I want to bring up a subject on converter calibration. It’s not something I’ve really seen pop up and it’s a quest that I’ve kind of went down and discovered mixed material on the internet and I’m trying to condense it down in the best fashion I possibly can to show you where it could be applicable and useful.
So what is converter calibration? First off, every interface, audio interface that has a threshold of what the actual digital chips can take before they distort audio distortion. So for most prosumer equipment it’s anywhere from anywhere from 18 to 20 dB-s if it’s a newer piece of equipment. A couple of years ago, maybe 5 or 10 years it was smaller than that, 12-14 dB-s. So that basically means you have about 12 to 14 decibels before your transients and your average levels are going to distort and then you have very crappy audio recorded.
So what is your converter calibration? Most manufacturers will just stamp it on the back of the box. It’s measured in dBU or dBV. They’re very similar measurements but we use Apollo converters at Sacred Grounds and ours are 20 dBV which equates to about 18 decibels of headroom, okay?
So how did I measure that? Other than looking on the back of the box I took a 1K signal generator and I sent it out of Pro Tools, heads the D2A converter on the Apollo unit and I took a multimeter and I measured the tip and the sleeve of the quarter inch cable until it read 1.23 Volts on my multimeter. I think when you open the plugin in Pro Tools it’s standard as -20 dBFS but what you’re going to do is adjust that inside the box, inside Pro Tools until it does read 1.23 on the multimeter. You’re going to see that every converter that you purchase or use has a different calibration setting. And this is important to know because that’s how you know your average response for your signal path. So if you don’t know it and you’re tracking too high you’re not going to have any headroom. If you track too low you’re going to have noise floor issues possibly.
So as a rule of thumb -20 dBFS out of the tone generator should equal 1.23 Volts which equals 0 VU equals +4 dBU. Now, it’s a lot of numbers but you’ll get it as you use it more. If that changes you have to adjust your tone generator like we said but if I do that on the Apollo we get around -16, -18 dBFS. That’s where on my multimeter with our Apollo converters it says 1.23 Volts or close to. We do that because that’s our sweet spot. That’s where we want to record the average level of music. When we’re tracking it dynamically gives us enough headroom for transients to pass through and not be clipped and there’s enough signal that there’s no noise floor issues with the surrounding noise basically.
If you’re not getting the 1.23 Volts, just adjust inside the box or get it as close to as possible and whatever your negative dBFS number will be, if it’s say 14, that’s your sweet spot or your converter’s. I’m going to step back and show you this right here. Our converters I believe are about +18 dBU for the Apollo units, somewhere in there. So if I subtract +4 from the VU equation, that leaves us with about 14 decibels of headroom. So that’s where I measure my average levels when I’m recording and I usually don’t have any headroom levels or signal to noise ratio problem.
So find out what yours is. It’ll be very interesting. It may not be represented correctly on the box. And if you have any questions about the process, I know this is my take on it, feel free to email me at firstname.lastname@example.org or just leave a comment on the YouTube channel.
Steven J. Stanek
This is an unedited transcript from our video series from Acoustic Fields. There will be some errors in grammar and sentence structure that occur during this translation process.
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