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Tetra v1.2 VST


How it works
How to use Tetra
          Special Controls
MIDI Control Mapping
Version History


Sound demo of the new v1.2 presets: tetra12newpresets.mp3


Tetra generates a bandlimited quadrature signal, ie. a stereo signal whose harmonics differ pairwise by 90 degrees. By morphing between two definable timbres, Tetra can create a wide range of unique sounds.

Tetra was made using Synthedit. The sound engine isn't based on native Synthedit modules, though, but newly coded in C++. 


  • Stereo output of bandlimited quadrature signals with up to 1000 harmonics.
  • Morphing between two timbres, controlled by an envelope.
  • Copy & Paste buttons for quick storing and restoring of all control settings.
  • Control changes may affect the sound either immediately or when the next note is played.
  • Text field input for all controls.
  • Freely selectable polyphony with up to 16 voices. Note sliding. Adjustable pitch bend range.
  • Envelope and key tracking controlled volume.
  • Freely adjustable Attack, Decay, and Release slopes.
  • MIDI control of all parameters (NRPN).
  • 111 presets.

How it works

Feel free to read my article The Math behind DSF Synthesis if you're interested in knowing how the synthesis of Tetra works.

How to use Tetra

Special Controls

First some words on special controls and general control handling:
  • Copy, Paste: Pressing Copy stores all control settings, and Paste restores them. This is useful when you temporarily change some settings while playing and then want to return to the initial state. Or when you are creating new presets. In fact, it is possible to copy the settings of one preset to another preset. Copy & Paste only works when the plugin is active and ready to play notes. The leds below the buttons will flash to indicate a successful storing or restoring.
  • Automatable: If this is active, the Copy and Paste buttons are automatable, ie. they can be influenced by an extern control like all other knobs and buttons. However, Copy and Paste will not work properly when you are recording control events - provided your host is capable of that. When recording control events, Automatable should be turned off.
  • Change per Note: When this is turned off, changes of the controls will immediately affect the sound of all currently playing notes. When turned on, control changes will only affect the next played note and the notes after that.
All but the switch controls have text entry fields which you can use to enter values - just click on them and type. The sliders can be moved with more precision when you press CTRL while moving them. And to turn the knobs with more precision, click on them, then move the mouse pointer away while keeping the mouse button pressed; the farther away the mouse pointer is, the more precise will be the knob.


In the Oscillator section, the two knob rows marked as Set1 and Set2 define two different timbres. When a note is played, Tetra will morph between those two timbres as defined by the envelope generated in the Morph Control section. If a note is played, morphing will start at the timbre defined by the lower set, Set2. The timbre will then change into the timbre defined by the upper set, Set1, at a speed defined by the Attack slider. Morphing will stay at that level for the time defined by the Hold slider. Then it drops back to Sustain level at the speed defined by the Decay slider. Finally, when the note is released, the timbre will change back to the timbre defined by the lower set. If the LFO in the Morph Control section is active, it will add a vibrato to the morphing envelope.
Now, the "lower" parameter set, where the morphing starts, is actually not always Set2, but it is freely selectable anywhere between Set1 and Set2. The starting point of the morphing is defined by the Range Lower slider in the Morph Range section. If the slider is in the lowest position, the starting point is Set2, and if it is in the highest position, the starting point is Set1. Similarly, the "upper" parameter set, which is reached after the Attack phase, is defined by the Range Upper slider.

Oscillator Settings

The Harmonics knob defines the amount of harmonics generated by the oscillator. Other than that, the Oscillator section defines the parameter sets Set1 and Set2 that determine the timbre range. The knobs in Set1 and Set2 have the following meaning:
  • FreqRatio: These two knobs determine the distance between two neighbored harmonics, the frequency fr.While the fundamental frequency fis specified by the currently played note and the Pitch settings, fr is computed as
    fr  = fc * left / right + offset, where "left" is the value of the left knob, "right" is the value of the right knob, and "offset" is the value of the FreqOffset knob.
  • FreqOffset: See the description of FreqRatio above. Choosing non-zero values for FreqOffset: usually detunes the sound but can lead to interesting effects.
  • Pitch: Alters the pitch, measured in octaves. That is, a value of 1 means that the note is played one octave higher.
  • Round: If the Round button under the FreqRatio knobs is turned on, then the values of the FreqRatio knobs will be rounded to integers, causing a harmonic spectrum. Similarly, turning on the Round button under the Pitch knob will round the pitch value to an integer, so the pitch will increase/decrease by full octaves when turning the knob.
  • Brightness: This is the rate by which the magnitude of the harmonics drops off from the fundamental frequency. Low values make the sound dull, high values make the sound bright (or even unhearable, unless the overall number of harmonics is small).
  • Color: If this knob is in its leftmost position, the harmonics will be on the "left" side of the fundamental frequency only, ie. in the lower frequency range. If the knob is in its middle position, harmonics will be on both sides of the fundamental. And if it's in its rightmost position, the harmonics will be on the right side of the fundamental frequency only. Turning the knob to intermediate positions will continuously mix between those states.
  • CombFreq and CombPhase: The harmonics' magnitudes are altered by a comb pattern. CombFreq determines the distance between notches of the comb, and CombPhase determines the distance by which the notches are offset.
  • FreqMode: In mode Fixed, the distance between notches of the comb pattern does not alter when morphing rests at a certain level. In mode Shift, however, the distance between notches permanently changes; the CombFreq knob doesn't define the fixed distance between notches but rather the speed at which the distance changes. Track is similar to Shift, but the speed is not only influenced by the knob value but it also scales with the played note. Either way, the continuous changes of the notch distance create a flanger-like effect.
  • PhaseMode: FixedShift, and Track are analogue to the FreqMode settings, yet they don't affect the notch distance but the notch offset. Additionally, there is the setting Effect, which creates sort of a wah-wah effect that also can be altered by the FreqMode and PhaseMode knobs. Well, it's hard to describe, just play around with the settings.

