[LAD] AoIP question
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  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  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
 http://ww1.microchip.com/downloads/en/DeviceDoc/39700c.pdf page 10
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