Film-Tech Forum ARCHIVE: DTS XD-10 + Dolby CP650 + Dolby DMA8Plus = Buzz issue

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Posted by Jean-Michel Grin (Member # 1360) on 12-18-2009, 03:52 PM:

Hi Evryone !

In some of our theaters we use the DTS XD-10 as video media files player for the advertising. It's connected to the 6-channels analog audio input of the CP 650.

But since we added a DMA-8Plus between thes two appliances, to decode the digital audio channels of the Doremi server for the digital cinema, we have a ground loop issue in the "analog film" format: the sound of the XD-10 is with a loud buzz on all channels.

In the others formats on the DMA-8Plus, like: "Digital 1-2-3-4" we don't have any buzz or ground loop noise.

If I switch off the power / standby switch on the front of the XD-10, the buzz is canceled, but the XD-10 cannot play the advertising [Big Grin]

I know, it's silly, and that's not the solution to solve the issue, but it could be a cue to find a cure.

I tried to remove the ground connection on the main power cable, but the noise persist.

If I replug TH XD-10 directly to the CP-650, the sound is perfect. I read that we should use to connect the DMA8Plus audio output to the CP650 audio Input a special cable assembly delivred with the DMA-8Plus, But the guy who's did the job, don't had this cable.

So the best solution for us I think, is to make our own cable: the first one for connect the XD-10 and the DMA-8Plus, and the 2nd to connect the DMA-8Plus to the CP650 processor audio inputs.
Did someone experienced the same problem in his cinema ? Could you give me some advice or schematics to build them cables ?

One other solution could be to make a special asembly with an audio transformers on each channels to separate the audio ground of the XD-10 and the audio ground of the DMA-8Plus, but this solution is too expensive for my boss..!

Any help will well appreciated. Thanks for Your atention and sorry for my weak English !

Season Greetings,

Jean-Michel Grin

Posted by Steve Guttag (Member # 268) on 12-18-2009, 05:39 PM:

As for too expensive...then live with hum! What sort of response is that?

But looking at the situation...keep in mind, the DMA8 Plus went to some extremes to keep hum out though isolation. One of the ways they did that was to balance things up in the DMA8 Plus...it is balanced in/out.

Make sure your cable wiring matches up on its plus/minus pins. I would also be inclined to NOT connect the XD10 to the chassis ground pins on the DMA8 Plus...this may be where you are creating the ground potential difference (hum).

We generally make our cables and have found the DMA8 Plus to be hum free though I don't think I have an XD10 coming before it. We use Belden 8508 (twisted pairs, overall shield) as it keeps the balanced pairs together...as such we also don't connect to pins we are not using...a potential source for problems. Give it a shot.

Steve

Posted by Tony Bandiera Jr (Member # 2365) on 12-18-2009, 10:25 PM:

Odyssey Products makes a transformer isolation board that plugs directly into the CP650 (and I see no reason it wouldn't work plugged in between the DTS XD10 and DMA8 PLUS) scroll down to the bottom of THIS PAGE and it's the 4FXFMR board. IIRC it's about $150.00.US

If that's too much for your management to spend, then like Steve said, Just live with the hum. [Roll Eyes]

Posted by Michael Voiland (Member # 5386) on 12-19-2009, 04:07 AM:

make sure your xd-10p is on the same circuit at the proc and Demetri and dma8 if you cant do that make sure they are on the same phase as the same panel. You said diconnecting your digital server stops the buzz. what if you leave the dma 8 attached does it still buzz?

Posted by Steve Guttag (Member # 268) on 12-19-2009, 07:25 AM:

Tony,

Do you know the frequency response or brand of the transformers that Odyssey uses?

Steve

Posted by Tony Bandiera Jr (Member # 2365) on 12-19-2009, 04:03 PM:

Steve,

Don't know the freq response but I can tell you that the transformers are the following: (happen to have one of those boards here right now)

Brand: Tamura Microtran
Part# MET-09 196-0803 or PM-17M or TF5S21ZZ
10kohms center tap windings

I can probably do some measurements with my RTA to try to judge the response in the near future..I'll also look up the specs online.

Posted by Steve Guttag (Member # 268) on 12-19-2009, 04:48 PM:

Thanks...those people at Odyssey are always thinking up clever solutions to common problems. Kudos to them.

Steve

Posted by Demetris Thoupis (Member # 833) on 12-20-2009, 03:32 AM:

Another solution would be to give in the DMA8Plus unit and get a Cat 790 which will give you the AES input from the Doremi directly on the Dolby and the Analogue 6 Channel for the XD10. One question, where is the Doremi Server based? Did you try unplugging the Doremi AES cable from the DMA8Plus when selecting the film audio to see if the buzz/hum is still there? It sound like a ground loop the way you describe it. Are you sure the DMA8Plus is properly grounded to the same sockets as the rack? Is the Doremi on when you select Film-Audio? Did you try switching it off? In reality you should not have any hum/buzz if you connect directly the XD10 with the DMA8Plus and nothing else. I recon is a ground from the Doremi thats' causing the problem and not the DMA8Plus. but because the audio INPUT from the Doremi is in Digital format, a ground loop will not disturb that much.
Demetris

Posted by Jean-Michel Grin (Member # 1360) on 12-20-2009, 06:28 AM:

Hi Again !

