Here's what you'll need for starters:
- A clock source, such as a DTG.
- A sequencer to generate your melody and bassline. In this case I'm using a TKB.
- SSG: The Stepped section will be used to set the pitches for the bassline.
- Processor: This will be used to mix the triggers being sent from specific sequencer stages, and will drive the S&H pitches and rhythm for the bassline.
- A pair of VCOs: One for the melody and one for the bassline.
In this example, I'm using a TKB to sequence a melody (row A) and bassline (row B). Here's the basic patch:
The sequence is clocked by a DTG. If the pulses are thought of as 16th notes, then the tempo is 60 bpm, with the 16 stages of the TKB taking 4 beats to complete.
Row A drives the melody, which in this case is simply a C major scale: stages 1-8 climb up the scale, and stages 9-16 climb back down. Row A's output is connected to the 1V/OCT input of the first oscillator.
Row B is the bassline, which occurs on stages 1, 9, and 15, and whose tunings complement the melody. Row B's output is sent to IN on the Stepped section of the Smooth / Stepped Generator. The trigger outputs for stages 1, 9, and 15 are patched to one section of a Dual Processor, whose OUTPUT sends pulses at the designated stages to the SAMPLE input of the SSG. The Stepped OUT jack on the SSG then sends the bassline pitches to the 1V/OCT input on the second oscillator.
The purpose of the VC RATE knob on the Stepped section of the SSG is to control the size of the steps (i.e. the interval between pitches) to be allowed at the output. The further counter-clockwise the knob is turned, the smaller are the steps allowed at the output. Fully clockwise allows maximal step sizes.
The intervals in the bassline are a perfect 4th (C down to G), a major 6th (G up to E), and a major 3rd (E down to C). These intervals are larger than what would be allowed if the VC RATE knob was fully counter-clockwise, or even anywhere in the lower half of the knob's range. The lowest I could set the knob to allow the largest interval was at the noon position.
The VC RATE knob has an associated VC jack. The rate (step size) can be forced up or down with control voltage, which could be used to interesting effect. For example, if the rate is forced down slightly (smaller steps allowed), the major 6th interval (G up to E) becomes a minor 6th (G up to D#), effectively changing the key.
That's the simple version of how to accomplish this. There's a fuller version of this patch below, which illustrates why the Sample & Hold function of the SSG is a vital element in creating the bassline. I'll explain...
Here's what you'll need for this expanded patch, in addition to the items already mentioned:
- Two DSGs: These are used to determine the final amplitude envelopes for the two VCOs.
- Two VCAs or equivalent: In this case I'm using a pair of XFADERs.
- Optional: A Filter, such as a VCFQ.
The main difference in this fuller patch is that I'm using two DSGs to control the gain envelopes on the XFADERs. I also used a third row on the TKB to sweep the frequency on a VCFQ to give my dorky melody some shape.
Now I can explain why the Sample & Hold is so important in this patch.
If we skipped the SSG and sent the bassline triggers from the Dual Processor directly to the DSG controlling XFADER #2, we'd run into a problem: If the gain envelope has any amount of tail on it, then when stages 1, 9, and 15 fire we'll hear the pitches of the in-between stages trailing off with the envelope. That's just a function of working with sequencers like the TKB.
The Sample & Hold solves that problem by holding the pitch that occurs with each trigger until the next trigger arrives, ignoring the pitches of the in-between stages.
And that's really it! This basic idea could be modified to use different trigger sources for the bassline, such as a particular DSG firing at a chosen moment, or an envelope follower that triggers with a kick drum, or...