How to determine the calibration levels of your recording setup

Background

With the arrival of highly-accurate modeling technologies like NAM, there’s been an interest in ensuring that the gain staging of the signal entering and leaving models is accurate in the digital realm to what one would experience when plugging into the source analog gear.

As of version 0.10.0, the NAM file spec defines a pair of metadata fields, "input_level_dbu" and "output_level_dbu", that document the relationship between the digital and analog signal strengths. Specifically, these fields record the analog signal strength (in dBu) that corresponds to the loudest signal that can be represented digitally. With these two values recorded, one can calibrate the gain staging of the digital models to accurately reflect the behavior of the source in its native analog realm.

This tutorial explains how to take the measurements to fill in these metadata.

Notes

A few things before we begin:

• As with all metadata, providing calibration levels is optional. If you don’t want to do this, you can leave them blank. The models will still work in any plug-in that supports playing NAMs, but the gain-staging may not be accurate to the source gear.
• Pay attention to what’s in front of your model. If you are using a model with a calibrated input level, that level will only be correct if either (A) there are no effects in front of it, or (B) any preceding effects’ input and output calibration levels are known and accounted for. This is possible to do automatically within a single plug-in, but if there is another plug-in in front of your NAM whose calibration is not known, then all bets are off!
• You can still use your ears! Before getting too deep into this, remember that we’re trying to make music. Don’t let the “correct” answer get in the way of making the music you want to make!

Tools needed

In addition to your reamping gear (interface, reamp box, DI box, load box/microphone, cables), you’re going to need to know how loud things are. In the digital realm, the meters in your DAW tell you what you need to know; for the analog electrical signal, you will need a multimeter.

Procedure

First, set up your gear as you would normally for reamping:

• Mute the inputs to your interface in your DAW.
• Connect the output of your interface to the input of your gear via the reamp box.
• Connect the output from your gear (or load box, or microphone) to the input of your interface.
• Turn the output from your interface to its maximum value.
• Begin reamping and set the gain on the return input to your interface so that no clipping occurs.

Figure: Example reamping setup. The output from the Focusrite Solo passes through the reamp box into the input of the amp. The (effects loop!) send from the amp is returned to the recording input of the interface.

Go ahead and reamp your gear to get the data for your model. So long as you’re happy with your send level and the return doesn’t clip, you’re good.

From this point onwards, do not change any hardware gain levels. You already recorded your data; if you make any changes, your calibration measurements won’t correspond to your data.

Next, measure the send level. To do this, play a sine wave with 1kHz frequency and 0dBFS peak amplitude. Some plug-ins can do tone generation, or else you can just loop this 1-second file: sine 1k.wav. Unplug your cable from the gear you are reamping and measure the RMS voltage across its tip and sleeve.

Figure: Measuring the level of the jack that was plugged into the input of the amp. I measure 6.40 Volts RMS.

Convert the RMS voltage to dBu using the formula:

Alternatively, the bottom of this page has a table of pre-computed values you can reference.

Example: In the picture above, I measured 6.40 V. This corresponds to 18.3 dBu.

Next, measure the return level. To do this, start by connecting the send from your interface directly to the return you used for recording. Remember, do not change any hardware gain levels or else you’re going to have a different calibration from how you recorded your data!

Figure: My reamping setup, with the send plugged directly back into the return.

Play the sine tone, and note the return level (in dBFS) in your DAW:

Figure: Monitoring the return level. The “send” track is outputting a 1 kHz sine tone at 0 dBFS. The return track is clipping.

You may find that your send signal is so loud that it clips the return, like above. This is not a problem. If this happens, reduce the level of the sine tone in the DAW until the return doesn’t clip (You should do not change the “input.wav“ file as well. In fact, hopefully you already reamped so that you’re not tempted!)

Figure: Monitoring the return level. The “send” track is outputting a 1 kHz sine tone at -6.50 dBFS. The return track is measuring -0.5 dBFS.

Note the return level (-0.5 dBFS above) and the amount by which you changed the sine tone’s level (-6.50 dB).

To compute the return level, remember: what we are trying to do is figure out how loud an analog signal is (in dBu) that barely clips the return on our interface (i.e. achieves 0 dBFS). We know the analog level of the sine tone (at full volume, i.e. 0 dBFS) from above, so the formula to compute your return level calibration, in dBu, is:

where Δ is the amount by which you changed the sine tone’s level, and L is the observed return level in the DAW (in dBFS peak).

For example, using the numbers above, I get 18.3 + (-6.50) – (-0.5) = 12.3 dBu.

A note on updating old model files

If you know the levels from a model that you made previously, you can update old files to include this metadata. .nam files conform to the JSON file format and can be edited as plain text. If you want to do this, you can make a new model and use it as a reference for how to add the new metadata fields to your old files. Look for the fields "input_level_dbu" and "output_level_dbu" in the new file and copy them to your old file in the corresponding location, changing the numbers as necessary. (If you only know one, then you can leave the other as null.) As always, it’s recommended to save a backup of your file before you being editing it in case you make a mistake.

Appendix: Conversion table between RMS voltage and dBu