Morph Control

The Morph Control generates the envelope by which the signal is morphed between the two parameter sets. The envelope is computed adding the output of an ADSR and of a low frequency oscillator.

ADSR settings:
  • Attack, Hold, Decay, Sustain, Release: Once a note is played, morphing will basically follow the envelope defined by these sliders. Hold defines the time the envelope stays at maximum after the attack phase is finished. Otherwise, nothing special here. Except for:
  • Splope controls: Below Attack, Decay, and Release there are knobs by which you can change the respective slope of the curve. When the knobs are in the leftmost position, the slopes start fast and slows down at the end. Turning the knobs to the rightmost position will result in the opposite behavior.
  • No Release: If this button is on, the envelope will not drop when the note is released but it will stay at the current level
LFO settings:
  • Speed, Phase, Amount: Speed and Phase define the LFO frequency and the LFO phase, respectively. Amount determines how much of the LFO output is mixed with the ADSR output to create the envelope. The higher the value, the more LFO you will hear.
  • Mode: The Mode switches define the method by which the LFO is mixed with the ADSR.
    • Off: Turns the LFO off.
    • Normal: In this mode, the LFO amplitude is not affected by the ADSR envelope but only determined by Amount.
      In the remaining three modes, the LFO amplitude is dynamically scaled by the ADSR signal.
    • Bottom: The lower bound of the LFO amplitude is constantly zero. The upper bound is equal to the current value of the ADSR signal.
    • Center: The LFO amplitude is scaled by the ADSR signal. The lower and upper bounds of the LFO amplitude are not constant but are moving with the ADSR signal.
    • Top: The upper bound of the LFO amplitude is constant at maximum. The lower bound is equal to the current value of the ADSR signal.

Volume Settings

  • Attack, Hold, Decay, Sustain, Release: These are working is usual.
  • Level: The maximum volume level. This control differs from the Volume control in that Level is stored individually with each preset, while Volume is not affected by preset changes.
  • Slope Controls: Below Attack, Decay, Release there are knobs to control the slope of the respective curve.
  • Full Attack: When this button is turned on, the volume envelope will always complete both the Attack and Hold phase, even when the note is released meanwhile. This comes in handy when you have a long attack and hold time but don't want to keep the key pressed so long. 

Stereo Settings

Each harmonic pair of the quadrature signal is perfectly balanced in stereo (in the stereo image, it describes a circle). However, the overall signal, ie. the sum of the harmonics, may not be balanced. You can use the stereo controls to change the overall stereo image.
  • Width: When this knob is at the rightmost position, the harmonics are out of phase by 90 degrees (all cosines on the left, all sines on the right, by default). In the leftmost position, the image is inverted, and in the middle position, the output is a mono mix of both channels.
  • Angle: Angle by which the stereo image is rotated.
  • AngleIncr: The angle by which the stereo image is rotated further every time a note is played. At -90 and 90 degrees, left and right channel will be swapped with every note.


Voices: The maximum amount of voices that can be active at the same time. It can be freely set between 1 and 16. 

Slide: This is the portamento speed, ie. the speed at which notes slide. To make notes slide, a key must not be released before the next key is pressed. If the Slide knob is turned to the leftmost position, sliding is off.

Bend: This is the pitch bend range measured in half tones. If you're using a pitch bend wheel, you may have to alter this parameter to get the desired effect.

Volume: Limits the overall volume.

MIDI Control Mapping

This table shows to which MIDI NRPNs the controls are mapped

FNum1 (FreqRatio) 1 Harmonics 21 ReleaseAS 41
FNum2 (FreqRatio) 2 CFMode 22 NoRelease 42
FDenom1 (FreqRatio) 3 CPMode 23 LfoSpd 43
FDenom2 (FreqRatio) 4 AttackV (Volume Env) 24 LfoAmt 44
FOffset1 5 AttackVS 25 LfoPhase 45
FOffset2 6 HoldV 26 LfoMode 46
Pitch1 7 DecayV 27 Track 47
Pitch2 8 DecayVS 28 Slide 48
Bright1 9 SustainV 29 Bend 49
Bright2 10 ReleaseV 30 Voices 50
Color1 11 ReleaseVS 31 Width 51
Color2 12 Level 32 Angle 52
Fund1 13 FullAttack 33 AngleIncr 53
Fund2 14 AttackA (Morph Env) 34 Volume 54
CombFreq1 15 AttackAS 35 RangeL 55
CombFreq2 16 HoldA 36 RangeU 56
CombPhase1 17 DecayA 37 ChgPerNote 57
CombPhase2 18 DecayAS 38 Copy 58
FRound 19 SustainA 39 Paste 59
PRound 20 ReleaseA 40


Put the file Tetra.dll into your VST Plugins folder.

Version History

  • Fixed crackling noise when running multiple synth instances on multi-core systems.
  • Added 11 new presets, retuned old presets.
  • Minor speed improvements (about 10% less CPU drain).
  • Fixed potential pops and sticking notes.
  • Limited the maximum generated frequency to 21kHz even at sample rates greater than 44.1kHz to avoid volume change at different sample rates.
  • Downward compatible to v1.0/v1.0a.
v1.0a: Fixed the pitch bend knob and another potential glitch.
v1.0:   Initial release.


You can contact me by E-Mail at: (Burkhard Reike).


This software is provided as is, there is no warranty and nobody is responsible for any kind of damage. Use it at your own risk.
VST is a trademark of Steinberg Soft- und Hardware GmbH, Germany.

Download: (Windows)

Older Versions: (Windows)

This synth is free. If you like it, feel free to leave a donation.
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