Electrically Calibrating Your I/O

A few months ago, I posted a tutorial on how to calibrate your monitors to a standard listening level. [That post can be found here.] It’s an important step to take when preparing high quality mixes. People seem to have appreciated that post. So I thought I would follow it up with another calibration tutorial that’s a little more complicated and esoteric, but eminently more important. How to calibrate the trim levels of your DAW’s I/O.

Here are some of the tools you’re going to need:

In that picture you’ll notice I have a digital multimeter, a small flat-head screw driver, a “tweaker,”a pair of test leads and a DB-25 cable for my Digidesign/Avid 192 I/O’s. When you’re choosing a digital multimeter for this application. It’s important to note that you need a multimeter that can read “True RMS.” The less expensive multimeter’s usually can’t read True RMS, they use forms of “peak-to-peak” or “average” metering that are not accurate enough for what we’re going to be doing here. So, make sure your meter can read True RMS, or else you’re going to be adjusting your hardware to incorrect levels. A “tweaker,” if you’re not familiar with it, is a tool for making adjustments to tiny trim-pots in electrical devices. You can easily, make these adjustments with a small flat-head screwdriver, but a tweaker is constructed so that it will not slip off the post like a screwdriver can. Ultimately, you can use either, but a tweaker just makes things less frustrating. You should be able to find one at any number of online electronics component/supply dealers.

You may have noticed a blue cable in that picture. That cable happens to be the aforementioned test lead for the multimeter. If you’re wondering where I got a hold of a fancy XLR test lead combo…let me tell you right now, soldering to hobby/electronics store banana plugs is a pain in the ass! That being said, I pulled it off. So, you probably can too. I put in a little extra effort on the front end to save myself frustration during the calibration process. Your “red” (or positive…”+”) lead should be connected to pin 2 of your XLR connector, and the black lead (or negative…”-”) should be connected to pin 3 on your XLR connector. [Most XLR connectors have their pins numbered so you can read them, either inside or out.]

In addition to these items, your going to need some very fine motor control skills, high amounts of patience (or some combination of the two).

I’m going to be using the Digidesign/Avid 192 I/O and Pro Tools as my examples in this tutorial, but the principles apply to any AD/DA and DAW combination. The 192 I/O comes from the factory calibrated for -18dB. Obviously, this is the standard video reference of -20dB. This is one reason you may want to go through this calibration. Now, I’m only showing one DB-25 cable above. The one in the picture is an output cable. So, it goes from a DB-25 connector to 8 female XLR connectors. I am going to be using two of these cables in this process, as I’ll be calibrating my inputs as well. So you may want both if you’re going to calibrate inputs as well as the outputs. Of course you may not need any spare cables like I do. I’m using them to make my life a little easier. Here’s a shot of the back of the rack where my I/O’s are housed in our central machine room.

As you can see, while well organized (thanks DSI), they racks are densely packed. In addition to that, the cables run down into the floor to reach the rooms and panels they are associated with.

I’m either too smart or too lazy to bother trying to track these things through the floor. So, I’m going to replace the analog output with my spare DB25 cable. Now I have easy access to my 8 output channels. Before I hook up my test leads to my multimeter though, I’ll need to setup my session in Pro Tools. In my session, I’m first going to go to the Preferences setup window, and set the “Calibration Reference Level” to -20 in the lower right hand corner of the “Operation” pane.

Then, I want to set up 1 track with the Signal Generator plug-in. It should be set to a 1kHz Sine wave at -20dB PEAK! Don’t set the Signal Generator to RMS. This may seem confusing considering the True RMS bit I mentioned earlier. To put it simply, our test signal (audio in the DAW) is set to Peak, but we’ll be measuring the RMS (root mean square) of our output ELECTRICAL signal.

This signal should be set to output on one mono channel only. Don’t pan this signal between multiple outputs, because it will give you a false reading. If you like, you can set up 8 mono tracks, each routed directly to a separate output channel, and instantiate the Signal Generator plug-in with the previously mentioned settings on EACH track. This may help you move faster, but having one track and routing to each output in turn will help you keep track of which output you’re measuring. It’s up to you how you do this.

I’m going to start out by doing all of my “A” output trims. The 192 I/O has two output trim levels that are selected in the hardware setup menu. Before you start, make sure you know which set of output trims you’re measuring and adjusting. The trim pots on the back of the 192 I/O are labelled directly underneath the DB25 connector as: 1A, 1B, 2A, 2B….and so on. I’ll talk about how you might make use of the “B” output trims a little later.

Don’t hook up your test leads yet. When you have the test signal routed to your first output, you can turn on your multimeter. Set it to read AC voltage. This means you’ll probably be turning a dial to “voltage” and using a different control (a button on my model) to switch back and forth between DC and AC. Now connect your test lead between the XLR connector and the multimeter. As I mentioned earlier you want to connect the RED (“+”) test lead to pin 2 on your XLR connector, and the BLACK (“-”) test lead to pin 3 of your connector. You can do this by building a test lead like I did, using a bare connector and test leads with alligator clips, or by stripping down an old XLR cable you don’t care about anymore and again using test leads with alligator clips. Start with your first output, and adjust the trim pot so that the multimeter reads 1.228 volts AC, then continue through the rest of your outputs the same way.

[note: 1.228 volts RMS is the calibration point for pro gear, or items that are designed to operate at +4 dBU. Consumer gear, which operates at -10 dBV, looks for a reference of .316 volts RMS. That's a HUGE difference. Honestly, I'm not even sure if this whole procedure is appropriate to apply if you're trying to calibrate for consumer level gear.]

This sounds easy, but it’s not. I’ve done a set of 8 outputs in 25 minutes before, and I’ve encountered times where it’s taken me an hour and a half just to get the same number adjusted. Ultimately, it’s up to how accurate you want to get. It’s perfectly acceptable to round to 1.23 volts; which honestly is what most people do. Getting all of your outputs aligned to the same THOUSANDTH OF A VOLT (1.228) can be very time consuming and frustrating. There is a certain satisfaction you may feel if you can pull it off though. ;)

Once all of your outputs are calibrated, you can get ready to adjust your inputs. [You won't need the multimeter for this part.]You’ll need to connect your outputs directly to your inputs, either with a DB25 to DB25 cable, or using a pair of input/output cables that you plug directly into one another. Pro Tools installs a calibration session with the software that you can use for this purpose. You can find it within the “Digidesign/Pro Tools/Pro Tools Utilities/Calibration Mode Sessions” directory.

Load this session and ensure that your “Calibration Reference Level” is still set to -20 in your preferences (it may be reset when you open this session). Enable “Calibration Mode” from the “Options” menu and un-mute the output channels.

The meters of the input channels should kick on, and the little triangles at the bottom of each channel strip will let you know if the trim needs to be adjusted up or down. If the up triangle is lit, you’ll need to increase the trim…and vice-versa. The meter will also let you know what the current level of the input is, so you can adjust directly to -20 without looking at the arrows if you like. When you do hit -20, both arrows will light simultaneously. Those inputs are now set. You’ll notice that these go a lot faster and are a lot less precise than our outputs. This is mainly because there’s no way to for us to measure the voltage after the trimpot without opening up the I/O itself and a much greater understanding of electronics. For the most part, the input trim level is less critical than the outputs for us anyways.

That’s all there is to it. Admittedly, they’re simple concepts, but their application is a bit more involved than they first appear.

So, with the 192 I/O, we have two output trim levels. What can we do with that second set of trim levels, labelled “B”?

This is an excellent piece of functionality if you work with two different mediums (i.e. film/TV and web). What I’ve done at work, is set my “A” trims to -20, and my “B” trims to -14. Because I went through, and calibrated all of the “B” trims to 1.228 volts while outputting a reference of -14, the “A” trims are electrically equivalent to the “B” trim. So, if my monitors are calibrated to reference = 78dB-SPL, the “A” trims will output -20dB-FS at 78dB-SPL, and the “B” trims will output -14dB-FS at 78dB-SPL…all without me having to recalibrate my monitors. So, I can mix on “A” with an amount of digital headroom that is appropriate for TV/film, and mix on “B” with an amount that is appropriate for web. The beautiful part is that I can mix for either medium using the same listening level. I will still get the dynamic range I want, but in an appropriate ratio for the respective mediums. I just need to keep in mind that I have less headroom in “B” (meaning I have to watch out for clipping).

It goes without saying, that you should always re-calibrate your monitors after doing something like this.

I hope you find this post useful. If you have any questions, feel free to leave a comment. If anything is confusing, let me know and I will clarify and update the above instructions.

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11 Responses to Electrically Calibrating Your I/O

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  3. Anton says:

    HI Shaun,

    You say electronically calibration your setup is “eminently more important”. However I dont quite understand why it is so.
    So my question would be. Whats the benefit of doing this?

    • Shaun says:

      The output level of your I/O affects the input stages of anything that is connected to those outputs. If the signal is too strong, you come closer to overdriving at the input stage that much sooner. If the signal is too weak, you run the risk of introducing more system noise because of a greater need to increase signal gain. Granted you may have to get pretty wide off the mark to experience it, but imagine that in your monitor path.

      Most gear you purchase (be it an amplifier, active speaker, or piece of outboard gear) is calibrated to look at a specific number of volts RMS…and yes, the levels are different between +4 dBU and -10 dBV. Different pieces of gear are more tolerant of variations in calibration level than others, but ensuring that everything is calibrated to the correct levels give you a more dependable and predictable signal path. On top of all that, those trim levels can drift (albeit VERY slowly) as the electronics age or are stressed. So, it’s a good idea to check them every couple years.

      Writing this make me realize I should have mentioned that I was calibrating my I/O for +4 dBU, so I’ll add that in.

      Thanks for the question.

  4. melvin rebiono says:

    how do you calibrate it to -14? i did tried this way and not sure if it right or no hehe, signal generatore set at -14 and adjust the trim until multi meter hits that 1.228 is that right?

    • Shaun says:

      correct, melvin. precisely how to do it. with the trim set in that manner, you won’t have to turn down the monitors to maintain the same listening level…just switch between the two trim settings depending on which reference you want to mix to. just be sure to remember that you have less digital “headroom” at the higher reference level (-14dB).

  5. Michael says:

    Hi thanks your article it was a big help…I was wondering when you made the cable for the multimeter what did you do with the ground wire…I know Red + and Black – but where does the other wire go that’s on the multimeter side…did you put it with the Black wire to the Banana Plug…or just disregard it (cut it off)? Thanks Michael

    • Shaun says:

      Hi, Michael. The black wire is connected to its own banana plug. It’s hard to see in the first picture, because it’s laying under the red one. You need both leads in order to take a voltage reading.

      • Michael says:

        I’m sorry I think you miss understood..the wire that goes to the sleeve..the one without shielding…not the black or red wire..I would think you could put the Black with the unshielded together in one plug and the red into the other…correct. what did you do with the third wire on the one you did? Thanks

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