FIG. 1: I use settings similar to these in Sound Toys Filter Freak for drum-to-bass conversion.
You can craft interesting quasi-random bass lines from drum loops with the aid of a full-featured filter plug-in such as Sound Toys Filter Freak (see Fig. 1). The only requirements are that the filter must self-oscillate when the resonance is turned up all the way, and it needs an envelope follower to modulate its cutoff frequency.
Select a drum loop that has an interesting rhythm, perhaps with snare beats on something other than 2 and 4, and with accents in unusual places. Load that loop into an audio track of your DAW and insert the filter plug-in on the track. Turn the resonance of the filter all the way up and play the drum loop.
Good, Good, Good Oscillations
The filter will self-oscillate, producing something close to a sine wave whose frequency is the cutoff frequency of the filter. You can moderately vary the waveshape — and hence the timbre — by changing the filter slope (which Filter Freak indicates as Poles, but other filters may display in decibels per octave). Set the cutoff frequency so that the pitch is in the bass range. The filter may continue to oscillate when playback stops, which is annoying but normal. To control that, automate the filter resonance, or simply turn it down manually at the end of the bass sequence.
To go beyond a 1-note drone, route the envelope follower to modulate the filter cutoff frequency. The pitch of the bass will now follow the dynamics of the drum loop. The amount of modulation controls the pitch range. Experiment with any other envelope-follower controls that the plug-in offers, such as threshold, gain, and attack or release time. The key is to adjust the available controls until you get an interesting bass line. You don't have complete control of the pitch, but the results can be useful just the same.
You might modify the drum loop used to generate the bass line in a variety of ways. You don't need to have the altered drum loop in the mix, so radical changes that may not work as a drum part are fair game. For example, you might cut the drum loop into 16th-note slices and rearrange them to improve the bass line. You could then make the bass line longer by varying the pattern of slices over four or eight bars (see Web Clip 1).
Bear in mind that the filter cutoff frequency will determine the lowest note of your bass part and the envelope modulation amount will determine the highest note. You can vary the notes in between by changing the volume of individual beats within the drum loop. To do that, insert a trim plug-in before the filter and automate the trim level to tweak the level of each beat to produce the desired pitch (see Web Clips 2 and 3). The process is time-consuming but gives you more control over pitch.
If you have Spectrasonics Stylus RMX, you can use it to mix things up further. Place the filter plug-in after Stylus. Choose a beat from Stylus, drag its MIDI file to a MIDI track, and play the beat in Slice mode. Then switch beats in the Stylus Beats menu, but use the MIDI file from the first beat you selected (see Web Clip 4). Because the two beats have different MIDI files, the resulting groove will be different from either of the selected beats, and the bass line will change considerably (see Web Clips 5 and 6). Use the Chaos feature of Stylus to create still more variations. You can do similar things with Submersible Music DrumCore, FXpansion BFD2, and other drummer software.
This technique is not limited to bass lines. Raising the cutoff frequency will produce a midrange or high part, but it might be harder to find an application for higher parts. Raising the frequency slightly will create a low midrange line. Filtering out the low end then leaves sonic space for a simpler bass line below the midrange part.
Quasi-random bass lines are not for every musician or type of music. But even if you end up not using the generated bass line, the other ideas that a random musical phrase can inspire may justify the effort.
Steve Skinner is a programmer, arranger, and producer. He has composed and produced numerous pieces for television, some of which use loop-created bass lines.
Sound Toys Filter Freak
Spectrasonics Stylus RMX
Submersible Music DrumCore
Hear audio examples of drum loops and basslines.