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...

image.png

[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