Film-Tech Forum ARCHIVE: Analog Echo Problem

This is topic Analog Echo Problem in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Ryan Navaroli (Member # 3477) on 01-25-2006, 10:55 PM:

Over the past few months one of our auditoriums has had a very bad echo when playing the Analog Stereo SR. The nonsync line for the theatre ads also has the echo on it, but when the DTS kick in there is no echo. To combat this I have been playing only DTS movies in that house and switching the trailers in mono, none the less I'd like to get it fixed.

Heres what we've got:
Smart MOD VI Stereo Processor
DTS 6D
Kelmar Analog Soundhead (I think ATD 1200 reverse scan)
QSC USA900 Amps (though I doubt this matters)

I thought this would just be an head alignment issue until the nonsynch was doing the same thing. The echo deffinitly occurs between the right and the left channels.

Coincindentally today we had our Zipper Changover start smoking tonight. I came into to try and solve the problem and it was forcing the automation to keep the processor on the nonsync. I tried using the manual bypass but it was too loud. I ended up unpluging the automation from the back of the processor and it worked fine for the show. I don't assume that these problems seem related, but it beats me. The changover will move one way but not the other and all the automation is out of commission.

Help on either of these situations would be greatly apprciated.
Thanks.

Posted by Demetris Thoupis (Member # 833) on 01-26-2006, 05:39 AM:

Hello Ryan,
First of all, any one of the DIGITAL formats that are connected on a processor do not pass through DSP Enchancement from a processor but only passthrough to the equalization stage and finaly to the preouts of a processor. That is one reason why you do not hear and echo when DTS is playing.
Now Secondly, the echo that you are referring can be a lot of things. The first one that comes to my mind is that the delay setting on your processor has been buggered to a maximum and that is why you get that echo. The second one is that if you have The SRS button pressed on the MODVI then you get "better" enchancement so if that is pressed, unpress it.

You fail to mention when the problem started though. Was it right from the start of the installation or was it after a few months? Did you alwasy has Reverse Scan reader or was it recently installed? One strange thing I found when I installed a BACP reader on 2 MODVI was the fact that the L+ L- (or R+ R- I dont remember exactly) had to be wired out of phase in order to have the processor playing in phase!! I has getting a strange curve at my osciloscope until I reversed the polarity of one of the channels and it went away. Having one channel input from a projector out of phase will end up what is supposed to be playing in the centre leaking all the way back to the surrounds and vice versa making the front speakers almost completely useless. Try out and give us your feedback.
D

Posted by Richard B. Perrine (Member # 409) on 01-26-2006, 10:13 AM:

The zipper coil is melted. Order a new coil for it.
What caused it......either a stuck relay in the automation or switch.
A zipper uses only a pulse (usually 110VAC) to open or closed it.
If the switch sticks (we used to have footpedels in the change-over days on 20 minute reels).....the coil melted. Replace the coil and the shorted switch or relay.

RBPerrine@aol.com

Posted by Ryan Navaroli (Member # 3477) on 01-26-2006, 11:50 AM:

All the equipment has been there for years. I noticed the problem in early Decemember playing a print of Home Alone for the holidays. I never use SRS..

We have all the same equipment on the other projector in the booth also. I feel as if I'm starting to detecet a slight echo in there but it is not very noticable and may be there due to paranoia.

Posted by Monte L Fullmer (Member # 2797) on 01-27-2006, 02:12 AM:

Sounds like it's time for an "A" chain alignment on the opticals due to the split cell isn't square to the soundtrack and you have a slight case of "out of phase" occuring where the surrounds are picking up center channel information.

Posted by Gordon McLeod (Member # 33) on 01-27-2006, 08:55 PM:

I would suspect the matrix board since on the mod 6 the NS feeds through the matrix
Is it a full 6 track or a centre surround only unit

Posted by Matt Fields (Member # 3211) on 01-28-2006, 10:31 AM:

I used to work at that theatre...it is a full six track unit.

Posted by Gordon McLeod (Member # 33) on 01-28-2006, 02:23 PM:

I would suspect the matrix especially if it is the older DSP based one they sometimes boot themselves into a noman lands state
try changing the delay setting