[LAU] open hw soundcard (was Re: status usb2 for sound)

Folderol folderol at ukfsn.org
Fri Nov 13 17:15:15 EST 2009

On Fri, 13 Nov 2009 22:37:33 +0100
fons at kokkinizita.net wrote:

> On Fri, Nov 13, 2009 at 07:56:13PM +0000, Folderol wrote:
> > The effect you can get with a system with poor stability but very strong
> > locking when incoming pulses are half way between the wanted times
> > (used to happen a lot with early discrete PLLs). The system will
> > alternately lock on the early and late pulses. The correction waveform
> > looks like a cog railway :)
> - To generate a clock that is good enough for audio sampling you'll 
>   need a analog PLL based on an Xtal oscillator, and with less than
>   1ns (nanosecond) jitter in the audio BW, and preferably even less.
> - All AD converters require a clock that is much higher than
>   the sample frequency. For example the TI ADS1278 requires 
>   27 or 37 MHz. 
> - If you want the soundcard to lock to a reference provided by
>   the master (PC), all you have is the timing of Ethernet 
>   messages. For example the PC could send a message every
>   millisecond. With the above clock frequencies that would
>   mean a PLL multiplier with a ratio of 27000:1, somewhat
>   more than the 1:1 you seem to assume. It would need a BW
>   of a fraction of a second. Can be done, but not simple.
> Ciao,

I fully appreciate what you are saying here. I have been largely
thinking out loud, and also mostly in the digital domain where
timing variations can be far more coarse-grained.

The fact there is software out there that enables collaboration via the
internet demonstrates that it is do-able. We just? need to think out
how it can be done with as simple hardware as possible :)

Will J Godfrey
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.

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