I give my Digitech RP355 and RP500 pretty good workouts when it comes to exploring their features and functions. One of the things I like on these devices is their twin LFOs–low frequency oscillators–which give you lots to work with when you’re looking for complex sounds.

An LFO changes (or “modulates”) some aspect of a sound–like distortion level, or pitch shift amount, or flanger feedback–on a periodic basis. In the RP355 and 500, you have two LFOs, and either one of them can modulate almost any of the parameters you use to set up an RP patch. About the only thing they can’t modulate is each other.

This lets you do some crazy cool stuff. For example, you can get around one of the big limitations of the RP, which is that you can’t have two modulation FX–such as pitch shift and vibrato–active at the same time. You have to choose one or the other, which is a drag if you wanted a low octave double with a big vibrato on it. But you can use one of the LFOs to rapidly modulate the output (or input) volume of the patch, which can sound very similar to either a vibrato (rapid fluctuations in pitch) or a tremolo (rapid fluctuations in volume), depending on what shape you give the LFO. With a triangle LFO, you get a smooth, steady transition from the bottom of the range to the top; with a sine, it’s a little more algorithmic. Either of those sound like vibrato. With a square wave LFO, you jump back and forth from the bottom to the top of the oscillator range instantly, with nothing in the middle, and the result is something more like a tremolo.

You could use the other LFO to modulate the pitch shift mix, which would make the pitch shifter fade in and out, either totally or within a certain range. That’s a lot of motion in your sound without doing anything more than a holding a note long enough to hear all that stuff happen.

Like I said: crazy cool. So here’s the bug I alluded to in the title of this piece: when you create a patch with a square wave LFO via editing from the front panel, no matter what that LFO is modulating, the RP won’t let you save it to anywhere besides user patch #1. I was apparently the first to report this bug to Digitech after I found it in the RP355, and they confirmed to me that they were able to duplicate it in their shop. Now I’ve discovered the same bug in the RP500.

This bug is a total drag when you’re editing from the RP’s front panel. If you want to edit two patches that both contain square wave LFOs, you’re only going to be able to save one of them before you turn the RP off; good luck with that decision. Once you’ve saved it to user patch location #1 (of course), you can move it to anywhere else from the front panel. (And if there happens to be something in user patch #1 that you don’t want to lose, unless you’re foresighted enough to move that one somewhere else before you start editing a square wave LFO, you have to decide whether that’s the one you save.)

The solution, of course, is to edit the 355 or 500 with Xedit, which will allow you to put the patch anywhere in the user area that you like. I also find that the 500, like the 355, gets flaky if you edit more than a few patches from the front panel, so get used to using Xedit for the serious editing sessions.

I intend to do a video discussing the RP355/500’s LFOs soon. In the meantime, remember: if you plan to get jiggy with the LFOs on your RP, do it from Xedit, not the front panel.