Re: [Jack-Devel] Ethernet-based audio interface using (net)jack idea

On Tue, Aug 16, 2011 at 12:27 PM, Fons Adriaensen <[hidden]>wrote:

> On Mon, Aug 15, 2011 at 11:03:15PM +0200, Jeroen Van den Keybus wrote:
>
> > It think this may eventually end up by you generating a square wave
> > synchronous to the Jack server sync packets, and feeding that signal back
> > (externally - FPGA PLL inputs are always dedicated inputs) into one of
> the
> > FPGA PLLs to generate MCLK. Beware that the square wave will really need
> to
> > be 'good' in a sense that simply clocking the FPGA PLL with sync pulses
> will
> > not do (too low a frequency - too much jitter). Therefore you will need a
> > digital PLL (= VHDL file) first. Beware that clock generation in this
> > fashion isn't common at all (let alone recommended), but since it would
> save
> > so much trouble processing the data, I think it's worth trying...
>
> I seriously doubt if you can get a sampling clock with the required
> low jitter using the FPGA's digital PLLs. The next thing to look at
> would be an external VCXO module to generate the master clock, with
> the loop closed by digital filtering in the FPGA followed by an
> analog lowpass driving the VCXO. Lock range for VCXOs is small but
> it should be enough to cover typical sound card frequency
>
> Ciao,
>
> --
> FA
>
>
My thought too was to use a circuit away from the FPGA - and a VCXO sounds
like a viable approach. Would it also be possible to have one clock as the
master for the entire system? I was googling and came across one of the
basic LM567 circuits...but for this project I'm sure that there is too much
'tolerance' in an IC like that.

As for Jeroen's approach in terms of having the FPGA just generate the data
that's needed for JACK...I'm not sure why I wasn't thinking of it that way!
A lot of things fell into place in my head when this was mentioned, so I've
started looking at the netsource.c file from the JACK source code (JACK1 for
now) to begin to understand how the packets are sent and received. For the
JACK programmers - based around this file, what other parts of the source
code should I be looking at?

Thanks everyone,

Dan

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