<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class="">On Aug 22, 2019, at 6:07 PM, Tom Wiltshire <<a href="mailto:tom@electricdruid.net" class="">tom@electricdruid.net</a>> wrote:</div><div class=""><div><blockquote type="cite" class=""><br class="Apple-interchange-newline"><div class=""><div class="">I don’t like to disagree with you, Don, but I’m not sure what you’re thinking. Moog or SVF are definitely *not* the only two filter options.<br class="">OTA+cap-to-ground+buffer? VCA+Integrators?<br class=""></div></div></blockquote><div><br class=""></div><div>Consider a description of a filter as a sort of "taxonomy" with three layers:<div class=""><br class=""></div><div class=""> Top Layer: the filter spec, number of poles, response</div><div class=""><br class=""></div><div class=""> Second Layer: the topology that implements that filter function</div><div class=""><br class=""></div><div class=""> Bottom Layer: implementation details, including the control element</div><div class=""><br class=""></div><div class="">So a Moog Ladder would be:</div><div class=""><br class=""></div><div class=""> Top Layer: 4 pole, low-pass, with resonance</div><div class=""><br class=""></div><div class=""> Second Layer: 4 single-pole low-pass sections in series, with feedback</div><div class=""> </div><div class=""> Bottom Layer: the ladder circuit</div><div class=""><br class=""></div><div class="">And a State Variable filter would be:</div><div class=""><br class=""></div><div class=""> Top Layer: 2 pole, multi-mode</div><div class=""><br class=""></div><div class=""> Second Layer: 2 integrators and an inverter, in a loop</div><div class=""><br class=""></div><div class=""> Bottom Layer: the circuit, perhaps OTAs </div><div class=""><br class=""></div><div class="">And so forth. </div><div class=""><br class=""></div><div class="">This analysis also works really well with oscillators and other functions.</div><div class=""><br class=""></div><div class="">Here's a Moog style VCO:</div><div class=""><br class=""></div><div class=""> Top Layer: VCO with sine, square, triangle, sawtooth waves</div><div class=""><br class=""></div><div class=""> Middle Layer: block diagram with exponential current source, sawtooth core, waveshapers</div><div class=""><br class=""></div><div class=""> Bottom Layer: the circuit details</div><div class=""><br class=""></div><div class="">So if I dismiss the implementation details, as defined this way, it limits the number of unique filter designs.</div><div class=""><br class=""></div><div class="">You know I'm a big fan of implementation details. And you'd want to make sure that the implementation details didn't have a significant functional effect as you draw these lines. That's part of the craft.</div><div class=""><br class=""></div><div class="">But if I'm characterizing filter types, I think it's reasonable to pay attention to the implementation topology and ignore the implementation details.</div><div class=""><br class=""></div></div></div><div class=""><div style="color: rgb(0, 0, 0); font-family: Menlo; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;"> -- Don<br class="">--<br class="">Donald Tillman, Palo Alto, California<br class=""><a href="http://www.till.com" class="">http://www.till.com</a></div>
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