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Audio sync of multi-OBs (Read 4912 times)
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Audio sync of multi-OBs
Nov 19th, 2012, 5:44pm
On a sister forum, questions have been asked about the audio content of large multi-source TV programmes such as the recent "Children-in-Need".

"When there are choirs all over the country singing together, how do they stay synchronised, bearing in mind all the distances and delays involved?"

Tony Revell, (Sound Supervisor) kindly described the techniques involved.

As it was I who was originally asked by the producers of Children in Need to devise a way of getting the 14 contributors into the programme singing live at the top of our show back in the 90s, so you can hear it from the 'horse's mouth'.

This was achieved by pre-sending a guide backing track up the clean feed line to the contributor early enough for them to sing to and send their voices only back down the line to TC1 in London. This is analogous to throwing many balls up into the air at different times (the earliest ball being thrown the highest) so they all land on the floor together.

We would start by laying up as many tracks on a digital multitrack (Akai DD1500s to start with - currently Pyramix) one track per contributor. Each OB's Clean Feed would be replaced by this track when it came to doing the piece within the show.

To get the delays correct, we would have to 'Pre send' each OB their own specially timed backing track. Originally we used a version of Hot lips to work out the time delays. This involved contacting each OB in turn and asking them to return our Clean feed back to us.  A tone blip was then sent out and Hot-Lips would measure the time it took between sending and for it to come back to us.  This time was noted and that OB's track moved forward in time by that amount.

When all tracks had been timed we would be ready to go.  These days with digital multi-tracks having moved on a pace, a guide tone blip is recorded early on a cue track.  The contributors are still asked to return our clean feed signal back to us when lining up, we then record their incoming feed onto the track that their nominated clean feed is to be on.  That OB's whole track is then moved, complete with its guide music, to align the received blip with the master sync blip.  This then ensures that the OB's contribution will time perfectly with the live band in TC1.

It is now worth noting that the band in the studio was to play the song live with children singing in TC1 as well.  To keep all of this in time, a click track was laid up in sync with the original backing track on the multi track and fed to the musicians headphones - thus enabling them to play in sync.

The multi track would be played early from TC1 so the OB's with the furthest delay had their guide track sent first - This would also allow the clicks enough time to count the band in.

It all worked so well, it has become a bit of an annual event now - rods and backs spring to mind!

The largest way this was used was for the Music Live 'Perfect Day' song where we had to sync 38 incoming sources - ranging from the banks of Lock Ness to a rooftop in LA with Lou Reed. This last one involved 4 satellite hops - 2 each way. This LA hop was about 2 1/4 seconds round trip!  For Perfect Day we set up a master control room on TC2's floor which looked after all comms and clean feeds to all contributors.

Thanks to Tony for this description.
A whole new meaning to the expression "It's all right leaving me".


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Re: Audio sync of multi-OBs
Reply #1 - Nov 20th, 2012, 10:44am
I was sent to TC (5?) in 1991 to organise satellite feeds during the first Iraq War when we did a 3way Christmas service for/with the troops.
We had to sync a church in Aldershot by terrestrial 2hop link with the orchestra playing with a one hop go/return satellite to Germany and a 2 hop (via Cyprus) go return with Iraq both carrying the reverse clean feed for them to sing to.
TC had just borrowed a very long video/audio delay line from Italy (where the cable companies used it to relay RAI channels so they "weren't live" to overcome the law).
We therefore delayed Aldershot so the loop via Iraq came out right and Germany via a BBC long delay line.
During the audience shots before the hymn, we mixed Aldershot through from "live" to the delayed version so the singing came out sync.
We tried the Designs tone "blip" device to do the timing adjustments but in the end on setup, speech came out BEST! Proving the human is better than the machine!

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