DIY Digitally-Controlled Analog Filter Design

In the Beginning

For the Rockit Synth, I designed a digitally-controlled analog filter.  I went down this path after a good amount of thinking, learning, and design attempts.  First, I looked into cribbing a filter design from some reputable synthesizer.  I found a few problems with this approach.  Primarily, voltage-controllable analog filters tend to be part-count heavy.  Look at most designs and you’ll find dozens of components, many of them requiring trimming to work properly, and many of them requiring many dollars to purchase.  The designs also tend to require high rail voltages, like +/- 15V.  And some require all of the above. Take a look at an old Moog filter.  It’s got boatloads of parts, high rail voltages, and in the end is only a low pass filter.  Sure, it sounds sweet, but my goal was to make an affordable kit synth.  Maybe one day, I’ll splurge and build a no holds barred filter, but it turned out to be interesting to live with some limitations and make my own filter.  Click through for a detailed discussion of the filter in the Rockit Synth.

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Rockit 8 Bit Synth: Filter External Audio Input Sample

I finally got around to testing the external filter audio input.  It had been sitting there untested for a couple revisions.  Other priorities generally carried the day, but, as I get close to the end, even the little things need to be tested.  I’m glad to say that it works as expected.  It’s actually a lot of fun to run new non-synth sounds through the filter.  Letting the LFO run on filter parameters generates fun wah sounds and sweeps and stepping through the different filter types is interesting.  I’m going to muck with the filters one last time tonight once and for all to see if I can get any little improvements.  I’d really like to get the maximum low pass frequency cutoff higher, but it comes at a price of audible frequency steps near the top of the frequency range.  I’ve changed values in there a dozen times and I keep coming back.  This will be the last time.  Anyways, here’s a sample of what it sounds like to run sounds through the filters. I run through the three filter types first, just moving the filter cutoff up and down and then I start using the LFO to modify the filter parameters.  Make sure to at least skip towards the middle to hear it get really crazy. I apologize for the noise. As I’ve mentioned in another post, it’s a problem that I’m aware of and fixing in the final revision.

External Filter Input Audio Sample