Syncing FOH console and video truck

[Primoz Vozelj]

I'm gonna be mixing a show that will be recorded for TV broadcast. Audio multitrack will be recorded with two PT rigs, one at FOH and one at MON console. Video will be recorded in video truck(8-10 cameras). I'm also responsible for audio postproduction. Problem is last time we did similar show, there was difference in length of video and my multitrack (12frames in 150 minutes). I solved it all in post, but this time I'd like to have FOH console synced with video truck. FOH console has both word clock in and out BNCs.
What is the standard protocol in this kind of situations? Who gives out master world clock? What happens with live sound in the venue if I'm receiving word clock from video truck and there's problem with word clock coax cable, will console (Vi3k) switch to internal clock without a glitch?

 

[Geoff Doane]

Standard procedure is for the video truck (or studio) to give you their house sync, and you lock to it. Their house sync is a video signal called "colour black" or "black burst" and is an SD video signal with a colour burst and just a 7.5 IRE level on all the visible lines. This is an analog video signal and contains all the timing information needed to sync two or more systems together.

Your console probably doesn't have a video input however, so you'll need an external box to do the math and give you a 48 kHz word clock for the console. I've used the Apogee Big Ben and Aardvark AardSync boxes to do this. We use a Mutec iClock in our audio Mobile to generate clock signals. It will sync to external video and generate WC, AES, SPDIF and video clock signals, but if it loses the video input, it will seamlessly start to free run. If the video input comes back, it will slowly resync to that signal until it can lock again. Since we typically record a 2-mix or even stems to VTR for video productions, it's imperative that we be locked to house sync.

We have a new Roland M-5000 console that does have a colour black input on its optional MADI card. I discovered (through a lot of trial and error) that not only does it need to be terminated in 75 ohms, I needed a video isolation transformer to remove hum that interfered with the successful reception of the signal at the console. As for how gracefully the console handles the loss of external sync, that would be a good thing to determine before show time. Hopefully, whatever sync box you use will do the "free wheeling" trick, so the console doesn't have to.

My experience is with NTSC video, but PAL should work the same way. The sync box just has to do different math to get a 48 kHz output. BTW, as far as I am aware, all embedded audio in broadcast video (either NTSC or PAL) uses a 48 kHz sample rate. There's no point in running at 96 kHz, since you'll just have to down convert at some point anyway.

GTD