Film-Tech Forum ARCHIVE: audio issue with cp65

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Posted by Peter Fagan (Member # 7134) on 11-26-2012, 12:39 PM:

Hello All.

I am having an issue with sound in 1 of my auditoriums.I am running a dolby digital projector with a dss 100 server in the screen.the audio is being broadcast through a DMA8 Plus.this last while on certain films i am getting what is sounding like an electric hum coming through the speakers.mostly on the right channel side of the screen.Did all the usual checks .I:E swapped the channel to a different amp to see if the sound followed to the other channel which it did not.Also tried turning off the amp so no signal should of been coming through or so you would think but to no avail the humming still persisted.If anyone could let me know there thoughts i would be very grateful to here from you.Also i would like to say hello to all & a big thanks to brad for helping me get logged into the site.Been looking for a site like this for a while now where i can have a chat with fellow projectionists.

Posted by Sam D. Chavez (Member # 1841) on 11-26-2012, 01:09 PM:

Welcome Peter,

First, does this hum/buzz only appear in one sound format, or in all formats; film, Non-sync, etc., and in all audio that passes through the DMA8, or just one in particular?

Posted by Peter Fagan (Member # 7134) on 11-26-2012, 02:02 PM:

Hello Sam

Yes the humming is coming through on all channels including non sync.I am using input 1 on the DMA8 for the film audio & then using the film input button for non sync.I am thinking it may be a problem with the dolby processer & the way certain films are being coded onto the hard drives as it is not there all the time.It seems to be only certain films are causing the hum to appear,as i was running silver linings playbook yesterday in that particular screen & there was no humming but the center channel kept dropping out.i switched the center card with my audio processer in screen 2 which is the exact same setup but it made no difference.i am running said film in screen 2 today & it is running fine.

Posted by Magnus Eriksson (Member # 6085) on 11-26-2012, 03:39 PM:

First post.

A common mistake when connecting a DMA8 plus (or ECI 60) to a Dolby CP65/55/45 is the balanced/unbalanced issue. These processors are unbalanced. Pins 1,3,4,6,8,12 (negative/audio return) in the DB25 between DMA & CP must be connected to gnd in the Dolby end.
Dolby provided an adapter (Cat no 757) for unbalanced processors, but it´s quite an easy fix if you are handy. The DTS breakout board for CP65 (D547) is very convinient to adapt for balanced/unbalanced audio.
If audio returns are left open on the DMA8 all sorts of strange sounds may occur.

Posted by Peter Fagan (Member # 7134) on 11-26-2012, 04:22 PM:

Hello Magnus.

As far as i know we are using the dolby (Cat no 757)adapter in the 4 back screens that all have cp65 processors.All our projectors & sound racks are grounded & running through a filter as well.The only difference between screen 3 & 5 here compared to the rest is that the are the version 1 series projectors where as all the rest are the series 2.The hum is not there all the time it only occurs on certain films which is the part that is baffling me.The version 1 projectors are using the dss100 servers & dsp 100 show players where as the version 2 is using the dolby dss200.An example is Madagascar 3 was playing for matiness & audio was fine.THe dark knight rises was playing for late evening shows in the same screen & once the adds finished & the main feature started the humming started.Granted it is not a loud hum but it is persistent all the way throw a movie when it does appear.I will check to make sure that the pins you stated are connected to ground though as well just to be sure.

Posted by Magnus Eriksson (Member # 6085) on 11-26-2012, 06:01 PM:

Hm, hm.
Cat 249 OK? Try swapping with another processor.
Try to insulate shield & ground pins from DSS/DSP100. Disconnect audio from all external souces (non sync, DVD, BD etc). Booth monitor can cause some headache, especially when connected to the power amp outputs AND the outputs of the processor. Loop amplifier (HI) interfering?

Posted by Tony Bandiera Jr (Member # 2365) on 11-26-2012, 07:19 PM:

Good suggestions so far, but I think everyone so far overlooked some key info from the first post:

quote:
...sounding like an electric hum coming through the speakers.mostly on the right channel side of the screen.Did all the usual checks .I:E swapped the channel to a different amp to see if the sound followed to the other channel which it did not.Also tried turning off the amp so no signal should of been coming through or so you would think but to no avail the humming still persisted.

My money is on something outside of the sound system....HVAC motor going bad, a freezer or refrigerator touching the auditorium wall on the other side, etc.

Look outside of the sound system for this one Peter. And welcome to the forum. [beer]

Posted by Sam D. Chavez (Member # 1841) on 11-26-2012, 10:08 PM:

Another thing to determine is if the hum level increases and decreases with the Dolby fader. It could well be the 249 power supply regulator.

Try bypass. This will only give you analog sound from the film projector but if the hum is gone, it tells you the problem is ahead of the 65 outputs.