linuxaudio.org

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

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> 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.
>

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> 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

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> 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

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> 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
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