B-Chain microphone issue

[Neal Scanlan]

Hello,
I am beginning the process of the B-Chain alignment for a new room with a Dolby CP650. I have the software and have a connection between the processor and the computer. Under the tab: ‘Room Levels’ when I ’calibrate internal meter with pink noise I cannot get any recognition of sound from the microphone. I checked the microphone under ‘Public address’ and it is working. Can anyone offer me guidance or suggestions as to how to resolve this issue?
Thank you

[Ryan Gallagher]

Not an expert:

Are you using a scope or relying on the RTA in the setup software? When you are in setup mode doing B-Chain I'm not sure anything in either the LCD or software is going to show you microphone response?... the old school method is with a scope connected to the RTA test points on the front panel. (New school would probably be with independent analysis using SMART or something equivalent).

Do you also have no signal on those test points?

Also the there is a phantom power dip switch for the mic if needed, but if you have it working in public address mode that is probably set correctly...

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[Neal Scanlan]

Thank you Ryan for the comments. It does seem that the software version does not use an oscilloscope. I’ll do as you suggest and see if I get reading from the test points.

[Marco Giustini]

I never used the CP650 RTA and I recommend you used an external RTA to be honest.

But if that is what you have, you need to plug the microphone at the back of the 650. If the microphone needs Phantom power, there is a dip switch you can lift at the front. But you say the microphone works so you're good to go.

What you need to do is to click on "Calibrate internal Meter with Pink Noise". That will enable pink noise from the centre channel. I don't remember exactly how the process works and the manual is not clear but I think you need to have a reasonable level in the room and then you adjust the microphone level using the trimpot at the back, by the microphone input socket, so that the bar on the software is between the two green lights.

650cal.jpg

Then if you have an SPL meter, you need to tell the software what SPL level you're reading with that level.

That should allow you to continue - you don't have to calibrate the SPL to be honest. You can continue uncalibrated and then use your SPL meter for levels at the end. But the RTA should work on screen as in the picture below, you don't need the oscilloscope.

One thing: if you end up raising the microphone volume a lot, the higher volume will also reflect on your PA format - so don't blow your speakers!

I hope this helps!

650rta.jpg

[Neal Scanlan]

Hello Marco,
thank you for this information it is very helpful. I’ve ordered a new better quality microphone with power supply and I am looking forward to hopefully moving ahead with the software.

[Marco Giustini]

Neal

Would you mind sharing what kind of microphone you ordered?

[Neal Scanlan]

Hi Marco,
I ordered this one:

Behringer ECM8000 Measurement Microphone

[Marco Giustini]

The ECM8000 doesn't have a power supply - but I think it needs phantom power. Take a look at the CP650 manual to find out which of the dip switches at the front enable phantom power.

[Neal Scanlan]

I ordered a power supply also. In your opinion will this microphone be fit for purpose?

[Ryan Gallagher]

The supply would normally only be needed if your equipment was unable to provide phantom power. The 650 is able with the dip switch set correctly, but this is all working towards determining if there is some issue with your particular 650 that is not letting you see a mic in level calibration procedures. Having a phantom supply inserter is a nice piece of kit to hang onto even if not needed.

[Marco Giustini]

But you don't need it. It's dip switch 3

As a previous ECM8000 owner, I'd say no. It's ok but with a little more you'd get a much better one. That said, the accuracy of the microphone is not the most important element in EQ'ing a room. It sounds like it's your first time so I wouldn't worry too much about that.

SCR-20250720-tppp.png

[Neal Scanlan]

Hi Marco,
Your help is invaluable to me, thank you for taking the time.

[Marco Giustini]

That said, the accuracy of the microphone is not the most important element in EQ'ing a room

Let me rephrase this:

AS LONG AS IT'S A DECENT OMNIDIRECTIONAL MEASUREMENT MICROPHONE, the accuracy of the microphone is not the most important element in EQ'ing a room

[Stefan Scholz]

The choice of microphone is normally not critical. You can use a $50 one or a $1500 one, the results should be pretty equal. The difference, the $50 one, once dropped, disintegrates into pieces.The $ 1500 type is nicer crafted, eventually has a higher usable dynamic range. Place both in the same spot, and calibrate the preamp to the same sensitivity, the frequency response seen is pretty close within a couple of dBs. Place it a foot away, it can be totally different.
I do not trust the auto EQ process in 650, especially with a single microphone used in a single spot. And using the pink noise method. If possible I'd always go for a multiplexed method, at least 4 microphones, or the hand held NTI, walking the room and integrating over 30 seconds.
WE were able to get pretty well repeatable results in multiple rooms by doing so.

CP 950 is a different story, using 8 microphones and a pink sweep. The results are pretty nice, and often only require minimum tweaking. But 650, a product discontinued around 2012/13, reflecting a 1998 state of technology works best with an external analyzers used. Just my opinion, mostly doing R2/D2 setups.

[Marco Giustini]

I concur with Stefan - my ECM8000 fell apart with light use ?

I also concur with everything else: a single mike in a single position is more or less useless. If run with the 650 auto-eq is even worse. I'd call it a starting point if someone has never done anything and has ordered an ECM8000! ? What matters is that it's understood that the outcome is going to be random and mediocre at best.

But hey, someone has to start from somewhere. The ECM8000 can then be wired to a sound interface and sound can be analysed using a software RTA etc.