December 05, 2009 - Sequencers and LFOs

Wow, it’s been a while since I’ve written in this… gotta update it more often :^)

Lately I’ve been playing around with my Poly Evolver quite a bit to try and get better at programming it. I bought the synth about a year ago, but I’m still just scratching the surface of all the things that you can do with it. So, aside from creating some drum sounds, I spent some tonight time messing around with the sequencer. It’s a 16-step analog-style step sequencer, which means that you have up to 16 steps per cycle in which it can control other things.

Normally I approach sequencers as simply a device that one can use to control pitch over time. The most simple example would be to generate a simple arpeggio, while more complex examples can sound more like actual elements of a song. Both of these - which are just presets in my Poly Evolver - are programmed such that the sequencer controls the pitch of the oscillators. This means that, when the sequence played back, each step acts as if a new key is pressed at a certain pitch, thereby generating a melodic sound. In some ways it’s like the tape roll on an old player piano, except that it can only have up to sixteen holes.

However, step sequencers can be used to control a lot more than just notes. The experiment I did tonight had the sequencer controlling the low pass filter instead of the oscillators. This meant that sound and pitch did not get generated when I played back the sequence. Instead, sound got generated the traditional way, with me holding down one or more keys. As I held them down, the sequencer acted as a retrigger, creating a straight rhythm. At the same time, I also had the sequencer modulating the lowpass filter’s cutoff frequency and (very slightly) the resonance, so you end up with this pulsing pattern that alternates between partially "open" sounding and partially "closed. To add a bit of flavor to the sound, I then finished off with some additional modulation from the LFOs.

Now for those who don’t know what an LFO is, the term stands for "Low Frequency Oscillator". Just like the name implies, it generates wave forms at low frequencies. However, you never actually hear these waves directly. Instead they act as control signals that can be mapped to other areas of the synthesizer. A good example would be mapping an LFO that generates a sine wave to an oscillator’s pitch. As the LFO’s wave rises and falls, the pitch of the sound will "follow" this, creating a sort of pseudo police siren sound. Also, "low" is somewhat of a relative term. On the Poly Evolver, an LFO can be set so slow that it generates a single cycle only every 30 seconds, or as fast as 261 cycles per second (hertz), which is almost the equivalent of middle C on a piano.

Anyways, for the test sound I made tonight, I had two LFOs controlling the volume of oscillators 3 and 4. The LFOs were set to a pretty slow square wave, which made one complete cycle about every six seconds. This basically gave the oscillators an On/Off effect. So, the final sound ended up being a pulsing sound, alternating between open and closed, with its timbre intermittently changing. You can hear the final result here, where I first play a single note/pitch, followed by a simple power chord.

This isn’t the first time I’ve messed around with step sequencers to control a filter, but it is the first time I’ve actually figured out how to program an analog-style one. Since the destination possibilities for each step are huge, I should be able to create some pretty interesting sounds using it. What’s more is that it can also be used to control other synthesizers… or at least I think.