Adjusting/standardizing Trim levels on Firepod

Posted: Mon Sep 22, 2008 9:44 pm

We have 3 Presonus Firepods daisy chained together that we are using to record birds in a specially designed room for our biological research. Currently for our setup we have XLR condenser microphones that require 48V phantom power plugged into the front of the firepods and are recording 44.1 @ 24bits. The sound experiments we are conducting (sound source levels and propagation) require all microphones to be recording at exactly the same levels. When I originally bought these units one attractive feature was I was told that they were "sweet" units and are very easy to adjust the trim levels to each microphone, such that I could be sure they were all standardized to record at exactly the same. However the trim adjustments on the front, even when set to the same place all are seem to be recording a slightly different level.

So my question is how can I be sure all channels are standardized to record at exactly the same level?

Posted: Tue Sep 23, 2008 4:40 am

If you are using identical microphones and set your trim levels identically, i.e. cranked all the way up? You are recording at identical levels. It's the environment and the acoustics along with the time delay differences from where the birds are to where the microphones are that create the differences. These differences cannot be made identical. That is unless you duct tape the birds to the microphones directly. Right. Not practical. Like the class you are taking. I mean, who records birds with multiple microphones and expects the levels to all be the same? Professors? People with fraudulent Ph.D.'s?? Haven't you ever heard of mathematics? The distance between here and there is different from the distances to there & over there and so, levels, response, will always be an infinite difference from one another. There is no way to accomplish your nonsense. Better go back to a better school?? Leave the damned birds alone and go get a real job.

The smartest dropout you'll ever know.
Ms. Remy Ann David

Posted: Tue Sep 23, 2008 5:55 am

Thanks, if I wanted a smart ass reply I would not have posted in this forum.
If you read my question it did not imply that I thought or was hoping to get identical levels from the microphones. I know and expect the levels to be different from each microphone, I just need to know that I am comparing the same thing. Basically, I just need to know that on the firepod the trim levels are all set to an identical level and how best to do it.

Posted: Tue Sep 23, 2008 6:34 am

Make a dummy microphone consisting of a par of 10K Ohm resistors and a pair of 47 Ohm resistors in series: 10K - 47R - 47R - 10K. Connect a battery-powered signal generator to the outer end of the 10K resistors and an XLR line plug to the 47Rs: pin 1 to the centre and pins 2 and 3 to the 10K/47R junctions. Set the generator to give 1V rms at 1KHz and plug the XLR into each FP10 microphone input in turn. Adjust the FP10 gain trims until you get the same reference level on each channel as measured by the computer. Your interface inputs are now calibrated relative to one another.

There is less of a case for calibrating the microphones for this type of work, as Remy indicated.

Posted: Tue Sep 23, 2008 9:05 am

There are also programs that can sweep the input/outputs of your FP10 I used one from hometheater.com I think. It will also check the output to input levels you have to set the trim to ensure that it is the same on every input. I would suppose you would need to use the same output on each different input.

But then again you'll have to make sure that each MIC cable is then checked (assuming they are all the same length and make) and you would have to check each MIC (assuming they are all the same make/model). With 24 inputs, 24 cables, and 24 MICs, this could take longer then the study. What is the margin of error?

Not that I know what I'm talking about but I can however see how this would be near imposible.

Posted: Tue Sep 23, 2008 1:28 pm

When using my Firestudio/Digimax FS setup, to set levels identically I count the steps (clicks) as I increase the gain on the preamps...if I raise channel one 24 clicks; I then raise channel two 24 clicks, and so on... I found counting clicks necessary because my Presonus gain pots don't line up perfectly.

This topic sponsored by: