Distributing sync via wireless?

[Dustin Moore]

I've got what probably is bad solution to an otherwise trivial
problem. I'm recording production audio for a long outdoor
event (3+hours) with multiple recorders spread out over
hundreds of meters of space. I really would like to hold all
of the recorders to within 5ms sync using some sort of word
clock distribution method.

The catch is, I need to do it without wires as the space
between the recorders is uncontrolled and any cables or
fiber I lay is likely to be rolled over by a tractor or messed
with by someone.

What would happen if I fed my master video black into a small
video transmitter on a pole and then put UHF TV receivers at
all the recording locations. If the output of the TV receivers were
then fed to an Aardsync or equivalent, would that give me a
distributed wordclock within my 5ms requirement?

Other than laying miles of mic cable, is there a right way to attack
this problem?

-Dustin
[The person with weird problems]

[drpro]

Dustin, I would suggest renting as many Ambient Clockit units as you need. These units will output TC and wordclock. Very stable.

David

[Dustin Moore]

If only my cheap prosumer video gear would take the genlock in.

I was hoping to drive everything off my primary video camera being
fed through a genlock-cleaner-upper. My video gear drifts about
0.3% relative to my 702 and in three hours that accumulates to
a decent bit of time. This is probably another case of "you can't
get that without decent gear".


If I owned my own engineering firm I'd figure out how to make something
like a lockit that uses GPS as the main timebase and then has
a 900Mhz transceiver to communicate with the other "Ubber-lockits".

The idea would be that there would be one Ubber-lockit taking
sync from video black and that it would calculate the difference in
frequency and phase between the genlock and the GPS timebase.
It would then transmit this freq/phase update to all the Ubber-lockits
in realtime so that everything would stay in sync.

Probably cost more than getting decent equipment or digging some trenches, though.

[drpro]

I understand. I primarily do location sound and when mixing fullsize cameras with the smaller nonTC ones, what we do sometimes is sync the Lockit boxes to the master whether its one of the fullsize cameras that has TC and genlock or another source such as your SD recorder. Then feed one channel of the smaller cameras the TC output of the Lockit box. Post if they are using Avid can read the LTC track from the small camera. There are a couple of programs on the market that allow FCP to do the same. If you are fortunate to be using one of the newer Canon HD XL cameras they have TC + genlock.

David

[Francis Vaughan]

What you don't say is what gear you currently have and what budget you have.

A GPS conditioned txco frequency reference can be had new for about $1000 and they are on eBay for $2-300 Trimble Thunderbolt or Jackson Fury. Both have stupidly accurate and stable clock outputs (actually better than rubidium atomic clocks) and are very small 12 volt powered systems - only needed a cheap external GPS antenna. But they only output a 10MHz signal. (And 1 pulse per second.) So you would need a clock generator that can derive a useful clock from that. The trouble with that is that it is too trivial - and you can't easily buy one - although you could make one for a few dollars. You don't need - or indeed even want - a frequency synthesising divider - all your converters need to be on exactly the same frequency. A simple divide by 208 would get you so close to 48kHz it doesn't matter. And with one of these frequency standards you would be within one microsecond for weeks. So - you could actually do it for not a lot of money, or for a lot of money.