[sdiy] 2164 tempco in VCF applications

Tom Wiltshire tom at electricdruid.net
Thu Nov 8 13:07:09 CET 2018

> On 8 Nov 2018, at 05:33, Michael E Caloroso <mec.forumreader at gmail.com> wrote:
>> The Xpander and Matrix 12 tuning routines include the filter cutoff and resonance.
> But not the older Oberheims.  VCO range only (scale is not autotuned)

Fair enough. There are different solutions to the problem. One way is to add temperature compensation to the circuits so they don’t vary (or only vary by a tolerable amount) or the other way is to allow them to alter but to provide a full autotune routine that enables the microprocessor to correct for any errors.

My view of synth history is that early instruments started with hardware solutions for everything (envelopes and LFOs are always hardware on earlier polysynths, for example) and that as microprocessors became more widespread and more powerful, more and more got moved into software, first with tuning routines (Prophet 5) and then later with LFOs, envelopes, and finally the entire modulation-side of the synth voice. The end point of this evolution is instruments like the Xpander where the voice hardware is basically two chips (CEM3374 dual VCO and CEM3372 VCF+VCA) provided with a handful of CVs, or the SixTrak, which even uses the processor to generate DCO timing pulses. Later synths avoided hardware wherever possible, and avoided trimmers by use of tuning routines whenever possible.

This question of whether you use hardware or software to sort out filter drift due to temperature (or anything else) is just a part of this larger story about how deeply you embed the microprocessor in the synth’s architecture, and how far you have a separate voltage-controlled analog synth with separate uP control.

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