Filters for the ages
The sound of the analogue filter has become synonymous with synthesisers. It’s the source of the squelches, quacks and chirps that punctuate our favourite synth tunes. In the early days, a manufacturer’s filter design was a closely held property. Robert Moog was justifiably proud and protective of his famous 24dB low-pass filter, as rival company ARP found out when they came up with a design that was a bit too similar to Bob’s!
Moog charted the map that would be followed by nearly every manufacturer, with most instruments sporting a low-pass resonant filter. Few designs were as beloved as Moog’s transistor ladder-based design, though a few are now recognised for their own unique qualities. Tom Oberheim quite sensibly chose a less-precise 12dB design for his SEM modules, as they were intended to be paired with a Minimoog or an ARP, and Oberheim rightly felt the contrast would be appreciated.
After being dissuaded from using a clone of Moog’s filter, ARP came up with their own 24dB low-pass jobs, though all were hobbled by a manufacturing flaw that reduced the frequency range ever so slightly. Their Odyssey added a non-resonant high-pass filter to the path, a combination that would become commonplace on Roland synths in the 1980s.
EMS went their own way, using a diode ladder arrangement for the supremely squelchy filter on the VCS 3 and Synthi A. Diode ladder designs would also be used by Nyle Steiner in the Steiner-Parker Synthacon, as well as Roland in their TB-303 – probably the most convincing argument in favour of this quirky, chaotic approach.
Some of the filters described herein were self-oscillating – meaning that the resonance could be increased to the point of producing a sine wave. This useful trick was the source of many an acid-drenched space jam in the ’70s.
Until recently, vintage filters have been notoriously difficult to reproduce in virtual form, partially due to the processing power required to do so convincingly. Luckily, advances in DSP have given developers the freedom to come up with some awesomely accurate emulations, such as those built upon zero-delay feedback algorithms.