Wizardry: Filters

Content courtesy of John Papiewski, except where noted

Voltage Controlled Q Filter (VCFQ)

Pro: Superb clean sound. Voltage Controlled Q. Perfect tracking. This filter will not oscillate unless you patch it up for feedback. Simultaneous lp/hp/bp/notch outputs. Separate audio inputs for variable gain and AGC, which compensates loudness for any Q changes. Pulse input for ringing the filter, variety of nice effects from pops to ringy bells to wood blocks. Hi-Lo range switch for fantastic and unique audio/cv applications - hit it with a pulse, get an unbelievably slow damped sine wave. Calibrated 1 v/oct and attenuated cv inputs, just like the oscillators. If you have only one filter in your system, this one is a great all-rounder.

Cons: Since it's a multimode filter, it's 12 dB/oct rolloff. You may want a LPF with more 'bite' in some applications. I think the bandpass and allpass sections of this filter have 6 dB/oct rolloff.

Wizardry: Must be patch-programmed to oscillate. Feed an output back into an input, then turn the Q up to get a nice sinewave. The various outputs will then be 90 degrees phase-shifted from one another.

Hey! Ain't there a way to hook up two 12 dB/oct filters together to get a 24 dB/oct with lotsa bite? There is, and here is the magic, courtesy of Harvey Devoe Thornburg.

4-Pole filter, by Harvey Devoe Thornburg

How to get a *killer* 4-pole filter sound from two VCQF's

2xVCQF, 1 VCA, 1 envelope detector, 1 dual processor/equivalent (used for offset inversion)

The VCQF is a state-variable 2-pole. Although it has a very nice, clean sound (great for processing drum loops) it falls short if you expect that Minimoog/VCS3/TB-303 type sound. By using controlled feedback around 2 VCQF's in series, you can get a nonlinear filter that IMHO sounds even better than any of the aforementioned filters. The patch may seem complicated so it's good to start with an overview. Clearly, two two-poles in series gives us a four-pole. The problem with this open-loop series configuration is that you'll get large amplitude swings when sweeping over a harmonic signal at high Q. Unless you like this sound, you'd remedy this by using a compressor. But what can you do about self-oscillation? The trick is to use feedback to generate Q externally, but control the amplitude of this feedback by monitoring the overall output amplitude. How is feedback used to generate Q? We use Moog's old trick of feeding back the inverted output of the open-loop 4-pole combination. By themselves, the filters provide 180 degrees of phase shift at the cutoff frequency. Inverting this generates another 180 degrees of shift, which regenerates the signal at the cutoff frequency, providing a resonant peak at that frequency. We get a screeching self-oscillation at uncontrollable high amplitude. This establishes the need for external amplitude control. If we can monitor the signal amplitude somewhere in the feedback path, then we can use a VCA to attenuate the feedback gain, keeping the "screeching" well-behaved. Now when we play with If the feedback gain gets too large (>4), this configuration will go unstable.

The input for this configuration will be the regular input (the "IN") for the first filter. The output will be the LO output of the second filter. Track the cutoffs of both filters in parallel (*make them equal at all times*), and set the Q to minimum, since we're generating the resonance externally. Also if there is a "gain" knob set this to maximum. (on the VCFX the gain is always set to maximum). Patch the LO output of the first filter to the input of the second. Now create the feedback loop. Patch the BAND output of the second filter to an inverter (use one of the dual processor sections, with the scaling knob full left). Take the output of the inverter and run it through a VCA. Patch the output of the VCA to the AGC input of the first filter, thus closing the loop. It is highly recommended to use a UAP as the VCA, because we want something capable of large gains (4x is necessary for self-oscillation). Offset the output of the second processor section to +5 volts, and patch this to the VC input of the VCA.

