[LAD] AoIP question

Len Ovens len at ovenwerks.net
Sun Oct 5 21:28:16 UTC 2014

On Sun, 5 Oct 2014, tom at trellis.ch wrote:

> On Sun, October 5, 2014 22:51, Len Ovens wrote:
>> On Sun, 5 Oct 2014, tom at trellis.ch wrote:
>>> Hi, i'm following the thread of the ongoing topics around transmitting
>>> audio over IP infrastructure. 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?
>> AES67 (and other formats) use PTP (IEEE 1588-2008) to keep the system
>> clocks aligned at a usec level. The media clock is then derived from that.
>>  The media clock on both systems should therefore be syncronis.
> ok, you can make the systems "in-sync" with PTP. but are there audio
> interfaces that can use the clock from the host? i.e. the host drives the
> audio interfaces clock.

Yes, look at the rednet series as an example. However, I think you are 
talking about ALSA interfaces. The audio that comes from the audio 
interface is already very loosely synced, One transfer in 32 samples or 
more and expects the data from the computer to arrive in time for the 
clock on the IF to clock it out. So the IF has to be built for network 
timing. It may be possible to generate a clock signal using a serial or 
parallel port from the ptp clock to externally clock an alsa AI, but I 
have not tried it. The audio interface would have to have a clock input 
for this to work and it may require some external wiring, etc. for 

It is not likely that it is possible to use an ALSA audio interface on two 
machines and link them syncronisly. Computer audio cards in general are 
not built to work that way. However, there are starting to be a lot of 
AoIP interfaces out there that could be used as the computer AI, but don't 
expect to use them along with the average ALSA card. However, if you wish 
to have an audio IF in your studio, one in your control room and one 
somewhere else... with AoIP this is possible and all computers connected 
will see the three IFs as if they were one.

Len Ovens

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