25 Jan
2013
25 Jan
'13
2:43 a.m.
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)