This is topic Setting output on Dolby processor in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Marco Giustini (Member # 4544) on 08-11-2008, 05:01 PM:
Someone teached me to set the output of the dolby processors to 300mV to optimize S/N ratio.
Just yesterday I was checking 650's output to confirm that the 300mV is at "103" channel level, flat EQ.
I wonder if the same level is correct for all dolby's processors. On CP65 if I play pink noise and check output from the processor, I have 300mV with 2-3 turn of the channel's level. That are a 25-turns trimpot if I'm not wrong.
Never tested a CP500.
Which are the correct levels? Should I check the levels BEFORE or AFTER the system is equalized?
Marco
Posted by Louis Bornwasser (Member # 3063) on 08-11-2008, 07:40 PM:
On U.S. amps, full output is 1.0v or 2.0 volts. I turn the amps full up and set Dolby to 85dba, see book. Louis
Posted by Marco Giustini (Member # 4544) on 08-12-2008, 01:09 AM:
Hi Louis
Sure, but this will not optimize S/N ratio. Manuals says to put the amps to maximum but it also says that "if a different setting is require in order to optimize the noise performance of the sysyem...".
But even if your amplifier does not have noise at all, I knew that setting the correct output level will increase the sound dynamic. Is it just an urban legend?
Marco
Posted by Louis Bornwasser (Member # 3063) on 08-12-2008, 05:27 AM:
Most amps do not have anything active before the input control, hence, no problem there. The noise/dynamic problem would then lie in the output of the Dolby processor. Older processors (CP-100-50-200 DID have a serious noise problem. Later ones (CP-55 onward) seem to have solved this nicely.
Amplifiers with very high gain inputs (nothing American) would make your problem with noise/dynamics worse. Only way to solve is to add an input "pad." (Drive the Dolby hard, pad it down to allow the high gain amp to function properly.) Louis
Posted by Steve Guttag (Member # 268) on 08-12-2008, 06:42 AM:
Actually, A CP200 (w/Cat 517s/Cat 560) remains Dolby's quietest processor, to-date.
The CP55/65 are definitely not quiet...especially on L,C,R and Ls. Neither are the CP500 or CP650. In fact, one reason I went to Panastereo was due to the noise that is seemingly inherent in the Dolby processors. When you do screening rooms where the speakers are rather close to the listener, the noise of the processor becomes quite apparent. The Panastereo CSP1200 was VERY notably quieter. Its output is referenced at .775mV (0dBu). This, no doubt also helps it in general achieve lower noise levels as it also has true active balanced outputs (behave like transformers).
I have to disagree with Louis on this one. To get the best noise floor, you really do want to find the optimal noise out of each unit. Normally, this means driving the output stage to the point just before it clips on the highest peaks...sometimes distortion creeps in just prior to clipping and you want to avoid that too.
For digital stuff, you want to use as much of the digital resolution as possible...again...you want to use that very last bit on the very highest peak. To use any less is to increase one's noise floor and, in the case of digital, loss of resolution in the signal.
One has to watch out when there are gain stages (like VCAs) and processing (like equalizers) as they will alter the effective gain of the system.
Another rule of thumb is...do as much amplification as early as possible and avoid several stages of amplification...this will generally reduce the noise of the system as well as distortion. It is one of the flaws of the red-readers in that we now have a pre-preamp...a better solution would have been to alter the gain of the optical preamp in the cinema processor...but think about all of those modifications on possibly obsolete processors. None the less, you would lower your noise floor by having the cell of the reverse scan reader connect directly to a good preamp with the proper gain. Also, though not as good for long life, increasing the LED intensity to have the same gain as the exciter lamp would improve the noise floor of the system. An arguement there is just what is the noise floor of the optical recording process to begin with....but I digress....
For most Dolby brand processors...they were designed around a 300mV buss level, including their outputs (The Cat 64Bs on the other hand were designed around a 170mV input/150mV output). So yes, going to 300mV (without any EQ applied) should yield you a lower noise floor than just cranking the amps and lowering the outputs of the Dolby processor. I don't doubt that experimenting with each specific processor one could find an even more suitable target level for lowest noise.
Rarely, if ever does raising the amplifier attenuators achieve the best noise floor for a system. I do believe a better term for the attenuators would be "sensitivity" controls. That is what they are and what I think Louis is trying to say. The amplifier has a fixed gain so it doesn't matter where you set them as far as the amplifier's contribution to the noise of the system...they DO affect the noise floor of the thing feeding the amplifiers, however.
Also note, the CP650 has two output modes (now)...Normal and "Noise Floor optimization." The difference is where they place the nominial level in the digital domain. In effect, they raise the input from the A/D converters and subsequently lower the output of the D/A converters identically (overall level does not change regardless of setting). However, by increasing the level in the digital domain, you are MUCH closer to clipping the digital processing. As such, if you over-EQ and play the volume anything above "7.0"...you will likely clip the system. The "normal" setting allows you to be stupid and not get into too much trouble.
