What a snapshot can and can’t capture
This chapter uses Black Diamond Probe, the free companion plugin. Probe is designed to visualize distortion, but it can also capture a snapshot of the distortion curve and save it as a preset you can load in Black Diamond Distortion.
Routing audio to Black Diamond Probe
All you have to do is send a test signal to your hardware and read it back into your DAW. Probe listens in two places: its main input reads the device’s output, and its sidechain reads the device’s input, so it can compare the two.
The routing, using Ableton Live as the example:
- Generate a test tone.
- Place an External Audio Effect after the test tone. It sends audio out to the hardware’s input and brings the hardware’s output back into the DAW.
- Place Black Diamond Probe after the external effect.
- Route Probe’s sidechain input from before the effect (the channel’s Pre-FX signal), so Probe can compare input against output.
The same routing works in other DAWs. It just takes slightly different steps to achieve the same routing. If you ever forget, Probe shows a routing diagram when you open it.
Once routed, you’ll see the distortion visualized in Probe: by scanning input against output, it maps out the transfer function.
Tricks for a good capture
Getting a good capture is mostly about setting up the test so that the transfer function is a solid/static as possible prior to capturing. A common deviation is loops in the curve, and we want to collapse these loops as much as possible. Loops can come from latency in your interface, and even the cable length.
In these cases the latency is a simple static delay and the solution is simple: latency compensation. You can compensate the latency however you want, but black diamond probe does provide X lag and Y lag controls to compensate the input or output respectively.
Another possibility is phase rotation caused by filters in the chain (like the DC-blocking filters hiding in many distortion circuits). These are more complicated because filters can delay each frequency differently (the phase response of filters).
Here’s why a sine wave is such a good test signal: when a filter before the distortion phase-rotates a sine wave, the result is still just a sine wave - only quieter and slightly delayed. In other words a sine wave is a single frequency so the filters phase shift only applies to this frequency and it looks like a static delay. So we can compensate for this issue the same as with latency, and use the X lag and Y lag to collapse the loop.
However, phase rotation after the distortion is still a problem. Because the distortion will introduce harmonics - and a filter will phase shift each harmonic by a different ammount. In this case there is not much we can do except bypassing post-distortion filters when possible, and by trying different test signal frequencies to avoid the cutoff frequency as much as possible. For example when I capture the multi-drive in the Moog Sub37 I notice there is a DC blocking filter after the distortion I cannot bypass, but by choosing a test frequency that is far away from DC (500 Hz) then I mitigate the issue.
If you cannot perfectly collapse the loop you can still get good captures. Black Diamond Probe will perform some curve fitting, averaging, and smoothing to approximate the transfer function to the best of it’s abilities.
The checklist:
- Disable any EQs in the effect chain - limit the capture to just the distortion.
- Try different test signals - when in doubt, a sine wave is the best choice.
- Try different test frequencies to steer clear of phase rotation - for example, 500 Hz sits comfortably above a DC-blocker’s rotation and below most lowpass filtering.
- Compensate what’s left with the lag (and gain) controls.
- When the loop is collapsed, you’ll get the cleanest snapshot.
Saving a snapshot as a preset
When you’re happy with the capture, press the pause button to freeze it, then save it to disk. Load it in Black Diamond Distortion from the Transfer Function section of the preset browser.