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Author Topic: DCM Input Clip Antonio Marcheselli Phenomenal Film Handler Posts: 1260 From: Florence, Italy Registered: Mar 2000 posted 05-30-2003 08:09 AM                        Seems that my post has disappeared... Perhaps the last server upgrade caused dome casualties? Steve, did you reply to my last post? Did you mean that I cannot overload DCM inputs with CP650? For whom that didn't read my original post, I'm experiencing INPUT clip on DCM in one theater, on louder subwoofer scenes. DOlby's SW level is set to 100, and during louder scenes CLIP led is bright and steady. I wonder if there is a "safe" number on CP650 that souldn't be crossed. Bye Antonio  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 05-30-2003 12:10 PM                     The CP-650 can certainly clip the DCM...it isn't too hard. You MUST and I mean MUST have the DCA amps all of the way up and even then you can clip the DCM. Its that simple. The DCM does not have that much headroom and clips easily. Steve  |  IP: Logged Antonio Marcheselli Phenomenal Film Handler Posts: 1260 From: Florence, Italy Registered: Mar 2000 posted 05-30-2003 12:35 PM                        Steve I do not fully understand this. You say that DCM does not have too much headroom and the dolby's outputs has to be set not too high. But I do not believe that setting all the amps (DCA or not) is the right way. I'm not a fan of "all amps up". I believe that there should be a maximum output of CP650 that should not be crossed to have maximum outputs without clip the DCM. THEN, I can set the room levels by amps (and/or minimal corrections of dolby). I've been told that maximum input level of DCM is 14.2dBu for screen channels and 18.2dBu for sub. What is the correct Dolby setting so that maximum dolby's output does not cross 14.2/18.2dBu? Do you agree with me? Bye Antonio  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 05-30-2003 05:59 PM                     No. I'm not a fan of blanket "amps all of the way up" either...but the DCM's gain structure and DSP doesn't work any other way and even with the amps up all of the way it runs out of digital headroom. Steve  |  IP: Logged Antonio Marcheselli Phenomenal Film Handler Posts: 1260 From: Florence, Italy Registered: Mar 2000 posted 05-31-2003 04:05 AM                        Thanks Steve. QSC confirmed this too. I'll do as suggested but... I'm still not too convinced! I cannot understand how setting all the amps up can be the winner procedure in ALL situations. Let me explain. Let's suppose I have a 400 seats screen with 2 4645C subwoofer and 1 2422 amp. Probably the amp set all way up is not capable to reach the desired SPL and in this case "all the way up" is the only way to proceed. Same screen, 12 subwoofer 4645C and 12 bridged 3022. Probably if I set the amps at full gain I have to lower the Dolby input to compensate... I don't know if I'm clear... I would say that there should be a maximum setting on DOlby to work weel with DCMs... THAT usually require the amps to stay all the way up... Where I'm wrong? Thanks for your time! Bye Antonio  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 06-01-2003 06:11 PM                     This really goes to the DCM's design more than anything. It is your weak link in the audio chain...the first thing that will run out will be the DCM's headroom in the digital domain and that will happen long before most of QSC's amp line will clip (run out of power). As such, you have a bottle-neck going into the DCM. What QSC really needs to do is to do what RANE has done with their DSP systems (see the RANE RPM-26 series and it associated RANEWARE and DRAGNET software). They provide the installer to see what the level is going into the DSP system...maximize the signal before clipping going into and throughout the Digital domain...then maximize the S/N on the output so you could maximize your S/N on the amps. Remember anything you do to the signal after you set the gain structure will affect your overall signal....thus if you set you system...then tune it...any boosts in the EQ will come off your headroom and potentially clip the system. Any cuts, while increasing headroom do so at the potential of S/N improvements. Moving down to the crossover...if, like the DCM, the DSP crossover has EQ within it...that also comes off the DSP headroom. So any "screen loss" or "CD horn compensation" you