25 Jan
2013
25 Jan
'13
3:45 p.m.
On Thu, January 24, 2013 5:43 pm, Fons Adriaensen wrote:
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.
Our on air was 2" machines. 5 of them. A very busy place during commercial
breaks.
For out of station feeds we had a (at the time very expensive) frame store
unit for sync.
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
And the frame which is actually two vertical scans... at least over here
in NTSC land. (NTSC = Never Twice the Same Colour)
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.
Yes for audio (MADI, ADAT, AES3 or whatever else) phase is not the problem
it once was for video. I would imagine the video world has added frame
store to all the inputs of the switchers by now.
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.
This is the difference from video. Video can actually be somewhat out
frequency wise as the frame store can "cheat" during the V retrace to keep
things aligned. Audio can't do that, it has to be the same frequency (if
not phase) or there are artifacts.
--
Len Ovens
www.OvenWerks.net