DCM-30D digital input level calibration

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  • Hi,
    I recently did the B Chain setup for a couple of small cinemas, a screening cinema and a mixing cinema. The clients wanted to use the digital inputs so the DCM-30D was configured appropriately using the slide switch on the rear panel. The mixing cinema is to be calibrated to Dolby Premier standards. During a recent visit by the local Dolby person to check the RT60 times, the clients were unable to get enough SPL through the digital inputs, and used an outboard D to A convertor into the analogue inputs. They then had plenty of SPL available.

    Whn I did the B Chain EQ, to my surprise, the -20dBFS pink noise digital output from the Dolby CP-650 could not make the customary 85dB SPL!
    The only way I could get enough SPL was to adjust ALL the DCM-30D gain adjustments to maximum, also the DCA amplifier gain controls to maximum (fully clockwise). When I say ALL the DCM gain controls, I mean the overall channel gain to +6dB as well as the individual gains for LF, MF and HF. I usually set these to 0dB for the LF, and about -6dB for the MF and HF horns. They are currently set to +6dB for the LF, and 0dB for the horns. I could then just achieve 85dB SPL on the screen channels, but not 85.5dB!

    My design calculations for the cinema indicated that to get 105dB SPL I need 220W LF, 35W MF, and 16W HF, so the 85dB SPL should have been easily achieved.
    Is there something I should know about digital levels? Are the input signal levels markedly different between analogue and digital on the DCM-30D?
    please advise
    David Murphy
    Krix Loudspeakers
    Audiophile_AU
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  • Hello David,

    My name is Sam Hynds, I'm the product specialist for cinema here at QSC audio.

    I'm thinking your problem has to do with the different impedance values between the output of the CP650 and the input of the DCM30D (or DCM10D)

    The DCM digital inputs are balanced - with an impedance of 110 ohms.
    The Dolby digital outputs are unbalanced - with an impedance of 75 ohms.

    It looks like you'll need to get some impedance matching transformers - also when the cat.778 card is installed - the CP650 drops its outputs by -10dB; this gives the CP650 DSP the necessary headroom needed for any EQ adjustments.

    Hope this helps,

    Sam Hynds
    Product Specialist, Cinema
    QSC Audio Products, LLC
    Sam Hynds
    Product Specialist - Cinema
    QSC Audio Products, LLC

    Direct: +1 (714) 327-4640
    Cellular: +1 (714) 227-1344

    QSC Cinema - www.qscaudio.com/cinema
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    Sam Hynds
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  • Hi Sam,
    thanks for your prompt response.
    However, it seems to me that your answers don't solve the problems.
    Re the statement that it could be a 75 ohm or 110 ohm line impedance, or the unbalanced to balanced problem, I don't quite understand why having less voltage change between logic zero and logic one would affect the numeric value of a digitised number (the audio voltage at that instant). It might affect the error rate, or it might not work at all, but the actual numeric value of the data? Hmmmmm. I don't think so.

    According to the Dolby Cat No. 778 data sheet, CP650s with later than ver 2.2 software (and this CP650 has been purchased recently), the 778 provides Unity Gain and Noise Floor Optimisation feature. The "Unity Gain maintains the original level of digital signals as they pass through the CP650, while...."
    And in this installation it is highly likely that the CP650 has correct levels is shown by the fact that when an external D to A was used on the digital output of the CP650, there was an ample amount of analogue signal available for the DCM-30D.

    Our clients (the South Australian Film Corporation) will perform some testing and monitoring of signal levels and communicate with you directly.

    Meanwhile, could you ask the QSC R&D department to check the firmware /software programming to see if the problem is there.
    You may recall that Krix purchased an early production model of a DCM-30D, and when I checked the crossover filters they did not conform to the Linkwitz-Riley characteristic. You issued a firmware update which corrected the problem, and my loudspeaker systems measured (and sounded) much better.

    regards
    David Murphy
    Loudspeaker Design Engineer,
    Commercial Cinema
    Krix Loudspeakers
    Audiophile_AU
    Newbie
     
    Posts: 2
    Joined: Wed Apr 14, 2010 4:03 am
    Location: Adelaide South Australia


  • David, do you have another -20 dBfs pink noise source you can check it against, just to make sure we're looking in the right place? I agree with you that an impedance mismatch would only affect the reliability of the digital datastream transfer and not affect the analog output signal level.

    Thanks.
    Bob Lee
    Technical Communications Developer
    QSC, LLC
    Fellow, Audio Engineering Society
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    Bob Lee
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  • OK. In looking at the data that is available from Dolby - it looks like their output is 110ohms.

    Now the question is their output - with the cat.778 installed the internal DSP will clip the output stage of the CP650 if the level is not reduced by 10dB.
    Sam Hynds
    Product Specialist - Cinema
    QSC Audio Products, LLC

    Direct: +1 (714) 327-4640
    Cellular: +1 (714) 227-1344

    QSC Cinema - www.qscaudio.com/cinema
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    Sam Hynds
    QSC Audio Products
     
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    Joined: Thu Apr 10, 2008 1:01 am
    Location: Costa Mesa, CA


  • Hi David,

    Laura Mercs, one of our DSP engineers, provided this insight:

    We have this issue with the CP650 all the time. Here's what's going on. The 650 uses a fixed point DSP. In order to avoid clipping when adding gain during EQ, a 6-10 dB pad is applied to the input of the digital processing for headroom. On the analog output, gain is applied post-DAC to compensate for the pad. For obvious reasons this can't be done on the digital output, with the result that the digital output signal level is lower than the corresponding analog output. There is an option in the 650 to disable this pad but then you are not allowed to add any gain in the EQ.

    In most instances the channel gain on the DCM or DCP can be used to compensate for the Dolby pad. Many users compound this problem by setting the gain pots on the amps to less than full up.
    Bob Lee
    Technical Communications Developer
    QSC, LLC
    Fellow, Audio Engineering Society
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    "If it sounds good, it is good." —Duke Ellington
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    Bob Lee
    QSC Audio Products
     
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    Joined: Thu Mar 06, 2003 2:01 am
    Location: Costa Mesa, California



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