Comb Filters

A few days ago, someone asked me something interresting : why a Feedback Comb filter was producing a lowcut effect…
I took a deep breath and a few minutes to think and, I remembered myself the theory about Comb filtering and especialy this part :

Fundamental frequency

The fundamental frequency of a comb filter is the inverse of the delay time. For example, if the delay time is 2 milliseconds (1/500 of a second), the accentuation occurs at intervals of 500 Hz (500, 1000, 1500, etc.), and the attenuation occurs between those frequencies. The extremity of the filtering effect depends on the factor (between 0 and 1) by which the feedback is scaled.

Source : MSP Tutorial

If I use a Comb+ filter (feedforward) and if my delai is 2 milliseconds, the first “1” will be at 500Hz and the first “0” will occurs between 500Hz and 1000Hz…

If I use a Comb- filter (feedback) with the same settings shown above, the first “0” will be at 500Hz and the first “1” will occurs between 500Hz and 1000Hz…

That’s it ! The Comb- filter is acting like a lowcut because its starts with an attenuation and not an accentuation…

I’ll try to edit this post later and give it some visual example…

Stay tuned !

“Analog” Combinator TR808

I’m trying to emulate the TR-808 with a “Combinator” mainly containing some Thor synth and 2 Redrum beat box. Currently, I already have really convincing results with the “Kick”, the “Snare”, the “hit-hats”. I’m currently working on the “Handclaps” and the next one will be the “Cowbell”.

TR-808 “Handclap”

Roland’s handclap sound is made up of two elements layered together; the “strike” and the “reverb” portion. The circuitry has different sections for these, and both are triggered simultaneously. The strike is a fast envelope with four closely spaced peaks. Both the 808 and 909 use a similar scheme to generate this envelope. The strike envelope controls a VCA (using an OTA) to shape white noise. Its multiple peaks simulate a group of people clapping. The “reverb” part of the clap simulates sound bouncing off the walls of a room. Its so minimal – the white noise source is high-pass filtered, and fed to an envelope-generator and VCA… but the VCA in this case is just one transistor.

Source : Eric Archer

TR-808 “Cowbell”

The Roland TR808 cowbell is the 808’s cheeziest sound, but its design is interesting to study because Roland engineers were masters of minimal analog synth design. They rolled this one up tight! … with just 2 simple chips and 4 transistors, the 808 cowbell contains two square wave oscillators, two VCAs, a triggered envelope generator, Hi-Q bandpass filter, and buffer amp.

The square wave oscillators are the usual CMOS schmitt-trigger type that you can build with 4584, 40106, 4093, etc. The schematic specifies their period instead of frequency. But you know that frequency = 1/period, so what they’re really asking for is FREQ.1 = 540 Hz and FREQ.2 = 800 Hz. These frequencies are variable with internal trim controls TM1 and TM2. As specified, the ratio of FREQ2/FREQ1 is 1.48, a detuned perfect 5th. This seems like its intentionally out of tune to enrich the tone.

Source : Eric Archer

Here is a nice “Sound On Sound” article of year 2002 : SYNTH SECRETS

Stereo Generator

I really like to work with mono samples because I master better the stereo space by re-creating this one in function of my needs. So here is a Combinator that simulate the stereo from a mono signal.

Fake Stereo

The process is very simple, I duplicate the mono signal, the first signal passes directly in a Comb filter and the second signal passes at first in a delay device before passing thru another Comb filter.

The result is a stereo signal nearly without the Chorus/Flanger effect that you may encounter if you try to mix the left channel and the right channel into a single channel. That the magic of the comb filter…
Of course you’ll have to set your own settings to achieve a good result !

Stereo generator

Enjoy !