques!
> 
> http://archive.org/details/06_l_ensemencement_de_la_tekno_2012_05_05
> http://archive.org/details/FolieFurieuse_mixset_madhouse_dirtycake2012-05-31
> http://soundcloud.com/foliefurieuse/springdaklub-mix
> http://archive.org/details/Kalabrese-SummerStretchingMix
> 
> Go post more of your mixes on http://wiki.xwax.org/ourworks !

Thanks for the mixes! Nice one.

-- 
Mark