On 2014-04-27, Rob <nom...@example.com> wrote: > Paul <tik-...@bodosom.net> wrote: >> I don't know what "terribly accurate" might be to you but in the real world >> sufficient accuracy depends on the circumstance. >> >> Someone should conduct an experiment. > > I am in a group that works on a project that needs synchronous audio > on geographically distributed PCs. > > Of course there are several hurdles. > First, we sync all machines to locally connected GPS receivers with > PPS output. We use ntpd and kernel PPS. This is wellknown territory. > > In the ntpq -p stats this appears to bring the systems within 10us, > often within 2us, of the PPS signal. We still have to find out if this > is reality or just output of a program.
reality. You can test the interrupt latencies on the machines (a few us) > > The next problem is to send output to a soundcard and making it send > a sample at the sampling clock edge closest to a specified time. > (48kHz sampling rate corresponds to a sampling clock period of 20.8us) It will certainly depend on the sound card. AFAIK most have their own internal oscillators, that are not adjustable. I suspect you will have to build your ownsoundcard to accomplish this. > > When that has been achieved, it of course is easy to wire up two of > those systems, place them closely together, and check on a scope. > > It could be further improved when we can somehow lock the sampling > clock to the PPS signal, so another +/- 10uS uncertainty/jitter is > removed. 48KHz is 20us. Why are you worried about a few us? What are you trying to do? _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions