[LAD] AoIP question

Winfried Ritsch ritsch at iem.at
Fri Oct 10 19:35:06 UTC 2014

Am Sonntag, 5. Oktober 2014, 23:35:21 schrieb tom at trellis.ch:
> On Sun, October 5, 2014 22:25, Fons Adriaensen wrote:
> > On Sun, Oct 05, 2014 at 08:39:11PM +0200, tom at trellis.ch wrote:
> >> As a scenario, at point a) an analog signal is injected that will be
> >> played back (analog) at point b) with the lowest possible (and constant)
> >> 
> >>  latency. How do you intend to handle diverging clocks of the audio
> >> 
> >> interfaces (ADC/DAC) at both (a/b) ends?
> > 
> > Either
> > 
> > 
> > 1. Sync the HW sample rates to an explicit or implicit reference
> > provided by the network protocol. Requires special audio HW. A few normal
> > audio interfaces (e.g. some RME cards) would allow to do this as well, but
> > I know of no software that uses this
> > capability.
> Sounds interesting. I asked myself if it would be possible to drive the
> audio interface clock by the host (not by a wordclock coming from another
> audio interface which makes the "network" case local again to some
> degree), i.e. through ticks sent via FW/USB.
just to add another options and be a wiseacre :

On a microchip dsPIC [1] you can adjust the internal oscillator (RC) (where 
also the sample clock for DA is derived) in a small range of 10%, so it is 
possible to use PTP for sample-exact synchronization (I did do a "soft PLL" 
for a low cost solution of a DA to avoid re-sampling and big buffers). I was 
told on ARMs Oscillator tuning is also possible, and if audiohardware derives 
clocks from it, it can also be tuned and used as soft PLL. 

Anyway there are Chips [2] which can generate sample-clocks (Master-Clock) 
from a PTP signal, so if your audiohardware supports Master-Clock, you can add 
such a chip ;-).

Anyhow PTP aware switches are only needed if absolute time is important 
(tracking systems), so with a constant latency, PTP also over a Switch border 
is no problem for sample accurate synchronicity, a router could be a bigger 

[1] http://ww1.microchip.com/downloads/en/DeviceDoc/39700c.pdf page 10
[2] http://en.wikipedia.org/wiki/List_of_PTP_implementations#silicon

mfg winfried

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