Film-Tech Forum ARCHIVE: CP65 non-sync surround gain adjustment

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Posted by Scott Norwood (Member # 30) on 08-31-2007, 07:55 AM:

How does one determine the proper setting for the non-sync surround gain pot on a CP65? Specifically, how does one set the correct surround level when using the non-sync input for video and other matrix-encoded sources (jumper set for 2:4 mode)? There must be a proper way to set this up, but the manual doesn't say much about it. Is there a test tape/DVD available that plays pink noise through each channel at 0vu/-20dBFS to allow one to set non-sync levels properly?

Posted by Louis Bornwasser (Member # 3063) on 08-31-2007, 08:42 AM:

Turn off all screen channels, then adjust the volume on the incoming signal to give you the surrounds you want. Turn on screen channels. Use the "non-sync" controls on the Dolby to set the screen volume. Note that completely off is not possible on these controls. Louis

Posted by Richard May (Member # 2627) on 08-31-2007, 11:43 AM:

Scott,
I do this daily with the 65. I am running through the non sync input with the jumper in 2:4. Most videos we get in have tone and color bars at the head of the tape. You can run tone and adjust the pot for double green level on the noise reduction card. Since EVERY video we get in has different levels, we do this for every video we get. Another way to do it if you dont have tone is to play the tape and listen for a scene with dialouge. Then adjust either left or right n/s pot until the dialouge only comes out of center channel. Just look at the little green led's on the edge of the output card.

Posted by Steve Guttag (Member # 268) on 08-31-2007, 12:23 PM:

....and then Richard reads...that Scott was referring the SURROUND level in Non-Sync, which has its own trimmer on the Cat 441 card.

Remember, cinema is the only form of surround sound where the surrounds are not balanced equally as the stage channels. That is, we set surrounds to 82dB/side with discrete sources for a combined level of 85dBc with the stage channels referenced to 85dBc as well.

Other forms of surround sound and in particular consumer based surround sound (as well as broadcast) has all channels equally balanced. As such, if a cinema system has a properly calibrated surround system, then the non-cinema sources will need a 3dB boost on each surround channel.

To muck it up yet further...on a CP65, there is a permanent boost put into the mono surround level of 3dB to compensate for well, the Cat 150 decoders that were in use BEFORE the CP65. I've never found that the Cat 150E (what originally came with the CP65) or the Cat 150F (which was in production until the end of the CP65) needed this extra boost that the earlier forms (Cat 150 - Cat150C) needed. Anyway, for this reason, when one tunes a CP65 and since one selects format 04, one must tune their surrounds to 85dBc/side for a total of 88dBc when both are playing...the processor removes this boost for digital.

If it were me, Scott, I would take my pink noise generator (external)...feed it in to left and right to take a reading of the SPL, then invert phase to one channel and set the surrounds. Note, the CP65 is unbalanced so you will need to isolate the channels.

Steve

Posted by Richard May (Member # 2627) on 09-01-2007, 11:57 AM:

What the hell was I reading. Sorry Scott.

Posted by Scott Norwood (Member # 30) on 09-04-2007, 07:55 AM:

Thanks, Steve. So, how do I invert phase on one channel of an unbalanced signal? (Probably a stupid question, but I've only ever made phase-flip cables for balanced lines.)

I'm presently running mono surrounds (with jumper properly set), so I assume that I should set the surround level to 85dBc for video/non-film sources?

Also, when running video, should the 1kHz tone be set to a specific SPL value (1kHz at 0vu or -20dBfs == 85dBc at average seat?). I do realize that video sound varies widely from one tape to tne next, but would this be a good starting point?

Posted by John Hawkinson (Member # 1135) on 09-04-2007, 10:48 PM:

Scott asks, "So, how do I invert phase on one channel of an unbalanced signal?"

The obvious way is with a transformer (I don't know if there's some cool audio hack that lets you skip that). The problem here is that even if your source simply allowed you to swap the signal and the ground (with would mean the CP65's ground was tied to the generator's signal and vice-versa), if you're feeding the same source into two channels on the CP65, then you're connecting the CP65's left-channel ground to the CP65's right-channel signal, and vice-versa, and that's just not going to work.

If you're a glutton for punishment, you should use a transformer for both signals, since in theory a transformer introduces some amount of phase distortion. But at 1 KHz, for anything decent, this effect is insignificant (under 2 degrees), so don't worry.

--jhawk

Posted by Steve Guttag (Member # 268) on 09-05-2007, 08:44 AM:

Scott, somehow I missed your response but yes John is correct, transformers are your best friends for something like this.

I'm thinking it should probably be possible for you to do it resistively using 4 resistors of say 1K-Ohms One from the generator's output to Lt and one from the generator's output to "E". Then for the reference side of the generator (or "-" side if diff out) again a resistor to "E" and one to Right. This would only work if the generator's output is truely floating with respect to the CP65 and it won't be the cleanest thing...transformers would be MUCH better.

If you are feeling in the hobby mood...use an op-amp to make an inverted output. Most any will do for this sort of thing.

As for level...I believe that if you were to run both signals in phase, measure the center speaker...then measure the same signal but out of phase, set the surrounds to the same level.

Note there are DVDs out there with rotating pink noise to calibrate with too.

Steve

Posted by John Hawkinson (Member # 1135) on 01-26-2008, 09:20 AM:

A slightly related question -- what's the recommended way to match the non-sync L/R gain pots to ensure that the matrix runs properly for center decoding? I can think of many ways to do it, but I'm not sure which one is easiest [Smile]

--jhawk

Posted by Scott Norwood (Member # 30) on 01-26-2008, 09:55 AM:

I play a mono signal through the left/right inputs and adjust the gain pots until there is no signal in the surround channel. This is easiest if you have a CD player that can be switched to mono output (I know that the Denon DN-C680 can do this).

If anyone has a better method, I, too, would be interested.

Posted by Louis Bornwasser (Member # 3063) on 01-26-2008, 09:56 AM:

Just jumper the l/r inputs together with a clip lead. Louis