[Asterisk-Users] Asterisk - fax - spandsp

[Peter Svensson]

On Mon, 16 May 2005, Steve Underwood wrote:

> >It is possible, though complicated, to synchronize the 2Mbit clocks on two 
> >unrelated cards by measuring the accumulated phase shift (difference in 
> >interrupt rate) over time and compensating, thus implementing a PLL in 
> >software. Digium has not shown any intereset in such a solution. It is not 
> >clear if the internal hardware clock generator can be fine tuned enough to 
> >implement this.
> >  
> >
> How can that work? You can measure the error, but you have no ability to 
> tweak the clock from software. Two cards could only be synced by hardware.

In most hardware the clock you use is not provided by a crystal. Rather
the crystal provides a reference for a pll. The conversion factor between
the crystan and the derived clock is usually tunable.

Whether the actual clock on the Digium cards is tunable enough I do not 
know. There are quite a few references to programming the clock in the 
source. 

Peter