Sunday, November 9, 2014

Using Key Follow/Tracking with a Modular

Key Tracking/Follow basics:

We've all seen Key Tracking or Key Follow on a synthesizer's filter section, right?   An analog synth's keyboard creates a voltage which is used to play the oscillators in tune up and down the keys.  Applying this voltage the the filter section can make higher-played keys sound brighter and lower keys sound more dull (in the case of a LPF).  If your filter can self-oscillate and is calibrated to 1V/Oct, you can use the oscillating filter as an extra VCO of sorts, simply tune the Fc to suit the pitch of your VCOs.  Some synths with multiple VCOs might also have a switch to turn Key Follow off for one or more of them, so that they can be used as FM sources.  These are the standard applications found on most synthesizers.  But we're here for modular applications, yeah? 

Example 1:
Key Follow // Logic (Reverb Send)

Have a look at this patch I created on  

Imagine that the Bus Access module is delivering the MAQ Row 1 sequence (or keyboard) CV and Gate to the system.  This CV not only controls the VCO 2 pitch, but also affects the Voice 1 Filter Cutoff, but the point of this patch is that this CV sequence is also sent to a Boolean Logic moduleA 16-step gate pattern from the MAQ Row 1 is also routed to the Logic module.  

When you play above a certain key (> +2.0V for the Doepfer A-166 used here) the Logic AND circuit allows the gate signal to trigger the reverb patch.   The trigger is passed through AND to the ADSR which controls the VCA at the Reverb's input, like an automated effects send.  (Note: this ADSR can be omitted if necessary and the gate/trigger can be fed directly to the VCA's CV input, or smoothed a bit first by using a lag processor / slew limiter)  

Now any low notes you play will remain dry and notes above +2V will have reverb applied (this is adjustable by using an offset generator, attenuator, or amplifier).  If you chose to use the ADSR for the send VCA, the full envelope will be heard with reverb.  If you omitted the ADSR, the reverb will simple be fed with the pitch that occurred for the duration of that step/key.  The Logic AND could be controlled by something else instead, like a joystick, LFO, sequencer, velocity CV, etc, but key follow works too and makes it all dependent on the key you press rather than something timing-based or something that would require a free hand.  Check out this audio/video example below.  

In the video below, at first the patch is the one shown above, but the patch changes to this towards the end which uses the other half of the Logic module (XOR output) and the /2, /8, and /32 outputs of an A-160 Clock Divider to alter the trigger pattern sent to Voice 1 (Bass). 

Example 2:
Inverted Key Follow // VC Decay

A modular system is open to many more voltage processing possibilities than most synthesizers on the market.  As I said earlier, most synths only allow you to use key tracking for VCO pitch and VCF frequency.  However, there are some that have mathematical functions as part of their modulation matrices (like my Waldorf Microwave XT and E-mu E6400 Ultra).   These will let you use Key Follow as a modulation source, process the signal, and route it to a variety of destinations.  Similar patches can be created in a modular system. 

One patch I’ve used takes the keyboard/sequencer CV signal and inverts it, so the higher up the keyboard you play, the lower the voltage becomes at the inverter output.   I’ve used this in combination with a Doepfer A-142 VC Decay module and a VCA to let low notes maintain the maximun Decay time (set by the Decay control) while high notes will have a Decay time altered by the voltage at CV In. Because the Key Follow signal was inverted, higher-played notes will have a Decay time equal to Decay Control value minus Sequence CV value.  This allows a single sequenced loop to provide both a bassline and higher accent notes which are easily distinguishable.  

See the basic patch example here:

In the video clip below, I used a more involved version of this same patch.  The patch you will hear in the video looks more like this (:

This first video first shows a rising progression of notes with Inverted Key Follow applied to the A-142 VC Decay/Gate.  Then the MAQ is switched to Random direction and sped up.  A joystick and VC Switch is used to disable the Inverted Follow, so all notes are of equal length while the joystick is held to the right. 

This next video shows the same patch driven by a different MAQ sequence.   While the modular patch was recorded, I used the Joystick to disable the Inv CV to the VC Decay, and LFOs were free-running to modulate the patch.  Other sounds were added after the modular take to show how it might work in a "track".

