On Thu, Jan 24, 2013 at 01:51:41PM -0800, Len Ovens wrote: > Just a note on external sync/clock. The TV stations have been doing it for > years, Master sync with failover to hot backup goes to every video source > in the building (Our sync was colour black). Cable lengths (includes the > length from the video source to the switcher as well) were all the same > (or had delays to look that way) to make things line up. The problems with > jitter from long cables don't go away from adding sync to the computer > interface, they just get worse. Learning to deal with external sync is the > answer. When I was working at BRTN (which was the in analog video era - I've even known those massive 2" VTRs before they were replaced by 1" machines) we used 'reverse sync' in most cases. The destination studio would provide an error signal adjusting the source's sync and subcarrier oscillators. This made the sync independent of cable lenghts, but of course you can feed only one destination that way. The situation for (analog) video and (digital) audio is quite different. For video you need exactly the same color subcarrier frequency *and* phase *and* the H/V syncs must be aligned. For audio the only requirement is that the sample frequencies match. MADI and ADAT inputs can be designed so they can deal with a recovered sample clock that has a random phase w.r.t. the local one as long as that phase difference is constant. The consequence is a range of one sample time uncertainty in the actual delay which is usually acceptable - and as long as the connection is not interrupted it will not change. For PRO audio the solution is indeed to use a central clock [*]. This can be done even for 'worldwide' connections. A GPS/Galileo/E-Loran receiver can provide a reference frequency accurate to 1e-11 which in turn can be used to sync a master word clock oscillator. With that accuracy it takes weeks for the delay to change by one sample, no resampling is ever needed. [*] Which is one reason why a studio operating in such an environment will not switch to 44.1 kHz to produce a CD. Ciao, -- FA A world of exhaustive, reliable metadata would be an utopia. It's also a pipe-dream, founded on self-delusion, nerd hubris and hysterically inflated market opportunities. (Cory Doctorow) _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
