Film-Tech Forum ARCHIVE: CP65 Hiss

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Posted by Markus Lemm (Member # 3456) on 11-23-2011, 11:33 AM:

Just working on a 3 screen install (2 35mm). All screens have CP65s. I am getting a lot of hiss out of the speakers when turning the volume up to 7 or higher. Tried the amps with a different signal from a CD player and get no hiss at full volume.
Tried the gain controls on the CP65 but that does not eliminate the hiss. Tried the Cat. 64 equalizer on C/L/R but could only lower the hiss a little bit.
This hiss is on all three screens in all formats.

Any suggestions would be great.

Posted by Gordon McLeod (Member # 33) on 11-23-2011, 11:42 AM:

That is where doing proper gain structure design comes in as one needs to choose components in a design fashion to run each one in its optimal noise area and head room
The THX rule of thumb was set the processor output to 300mv with the fader at 7 and then adjust house gains on the amplifiers

Posted by Tony Bandiera Jr (Member # 2365) on 11-23-2011, 12:59 PM:

What Gordon said. [Smile]

^^

Markus, we really need more info to help out better:

What brand/model of amplifiers?

What brand/model of speakers?

What are the current settings of the gain controls on the amps?

What sizes of auditoria are we talking about?

It would be very rare for more than one CP-65 to have issues, they are pretty much bulletproof. And they tend to be very quiet.

Posted by Markus Lemm (Member # 3456) on 11-23-2011, 02:29 PM:

Speakers are EV Live X speaker - Brand New
Amps are QSC 1400 - running at max
Auditorium is 50 feet to screen by about 40 feet wide.

Question about gain structure. Are you referring to the output from the CP65 to the amps? Can that be measured with a volt meter?

I've tried moving the gains up on the processor and lowering the power output on the amps but same result. All done to 85dB L/C/R.

Posted by Marco Giustini (Member # 4544) on 11-25-2011, 03:55 PM:

Both.
The CP65 has some noise that the amplifiers are going to amplify if the gain structure is not optimized.

Generally speaking, you want to raise the CP's gain (I am not talking about the main fader) and lower the amp's gain accordingly. You need to know what you're doing and you need some tools, if you don't have either of those, don't do it!

One procedure could be to set the fader at 7.0, inject 300mV into the CP65 and set the levels until you have 300mV from the outputs. Then you set the SPL in the auditorium with the amplifiers. But this is just an idea, it's not so easy.

Posted by Gordon McLeod (Member # 33) on 11-25-2011, 06:35 PM:

actually it is very easy and was the basis of THX alignments
the cat 85c was turned on with the fader at 7 and the processor gain was set to 300mv then the 85dc was set on the amplifier gain

Posted by Mark Gulbrandsen (Member # 72) on 11-25-2011, 07:29 PM:

E-V Live-X... what the hell is that? Some sort of live sound reinforcment speaker? E-V is pretty much OUT of the cinema biz so this can't be a cinema type speaker. If so it may not have a truely linear respoinse and may also be part of your problem. Be sure they are set up correctly and EQ'd properly as in the ISO-2969 curve!

UPDATE: I looked up the E-V speakers and they are sound reinforcement speakers.... not cinema speakers.

Posted by Marco Giustini (Member # 4544) on 11-26-2011, 04:54 AM:

Hello Gordon

I did not know the cat85 was a reference for gain structure, thanks. Yes, it's easy if you know what you are doing.

Posted by Tony Bandiera Jr (Member # 2365) on 11-26-2011, 01:22 PM:

What Mark said. ^^

I can promise that the speakers are a HUGE part of the problem.

Live sound reinforcement speakers should NEVER, EVER, be used for a cinema application.

The two disciplines are so far apart that it is not even worth the debate. Live sound is after filling a large space with a lot of SPL, with linearity being an afterthought. The emphasis is on projecting vocals, so the midrange and high end tends to be exaggerated.

Cinema is about accuracy and linearity to realistically reproduce the sound image of the film.

So Markus has two choices, really:

1: Get rid of the EV's and get some REAL cinema speakers installed (and with upgrades happening it should not be too hard to find good, used JBL's or maybe even QSC's).

2: Keep the EV's, live with the hiss and realize that to get the EQ to conform to 2969 your EQ will be so forced that you will have phasing and imaging issues.

I would love to personally kill the buttheads who continue to try to use live sound speakers in a cinema application.

Posted by Markus Lemm (Member # 3456) on 11-26-2011, 05:28 PM:

Thanks for the replies.
I used a 300mv signal to the digital input and reset the gain to also output 300mv to the amps and it helped with the hiss, still not perfect. Nothing I can do about the speakers so I guess it will have to due.

Thanks again.

Posted by Louis Bornwasser (Member # 3063) on 11-26-2011, 07:56 PM:

Can we assume that this was done with the cat 64s set flat? Otherwise, no go.

I have purchased many used Dolby units, some from decent engineers. You cannot IMAGINE how far the EQ was set. Beyond any possibility of correctness. Louis

Posted by Richard Hamilton (Member # 321) on 11-26-2011, 09:23 PM:

Louis, I got a call years ago from an ASC tech. He was training a new ASC tech at one of Kerasotes theaters. He said they were having problems with the CP500s. He said the highs were off the charts on their analyzer. Of course I asked him what the hell he was doing checking the EQs. The theater was only a couple of years old and had no sound problems. On 4 units they turned everything from 250hz all the way down. I told them not to touch anything and made the 5 hour drive to the theater. The first evening shows were playing and the sound was so muffled I don't know how people were watching them. I set everything flat and told the manager to give refunds and passes to everybody, tell them they are also welcome to stay and watch another movie also.

The rooms were well designed and needed very little EQing. From that point on, everything was password protected. On the special venue projects I had a couple theaters insist on the passwords for the Peavey files I wrote. I explained that if they changed anything and I had to come fix it, they would have to pay all expenses. (Travel, food, booze and hookers) [Razz]

Rick

Posted by Bernie Anderson Jr (Member # 410) on 01-27-2012, 03:41 PM:

What should be the steps to properly setup a CP65:
1. Flatten the EQ on all channels
2. set all outputs from the 65 to amps at 300mV?
3. Set SPL at amps to 85db
4. then EQ each channel
5. check SPLs again?

Posted by Marin Zorica (Member # 1716) on 01-27-2012, 05:14 PM:

Also, check grounding, not only ground loops, but sometimes grounding can be issues of hiss.....also, how "high" hiss is? I recall when working with CAT64's that they didn't produce much hiss, but when CP was properly set as guys told you before. But if you don't get solution, try to unplug all inputs from Cp and see if hiss is gone by then.