There are a few manual controls:   

-  The Manual Gate's Output 2 is used to reset the Clock Divider and Transpose Sequencer before pressing PLAY (since I did't have a Reset set up from a MIDI/CV converter).  

-  An A-174 Joystick was used to toggle one half of a Dual VC Switch.  This half of the switch module toggles between the Inverted Key Follow signal and NO signal, so that I can remove the Inverted Follow effect momentarily and then reactivate it at the same depth.  

There is Logic at work:

In this patch, the top half of the A-166 is used to "mute" the sequence.  Input 1 is the Gate Out pattern from Row 1 of the MAQ16/3.  Inputs 2 and 3 are the /4 and /32 Outs from the A-160 Clock Divider.  This way, the envelopes are only triggered when all three inputs are HIGH (this changes for a section of the video, when just Row 1 gate pattern is used).

The bottom half of the A-166 is used in combination with a modified A-119 Envelope Follower.  I modified the module to let me use the internal comparator (which normally tracks the follower signal) with any external signal, in this case, the main CV sequence from the A-156.  The THRESHOLD is set so that only the highest notes of the sequence create a GATE OUT signal.  This gate is sent to the bottom Logic.  The AND output from the top Logic is also sent to the bottom section.  Now, only the highest notes of the sequence will trigger the ADSR/VCA which feeds the SR-01 Reverb, and only when the trigger sequence is "unmuted" (top, Logic AND).  This is all similar to Example 1, above, but uses a comparator for its threshold setting, rather than being stuck to a 2V threshold if using the Logic module alone. 

There are three voices, all playing simultaneously from the same 16-step sequence.

  -  Voice 1: Z3000 Saw and Square waves through A-109 (#1) VCF into A-125 Phase Shifter, then to A-199 Spring Reverb, finally back to the A-109 (#1) VCA.  The TipTip Z3000 receives CV sequence from MAQ Row 1 via A-156 Dual Quantizer, transposed by A-155 Row 1.  A-109 Fc is modulated by A-156 CV sequence (key follow) and by A-140 ADSR.  A-109 VCA is controlled by A-142 Env Out.  VCA out to A-138 Mixer.  Phase shift is controlled by an A-148 Dual S+H module, with an A-143-3 LFO as the sample source, and the MAQ Row 1 Gate pattern as the trigger.  The Reverb feedback is patched through an Analogue Solutions SY02 Multi-mode filter (MS-20 clone) and modulated with another A-143-3 LFO to the HPF CV In. 

  -  Voice 2: A-111-1 (#1) Sine to A-131 (#1) VCA, Audio In 1.  A-111-1 (#1) Saw to A-124 Wasp VCF (HP) out to A-131 (#1) VCA out to A-138 Mixer.  A-111-1 receives sequence CV from A-156 via top half of A-170 Dual Slew.  A-124 Wasp is modulated by one unit of an A-143-3 Quad LFO.  VCA is controlled by A-142 Env Out.

  -  Voice 3: Z3000 Saw to A-115 Audio Divider out to A-109 (#2) Audio In.  A-109 VCF Out to A-131 VCA (#2) to A-138 Mixer.  VCA is opened by an A-140 ADSR controlled by the Gate Output of the A-142 VC Decay module.  A-109 VCF Q is moduated by an A-143-3 LFO.  VCF Cutoff is modulated by a combination of sources.  An A-146 LFO Pulse is sloped by the lower unit of the A-170 used in Voice 2, and then attenuated into CV2 In.  Next, outputs from another LFO and another VCO are patched to an A-135 VC Mixer.  Their levels are modulated by another two LFOs.  The Mixer output is send to VCF CV1 In.  The Pitch of this VCO is also controlled by the main CV sequence from the A-156 Quantizer.  The VCF Out is also sent to an Analogue Solutions SR-01 Spring Reverb.  The SR-01 output is boosted by an A-119 Ext. In module and fed to an A-123 24dB HPF.  The HP output is sent back to the A-109 VCA In controlled by another A-140 ADSR.  VCA out to input 4 on the A-138 Mixer.  The A-140 controlling this VCA is triggered through the bottom half of the A-166 Logic (inputs = AND from top of A-166 + A-119 comparator out, see above)

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