Film-Tech Forum ARCHIVE: Dolby CP750: calibration Mic recommendation?

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Posted by Alan Gouger (Member # 543) on 11-14-2012, 05:26 PM:

Dolbys CP750 has a balanced Mic input for the rooms EQ calibration process can someone recommend a good for this purpose. I have a Shure SM58 laying around would this be a good candidate for calibration.
Anyone use the 750s on board calibration can you give a quick step by step.

Thank you!

Posted by Scott Norwood (Member # 30) on 11-14-2012, 06:46 PM:

If Dolby sells a microphone that is specifically intended for use with the CP750, use that. Otherwise, you want the one with the flattest possible response curve (a "measurement microphone"). The SM58 response curve is anything but flat, so you definitely do not want that.

What you really want is a microphone multiplexer and set of several (usually four) matched microphones. Dolby might sell one. If not, Ultra Stereo sells a nice kit for this purpose. This makes it easier to avoid room-specific dead-spots when doing the B-chain alignent.

Posted by Manny Knowles (Member # 1171) on 11-14-2012, 07:31 PM:

Ultra-Stereo Microphone Multiplexer MMP-10

The one that Dolby made was specifically for the CP-500 and I'm not sure if it's still available. If it is, it would most likely require an adapter cable.

Dolby Multiplexer MPX-500

Posted by Alan Gouger (Member # 543) on 11-14-2012, 08:09 PM:

Thanks for the information and recommendations. I found a Behringer ECM8000 Condenser Ultra-Linear Measurement Microphone. A few of these along with the multiplexer it should do the trick.

Posted by Dave Macaulay (Member # 813) on 11-15-2012, 07:21 AM:

In theory you could use any flat response omnidirectional microphone, and even cheap condenser mics are pretty flat.
Using one mic is dicey, you are basically tuning for one seat and there will be standing wave patterns in the room that cause response variations at each point in the room. Using a multiplexer is the least difficult way to minimize that, and it's the only way that will work with the CP750 analyzer function.
I recommend just getting the complete USL kit. I haven't tried using just their multiplexer with different mics, but probably adapting them (to the micro XLR input connectors) and calibrating the levels would be a chore.
You will also need a handheld SPL meter to calibrate the CP750 to the alignment mic(s) used.

Posted by Sam D. Chavez (Member # 1841) on 11-15-2012, 09:28 AM:

What Dave said.

Posted by Tony Bandiera Jr (Member # 2365) on 11-15-2012, 12:32 PM:

x2 what Dave said.

And, the best part....if you buy the USL kit with the mics they are matched for proper frequency response and set up with the levels all balanced as well.

That kit is well worth the money.

I cringe at the idea of using ANY other mic not specifically approved/tested by Dolby.

ALL "standard" microphones have some sort of irregularity in their frequency response. To even come close to doing it right the mic MUST be flat within 0.5db from at least 20-20k Hz. Such mics do exist but they cost as much as the USL kit will.

I guess all that is really saying to Alan: Don't jerk around on this, buy or borrow the USL multiplexer and mic kit and do it right.

Oh, and even if you go the Beheringer/multiplexer route, how are you going to match the levels and be sure the frequency response is right on ALL FOUR mics? [Wink]

Posted by Alan Gouger (Member # 543) on 11-15-2012, 10:58 PM:

Thanks Tony I am looking into the USL option.

Posted by Marco Giustini (Member # 4544) on 11-17-2012, 04:30 AM:

x3 what Dave said.
Single mike is usually a bad idea unless you walk through the auditorium with it and check for response.