Can you define modulation depth and speed?

This post was first published in Aug 2014 at projectstudiohandbook.com/PSHforum

In the recording studio we are surrounded by hundreds, if not thousands of switches, knobs and faders. If you've ever invited a layman into your space you will undoubtedly have been asked "do you know what they all do?". Like us you will have probably replied "of course!" (we all like to be thought of as terribly clever!).

But do you know what every feature of every studio device you own does? Looking around the Project Studio Handbook studio there are a number of pieces of equipment with feature we are clueless to explain. They fall into these categories ..

1) features we never learnt about

2) features we learnt how to use from the manual, but having never used, have forgotten

3) features we never understood and never will

4) features we don't understand but we kinda know what effect they have

Sitting in front of me as I write is a Mackie C4 controller which we use daily, but I've just noticed 3 buttons labelled "Split 1/3 2/2 3/1", "Lock", and "Spot Erase". What do they do? Haven't a clue. (We've made a note to read the manual and find out).

It's easier to remember features if you can visualise what they do. We struggled for years to visualise time domain FX modulation depth and speed (important parameters for phasing, chorus and flanging) until we related them to tape delay. These effects were first created with tape recorders. 

The recorder is put into record mode, a signal sent to its input and thus to the record/sync head. The recorded signal is monitored from the repro head, but with a slight delay (the repro head is "downstream" an inch or two away). To control the delay time you adjust the speed or pitch control (also known as varispeed). 

Now for the clever bit! If you constantly turn the speed control back and forth you create pitch and time modulations in the signal. The speed with which you turn the control is the modulation speed, and the distance you turn it is the modulation depth (or width). Faster turns create a more pronounced vibrato effect, and turning the knob further creates greater variations in pitch and delay time.

Combine this with a feedback loop and you can create a wide variety of time domain effects. 

These controls are common place in plug-ins and digital hardware, but visualising your hand on a tape machine pitch control makes it easy to remember what they do and get predictable results.

Thanks for reading.
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