Film-Tech Forum ARCHIVE: Buzz during lens/masking changeover

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Posted by Thomas Pitt (Member # 4322) on 03-15-2008, 04:26 PM:

I've seen (heard?) this effect at several cinemas now. Sometimes when the lens turret and masking change over from flat to scope, there's a 'buzzing' from the front speakers, which stops after the BBFC card appears in scope format.

It sounds more like rapid pulses of sound than an actual tone, as if the sprocket holes are being read by the analogue optical soundhead. The odd thing is it seems to fade in and out as the douser drops and comes back up, rather than suddenly starting/stopping.

Anyone know what might be causing this, and more importantly how it can be stopped? I suppose you could switch off the amps or turn them down during the changeover, but that would require a projectionist present at the time - not really an option in a huge 16-plex!

Posted by Benjamin John Doman (Member # 4365) on 03-15-2008, 04:42 PM:

Hi Thomas,
Can't say I've come accross any 'equipment failure' reasons for this, but could it be that the automation is not switching sound formats and you are hearing the leader going through the soundhead? Where have you heard this? Kinoton projectors, for example, are wired so that the LED's only come on when the light shutter is open.
Regards,
Ben

Posted by John Caswell (Member # 4024) on 03-15-2008, 05:28 PM:

Hi, We've had that problem a few times. Most of our screens are wired to mute the sound as the douser closes during a lens change, but occasionally when this doesn't happen, we find that you get a kind of skipping sound as the leader is read.
We've only got 4 screens here so all lens changes are manual, so we mute the sound out manually to make sure. [Smile]

Posted by Sean McKinnon (Member # 612) on 03-15-2008, 06:13 PM:

It is probably a grounding problem. Isolating the Changeover and lens turret/aperature changer on a seperate circuit from the soundrack could help.

Posted by Gordon McLeod (Member # 33) on 03-15-2008, 06:27 PM:

possibly they have black film inserted that has a white frame line across the entire width including the sond track area
a few years ago there was a lot of that stock floating around

Posted by Michael Brown (Member # 890) on 03-15-2008, 07:01 PM:

Thomas, Just out of interest, which cinemas have you noticed this at?

Posted by Thomas Pitt (Member # 4322) on 03-16-2008, 04:31 AM:

I've noticed it mainly at the Bradford Cineworld, but it's also happened at Vue Leeds Light and the Sheffield Vue (Meadowhall). It doesn't happen on every single movie that's shown, just a few of them.

Sheffield Vue used to use clear threading leader as 'spacer' film to cover the changeover, and you saw it projected on the screen for a couple of seconds before the automation triggered! Now they use black film though.

I'd say the frameline leader theory is the most probable. There's no Dolby Digital soundtrack, so the processor switches over to the analogue - and the framelines are read as a 24hz buzzing.

Posted by Michael Brown (Member # 890) on 10-24-2008, 06:23 PM:

I think I might have heard it. Last night I was watching Eagle Eye in screen 16 at the Bradford Cineworld. It's like a tapping, as if someone is tapping their finger on a desk, about 4 or 5 times a second and it lasted in total just under 10 seconds. It's not particually loud - it's not the sound of leader going through and i've not heard it the last times I was in this auditorium.

Posted by Tony Bandiera Jr (Member # 2365) on 10-25-2008, 04:03 PM:

It's also possible you're hearing a buzz from the masking motors.. quite a few installers don't mount them with iso-pads to cut the vibration on the rigging.

Posted by Joe Redifer (Member # 3) on 10-25-2008, 07:42 PM:

I used to have this problem at the mighty UA Greenwood Plaza 12 since the lens and masking would always reset to flat after each show (because we were awesome). You could hear the leader during the change being read by the optical analog reader, and the leader wasn't printed very well. I just rewired the automation to engage the MUTE button during the change and then go back to mono afterwards for the trailers. It required lots of wire, but it was worth it.