Agile Filter Bank

Included in the MRB Luxury Filter Bundle

CLICK ON THE SOUNDS TAB FOR DEMOS

INTRODUCTION

The MRB Agile Filter Bank (AFB) is a vastly improved version of the fixed filter banks (FFB) of old. A couple of the first were the modular Moog models 907 and 914 which provided 10 and 14 bands respectively. Each 4-pole bandpass channel used 2 coils and 2 capacitors for each band to set the frequency and bandwidth, and were quite difficult to build as most of the parts had to be measured and hand selected to get everything right. Even though the AFB doesn't have coils and capacitors, it is built with an identical architecture and is based on the same design principles.

As the name implies, fixed filter banks have a set or "bank" of filters which are fixed in frequency and bandwidth. Only their amplitudes are adjustable in a way similar to a graphic equalizer. The main difference between an FFB and a graphic EQ is that the amplitude of each filter in the FFB can be taken down to silence which is not possible with an equalizer with its +/-12db of adjustment range. This means an FFB can be used to admit only frequencies of interest while blocking all others, which is great for imparting multiple resonances to an otherwise boring signal. Body resonances of stringed instruments and the formants of a singing voice can be approximated.

The difference between an FFB and the AFB is that the filters on the AFB are definitely not fixed. They can move about in frequency and change bandwidth under user or voltage control. This breathes new life and motion into the once static sound produced by FFBs. If you leave the filters motionless, the module reduces to an FFB and can be used in the traditional way.

DESCRIPTION

The Agile Filter Bank is clearly divided into 2 sections through the use of 2 different color knobs. The white knob section can be recognized as the familiar 14-band fixed filter bank similar to the Moog 914. The 12 knobs arranged in a square block adjust the amplitudes of 12 bandpass filters. The knobs to their left and right adjust the amplitude of a lowpass and highpass filter that sit at the bottom and top of the frequency range. The center frequency of each filter is labeled. The jack below each knob outputs the pre-fader signal from each individual filter (ie. the knob has no effect).

The jacks at the bottom left, labeled ODD EVEN ALL are the audio inputs. The similarly labeled jacks on the right are the audio outputs. You can input to, or take output from, the ODD set, the EVEN set, or ALL the filters. It is possible to process one signal through the ODD and a completely different signal through the EVEN and keep them independent. Little ODD / EVEN labels are provided below each filter knob as a helper guide. Notice that the ODD filters are harmonically related, that is 125, 250, 500, 1000, etc. are octave spaced, and likewise the EVEN filters beginning with 175, 350, etc. This yields a total bank of 14 filters spaced 1/2 octave apart.

HOW TO USE

If you have never used a filter bank before, now would be a good time to plug a signal into the ALL input, listen to the ALL output, and fiddle around with all the white knobs to hear what this module does (while hopelessly fighting the irresistible temptation to touch those yellow knobs for now) . Pink noise is the perfect signal to use for this, and a single sawtooth oscillator is great to hear how the different frequency bands "voice" the signal and give it an organic character. After you've twisted enough knobs, listened to a few individual outputs and tried out the ODD / EVEN signal routing feature, you're ready to move into the future.

It's yellow knob time. The yellow knobs with their associated jacks are what make the AGILE FILTER BANK agile. The F ODD and F EVEN knobs and control voltages adjust the frequencies of the odd and even filters independently. This is a very powerful feature and means that multiple instances of the AFB don't have to be tuned the same! You can have one tuning in the right and another in the left stereo channel. You can also interleave the frequencies of one bank with another, effectively doubling or even tripling the number of resonant peaks, but this is only the beginning. Applying control voltages such as transients and LFOs to the ODD and EVEN frequency controls makes the AFB come alive with motion. Below these frequency controls on the right you will find the Q (resonance) adjustment and its CV input. (This was the hardest part of the project to do.) When you move either the F or Q knobs or change a CV, the filters in the AFB are totally redesigned on the fly. Remember how I mentioned the hand selection of components in the original FFB? This is kind of what's happening here. Try the pink noise again and move the Q knob. The sound changes drastically.

The ASYNCHronous LFO is the icing on the cake. This is not one LFO, but 14! There is one sine wave LFO assigned to each filter. They are free running and detuned from one another (asynchronous), so no two filters move the same. Their average rate is controlled by the RATE knob. The amount of frequency modulation is set with the DEPTH knob, and the BALANCE knob controls how much modulation is sent to the ODD and EVEN set of filters. The tremulants you can produce are amazing, and you can use the LFO on top of your own modulations, of course.

When you bypass this module, the ODD / EVEN / ALL routing will still be respected, but no filtering will be performed.

This description only touches on the use cases for this module. I look forward to hearing what you do with these new capabilities brought to our old friend, the filter bank.

All sound demos are recorded live using 1 track with no sequencing, EQ, or effects. Even though you may hear what sounds like FM or vibrato, there is none used. All motion in frequency and timbre is being provided by the Agile Filter Bank.

DEMO TRACKS

Lone_Sawtooth is exactly that -- one VCO sawtooth. An envelope is patched to the AFB CV inputs and there is a little LFO dialed up as well.

A_Study_In_Pink is a single pink noise source on the ALL input and some knob twisting. Real time, one take.

Big_Rome is 2 poly VCOs spaced 1 octave apart and the internal AFB LFO.

The ODD and EVEN outputs are patched left and right on all demos and no multitracking or effects are used.