apply in the crossover will come off the DSP headroom and put you in a clipping situation. Then next dirty little secret of the DCM/DCA system is its 6-dB hit caused by the design of the "data port" on the amplifiers themselves....yes if you use the data ports (the only way the DCM connects) it is as if you turned down the attenuators by 6-dB as compared to the XLR or phoenix input connectors. So with the DCM/DCA system you have two hits against you (gain wise)...the DCM clips easily (in the digital domain, and you have no way of knowing just how close to the "cliff" you are coming). Then you have a effectively 6-dB less gain in the amplifier due strictly to the dataport that the DCM uses. The net result...TURN THE AMPS UP ALL OF THE WAY and pray. You have no control over the system and no way of knowing if you are going to clip it unless you methodically, after tuning, check each input for clipping all of the way up to the maximum level that each channel will be expeceted to play. You will likely find that for many systems, there is no way to play a film at its maximum level without clipping the DCM. The only DSP crossover system that I personally like are those that I have worked with by RANE...they let you see what you are doing and virtually eliminate the 1/3rd octave EQ since you can talor the crossover to perfectly match the speaker it is connected to...you also can optimize the gain structure for maximum S/N. That being said...I'll use an analog crossover everytime if I can get away with it...more reliable and, to my ears, often better sounding with FAR less hassles. The QSC XC-3 are decent enough crossovers (and the SF-3 subwoofer filter is too). And, of course, I also recommend Rane's AC22B and AC23B are excellent choices too. Steve  |  IP: Logged Antonio Marcheselli Phenomenal Film Handler Posts: 1260 From: Florence, Italy Registered: Mar 2000 posted 06-02-2003 09:21 AM                        Steve Thanks for your long reply. I perfectly understand now. I'm a bit surprised that DCM has this behaviour. Bye Antonio  |  IP: Logged System Notices Forum Watchdog / Soup Nazi Posts: 215 Registered: Apr 2004 posted 11-04-2007 10:40 AM                 It has been 1616 days since the last post.  |  IP: Logged Nathan Tarpley Film Handler Posts: 12 From: Crystal Lake, IL Registered: Sep 2007 posted 11-04-2007 10:40 AM                    Time to dredge up this topic to inquire about the new DCMs. (10, 10D, 30, 30D). Has anyone installed any of these? Have they improved the design to accommodate more headroom? I noticed that the dataport output spec has gone up from 99dB to 103dB according to the literature. The maximum analog input spec is still listed at +14.2dBu. There is quite a bit more flexibility in the parametric EQ now, (which might be a bad thing, depending on your perspective) three bands per section (i.e. up to nine bands of EQ on a tri-amp system). Also, instead of simply routing the surround channels to the dataports in the analog domain, they are now included in the DSP and can be EQed. There is very little info on the spec sheet about the AES/EBU input on the D models. It doesn't even list what bit depths and sampling rates it can accept, i assume its up to 24 bit and 48Khz, which is the maximum a 650 with the cat.778 digital I/O board can output. Also, Steve, I am curious about your statement about dataport input on the DCA. Did you mean to say that the the 6dB hit is on the output of the DCM? Since the XC-3 and SF-3 also use the dataport input to the amp, do you see the hit when using those crossovers as well? Nathan P.S. Antonio, if your careful with it and implement it properly, the "Noise Floor Optimization" feature on the 650 can improve the system's overall s/n performance.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-04-2007 04:00 PM                     I have taken delivery, but not installed a DCM-30...as I have first hand information, I'll let you know. The 6-dB hit is on EVERY DCA amplifier...and yes it can affect the XC-3/SF-3 modules but...they do not suffer from digital clipping. Often, you do not need turn the amps all of the way up when using them so the 6dB pad does not seem to be an issue and actually can add into S/N. The problem with the 6dB pad with the DCM-1-2-3 is that you don't have the headroom to overcom the 6dB pad and can clip the input of the unit. Steve  |  IP: Logged

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