So, I'm sorry, but due to my weak English, It just a little misunderstanding, when i said that I removed the earth connection of the server: I talked about the earth wire of the DTS-XD10. But with the Doremi server we don't have any buzz problem.
So I keep on my mind that the ground loop problem comes from the DB25 audio cable between the XD-10 and the DMA-8Plus, who did a short circuit between the audio ground, and the chassis ground of the both units. (Read the famous RANE NOTE 101 about interconecting audio equipement)
On the DMA-8Plus installation manual, it's mentioned that digital inputs are galvanic isolated, and the analog inputs and outputs are fully balanced. But the audio outputs of the XD-10 is unbalanced mode.
So I'm on the job to build a cable to interconnect The XD-10 and the DMA-8Plus, and a another one with the specification of the Dolby CAT 757 dongle to try if the problem can be solved like this.
I Hope to try it this thuesday, because the theater who has the issue is not in Geneva, but in Lausanne, and I have to take the train to go there.
But if nothing works, I will make a transformer box, but the only model alvaible here is the Neutrik NTL-1 and each unit cost is ± $ 65 ! Can You imagine the cost for 6 channels ? That why i said it was too expensive for my boss..
So I do to finish the cable assembly and I will telling you the results of the tests in few days.
Thanks for all Your advices. Have a nice day !

Best regards,

Jean-Michel

Posted by Michael Voiland (Member # 5386) on 12-21-2009, 05:00 AM:

Try to add grounds. Make a common ground for the equipment electrically. Then make sure all your audio grounds are connected and now loose.

Posted by Jean-Michel Grin (Member # 1360) on 12-26-2009, 12:21 AM:

Hello everyone !
I hope you all have one happy christmas.
Sorry to give you a so late answer, but those last days I was really busy with various special events on diferent locations.
But here are the results of my tests to resolve the ground loop issue: I spent many hours to test diferents wiring configuration, and most difficult was to access at the DMA-8Plus, because the sound rack cabinet was really cramped.
The cable pinout that I did, no had any connection point between the audio grounds and the chassis ground of each unit like recomended by the theory. Only the used audio chanels was connecteds. I used a 6 pair-cable twisted pair with overall shield for the connection of the DTS-XD-10 and the DMA-8Plus and another cable with overall shield for the connection between the DMA-8Plus and the CP-650.

But I again the sound issue with a huge lot of noise... So I tried to make a star grounding of all chassis but the problem still the same
The only solution that I found who not really cancel the noise, but reduce him a lot, is to connect one of the audio ground pinout of the DTS XD-10 with the cable shield on the chassis of the DMA-8Plus ! Amazing is not it ? but it works !It seems that sometimes the pratice don't match with the theory 8-D
In the future I will consider to buy a 6-ways audio transformer, of odysey products to really solve this problem in a professional way. But is an another story. Only my boss can decide to buy it or not... if not, I should take my soldering iron to do a batch of cable for each cinemas...

Thanks for your attention and I wish You all the best for 2010 !
Best regards,

Jean-Michel Grin

Posted by Demetris Thoupis (Member # 833) on 12-26-2009, 08:56 AM:

Jean did you try disconnecting the AES cable from the Doremi server to see if the buzz was still there?
Demetris

Posted by Jean-Michel Grin (Member # 1360) on 12-26-2009, 10:15 AM:

Hello Demetris,
Of course I tried to disconnect all other equipment from the DMA-8Plus to see if the buzz was still present. The buzz still continued. :-(
But one thing I've had forgot to mention: The Doremi is directly conected to the Dolby CP-650 fitted with a Cat. 790 for direct AES inputs. The DMA-8Plus is only used to switch between the sound of the advertising who comes from the XD-10 and decode the PCM digital sound from two satelite receiver and a Blu-Ray DVD player.
Perhaps, a ground loop issue occurs from the chassis of the Doremi Server and the chasis of the Dolby CP-650. Next time I go there I will try to disconect the AES Cable on the Doremi side.

Best Regards,

Jean-Michel

Posted by Demetris Thoupis (Member # 833) on 12-26-2009, 10:55 AM:

In England and Cyprus where we follow the British standards for Electricity checks, we follow now the 16th edition of standards (I don't know if it applies to other countries as well). This edition notes that two grounds should exist in an installation. An Operational ground and a standard Ground. The operational ground should be used for server rooms, rack equipment e.t.c and standard ground for all other appliances. Try sending pre out from the server room anywhere else in the building and good luck! I had this problem in a residence and we had to redo a lot of power wiring because of that (although it was multiroom audio and home theater application) that is why I am suggesting that perhaps the problem was from the Doremi. It does not matter where it is connected in the chain but the as long as the ground is passed in an equipment is consecutevely passes to another. I would isolate all equipment except the DMA8plus and the XD10 and test to see where the problem is. On the other hand, a simple relay can do the trick. You could get the plate used for DTS which passes through the DA20 and that uses isolated relays. When the DTS kicks in the plate passes audio from the DTS. When the DTS is dissabled that board switches the relays to the other audio input (in your case the DMA8Plus). Good luck. You should eliminate the issue. The reason why you perhaps are not hearing it in the other sources is because they are Digital and ground loops are not hearable in digital.
Demetris

Posted by Michael Schaffer (Member # 1204) on 12-26-2009, 03:57 PM:

It is possible that the hum comes from an unexpected source - the video connection between the server and the projector. There is a ground on the video line, too, and if the digital projector is not properly grounded and there is a potential difference between it and the sound rack, you will get some pretty nasty hum in the whole system, even if you are not currently playing in digital.