At this point, the only thing left to do is to implement amplitude control. Monitor the VCA output by patching it to the input of an envelope detector. Invert the output of the envelope detector using the processor controlling the VCA. You won't need to do a full inversion: turn the knob only partially left. Now you should have a great sounding four-pole filter!!! You can play with the Q control on the first filter to vary further nuances in the sound. It's best that the input to this filter be full-amplitude, like a sawtooth or square wave from NTO.

Voltage Controlled Slope Filter (VCFS)

I have limited experience with this one, but here goes... Another nice filter. Cutoff slope is voltage controllable from a subtle 6 dB/oct to a more aggressive 18 dB/oct (?). More bite than the VCFQ, above. Handy little A/B input proportion mixer. Does not 'ring' or oscillate as far as I know.

From the catalog:
The VARIABLE SLOPE VCF offers unique control of sound quality offered by no other synthesizer manufacturer. All VCFs offer voltage control of the cut-off frequency, that is, control of which frequencies the filter lets pass. The VCFs allows the amount of filtering to be dynamically controlled as well, from barely perceptible filtering to highly resonant, sharp cutoffs. With the variable slope control in the center position, the VCFs acts as a typical flat-response VCF. with high, low, and and band-pass outputs available simultaneously. The slope of the cutoff is 12 dB/octave. As the the control is moved to the maximum position, the resonance of the filter increases, so that the cutoff becomes sharper. Although the VCFs will not ring like the VCFQ, it will resonate enough at the maximum setting to pick out harmonics from a complex signal input. As the control is moved to the minimum position, the cutoff slope will decrease to 6 dB/octave. This type of change of filter slope has been found to be an effective synthesis technique corresponding well with some of the transformations in acoustic instrument sounds.  There are two signal inputs to the VCFs which can be mixed and manually cross faded from the associated knob.

Variable Bandwidth Filter (VCF2)

Contributor Norman Fay says:
1. By mixing the hi-pass and lo-pass outputs together, a variable width notch filter is created. By turning the bandwidth control, the sound goes from a very faint fixed phaser type of effect to complete filtering out of the sound, with upwards and downwards filter sweeps clearly audible. If you are fading out a part during mixdown of a piece, doing this instead of pulling down your mixer fader creates a startling "how did they do that" type of effect.
2. Mixing the two fixed bandwidth outputs with the dry signal creates a pair of resonant-ish peaks in the sound without overwhelming it. Adjust both bandwidth and frequency controls to taste. At certain points you will find that almost ANY signal source takes on an eerie "vocal" quality, especially if you apply lots of reverb.
3. Mixing the lo-pass output with variable amounts of the two fixed bandwidth outs gives a response something like a cross between phasing and filter resonance.
>From the above, you'll probably conclude that a MIX2 module is an essential add-on to the VCF2, and you'd be right. The sound of the VCF2 is also very pleasing to the ear, and I'd strongly recommend anyone to add one to their Serge setup.

From the catalog:
The VARIABLE BANDWIDTH FILTER has a bandpass output which can be varied manually or with voltage control. This is a standard response synthesizer VCF, typical to filters used in many studio systems. In the VCF2, two state-variable VCF's are connected in series to produce a total of five outputs. High pass, low pass, two fixed bandwidth outputs and one variable bandwidth output are available. The outputs are all flat-response (no resonance) so the VCF2 is suitable for processing concrete sounds without introducing resonant coloration to the timbres. Under voltage or manual control, cutoff frequency of the high and low-pass outputs are affected, as well as the center frequency of the two bandpass outputs. Both center frequency and bandwidth are independently controllable on the variable bandwidth output.

Resonant Equalizer (EQ)

From Chris Whitten:
"Acts as a really nice desk eq. It's probably no better than a vintage high end desk eq but it certainly is more radical than my Mackie. I haven't tried using it on purely synthesized sounds yet. I have used it extensively on drums and drumloops. I often use it to add a little spice to a loop, the top end is incredible. You can get it into the realms of reggae dub eq, in other words really over the top highs and lows. I have also used it to add or subtract frequencies before sending a drum or loop to another module, the resonance is great for this. The module is much more spectacular than I expected."

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