Steve
Posted by Max Biela (Member # 1919) on 08-12-2008, 07:49 AM:
Steve,
I always thought the CP65s (or 55s for that matter) output is referenced at 0.775 V too? At least the 65s product sheet stated that noise is about 70dB below Dolby Level (With fader at 7 (approximately 10 dB below maximum), output set for -6 dBu (0.775 V) at Dolby level, Dolby A-type selected,equalizers set to flat.)
Posted by Steve Guttag (Member # 268) on 08-12-2008, 07:26 PM:
0dBu is .775V...-6dBu is .388V. so perhaps Dolby went up 2.2dB on the CP65/55.
As for reference...you will find that folks tend to reference .775V in professional circles as 0dB and consumer tends to reference 1.0V as 0dBV. Thus, when talking about consumer being -10dB they are referencing 1.0V or .316V at -10dB. Pros, when talking about +4dBu or 1.23V are referencing .775V. As such the real difference between Pro and consumer is not +4 - -10 or 14dB but 20Log(1.23/.316) or about 11.8dB which many round to 12dB. Dolby's cinema reference has been about -8.2dBu.
Steve
Posted by Max Biela (Member # 1919) on 08-13-2008, 08:38 AM:
Dolby said it is 775mV @ -6dBu @ Dolby Level (which is 20dB below full scale in SR*D IIRC as the maximum output level of a CP65/55 is 16dBu)...?
Posted by Steve Guttag (Member # 268) on 08-15-2008, 11:05 PM:
Max, you are misreading the spec....look again carefully.
Under "Signal Outputs"...it clearly states that the typical output level is -6dBu. dBu is referenced to .775V.
The statement you are grasping at is in reference to the "Noise Level".....it is actually consistant. What I think is throwing you off is that Dolby is stating that the -6dBu is referenced to .775V...that is, 0dBu is .775V (which it is by definition)...they are not equating -6dBu to .775V. Thus Dolby Level, when the fader is set to "7" will be playing at about -6dBu on the outputs (referenced to .775V) or 388mV.
Please also note that the spec also states that the MAXIMUM output is +16dBu. For digital, you need 20dB of headroom so if the outputs are set higher than -4dBu at fader 7, you will likely clip the outputs when playing a digital track at full modulation.
Steve
Posted by Marin Zorica (Member # 1716) on 08-16-2008, 05:50 AM:
Yea, this dolby levels did get tricky sometimes, same as 300mV on input of external processor, so try hook up SDDDS to it :-)
I work with cinema and broadcasting equipment, on professional PA and consoles 0dB is 1,55V so +4dB.
Also SONY did good job on DCP1000, you can chose output level vs. dB reference from -16 to 4dB and normally is more quiet than and Dolby made processor, as well external 8ch input is also selectable to get reference based on input device.
Posted by Marco Giustini (Member # 4544) on 08-16-2008, 06:38 AM:
I was following your thread about signal outputs. Thanks for the help.
I wonder why on the dolby's manual there is not stated clearly how to set the output, that should be a critical operation to do.
Thanks
Marco
Posted by System Notices (Member # 2357) on 05-11-2010, 10:21 PM:
It has been 633 days since the last post.
Posted by Tom Wienholt (Member # 1510) on 05-11-2010, 10:21 PM:
What is the process for setting outputs to 300mV in say, a CP55? I assume one would insert the pink noise generator card and then set the output levels for each channel with a meter attached to the output card test points? I have read other posts on this topic which suggest that certain pink noise generators produce a signal that is off by 1.5db. I have a component engineering generator for use in Dolby processors. Is this generator known to be "accurate"?
Posted by Monte L Fullmer (Member # 2797) on 05-11-2010, 11:59 PM:
What you have is the SG-1 from C.E. .. heard that their better than the CAT-85 that Dolby put out.
Did you get that C.E. unit calibrated before usage?
Posted by Tom Wienholt (Member # 1510) on 05-12-2010, 12:05 AM:
Yes I do have the SG-1. I bought it new so I didn't think calibration was necessary.
Posted by Louis Bornwasser (Member # 3063) on 05-12-2010, 05:22 AM:
Calibration was necessary when Bill Purdy owned the company. In fact there was a letter with the unit saying so. Louis
Posted by Tom Wienholt (Member # 1510) on 05-12-2010, 08:59 AM:
That's interesting. I purchased the generator about 8 years ago and don't remember anything in the package stating that calibration was necessary. Is this something that can be done on my own? If so, what is the process?
Posted by Louis Bornwasser (Member # 3063) on 05-12-2010, 09:01 PM:
Not to be glib, BUT. . . .
Borrow a Dolby pink generator and set up for 85dba. Remove Dolby and insert your generator. The small screw (at bottom, I think.) turn until you get the same reading. Louis
Posted by Monte L Fullmer (Member # 2797) on 05-12-2010, 09:09 PM:
Yea, that's what it was (for I've forgotten the technique) why I didn't post on how to calibrate the SG-1..now, good to know.
thx-monte
Posted by Sam D. Chavez (Member # 1841) on 05-13-2010, 12:15 AM:
The output level is supposed to be 300 mV with a true averaging meter, not RMS. The exact number is 287 mV I seem to remember, but who has a true averaging meter?
Steve will be along to enhance or correct me by sunrise.
Posted by Mark Gulbrandsen (Member # 72) on 05-13-2010, 08:43 AM: