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MIDI to Control Voltage Converter

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MDD Synthesizer Interface

Control Voltages and Signals

The MDD Control Voltage range includes the entire 10 octave range that can be MIDI encoded.  A GATE signal asserts as long as any Note is currently sounding.  A TRIGGER pulse is generated and is Velocity Responsive.  

The receipt of a NOTE OFF message (on the channel of interest) causes the negation of the GATE signal while the control voltage remains at it's current value (providing there is no other Note currently sounding).  If there is another Note sounding, the GATE will remain asserted and the Control Voltage will now change to a level corresponding to the new Note Value, and a new TRIGGER will be generated.  

Interface Voltage Response

The control of a synthesizers frequency usually conforms to one of two "laws" or response types:
    1) a fixed volt per octave or "linear" response, or;
    2) a multiplying volt per frequency "exponential" response.
An exponential oscillator (requires a "linear" frequency control, a linear oscillator requires an "exponential" frequency control.  While some variance occurs within the fixed "volt per octave" response type, most typical is 1 Volt per Octave (1V/OCT) this is this is the response type generated by the MDD. This interface conforms to requirements of exponential oscillators such as the MOOG, ARP, Emu, and others compatible synthesizers.  (SCALE Adjustment is provided for oscillators that require other fractional but linear response requirements).

Composite Voltage Control

In addition to providing MIDI Note to Voltage conversion, the MDD integrates a Low Frequency Oscillator(LFO), a Random Noise Source, the ability to respond to MIDI Pitch Bend commands.  These integrated modulation sources are "mixed" with the Frequency Control Voltage so that only a single control voltage connection is required to "drive" a synthesizers Oscillator(s) in a complex way.   This allows this interface to exist wholly "external" to the synthesizer so that alteration of a working and perhaps precious synthesizer is not required.

  • Voltage Output 1:   Raw 7 Bit MIDI to Control Voltage Output. MIDI Note values translate to 1 volt per octave VCO frequency voltage source.

  • Voltage Output 2:   Summed Composite of Raw Control Voltage, MIDI Pitch Bend, Integrated Noise, and Low Frequency Oscillator. Both outputs provide full 10 Octave control voltage Span MIDI 00 = 0 Volts.

  • Control Interface:   +5 Volt Positive going GATE and TRIGGER. S-TRIG (MOOG Trigger) is "selectable" to operate from GATE or TRIGGER. GATE asserts as long as any key is ON, TRIGGER asserts with any New Note On event.

  • Additional Interface Voltages:   The Trigger Pulse responds to Note Velocity. Selectable responses are Velocity Width or Velocity Delay modes.   Velocity Width causes the Trigger Pulse to narrow with increasing Velocity Values.  Velocity Delay causes the Trigger Pulse to delay it's activation with decreasing Velocity values(referenced to the leading edge of the GATE signal). Dynamic Trigger modes provide functionality similar to voltage controlled Trigger functions.

Trigger Mode Utilization

The usefulness of the Trigger Modes depends upon the nature of the Envelope Generator to which it is connected.
  • If the Envelope generator uses a monostable (one-shot) to drive the Attack/Decay settings then the Velocity Width feature will not cause any additional dynamism.  This type of Envelope Generators should employ the Velocity Delay Mode.
  • Envelope Generators that derive their Attack and Decay settings directly from the Strobe width can benefit from either Trigger mode.
Text Box:  The figure on the left depicts the typical functionality of standard GATE and TRIGGER signals in response to key activations.  As shown the press of a single key causes the assertion of a Gate and Trigger strobes and a Control Voltage corresponding to the pressed note occurs.   When a second key is pressed (while the first is still sounding and the new key is of higher precedence) a new Trigger occurs while the Gate signal remains asserted.   A new control voltage level is generated corresponding to the new note.

When the second key is release a new Trigger and a Control Voltage corresponding to the original key is generated.  The Gate finally when the first key is released.  From this depiction it can be seen that the Gate occurs when any key is sounding while the Trigger occurs with each new note.

Text Box:  The picture on the right depicts the Velocity Delay Mode of the MDD trigger signal. Notes sounding with large velocity (Fast activating notes) are accompanied by a fixed width trigger pulse which begins almost simultaneously with the gate.  Notes sounding with low velocity values produce a fixed width trigger pulse that occurs some time after the Gate occurs.   The distance is proportional to the Velocity Value. This acts to modify the contour of many Envelope Generators.

The Dynamism expressed by the Trigger Modes expands the expressive capability of the Envelope Generator allowing it to vary with the playing (or recorded) Velocity.  An example Envelope Generator design that exploits the two Trigger Modes is included below. Key Scaling is also implemented so that keyboards or sequencers that implement a static Velocity will alter the Trigger with correspondence to Note Value

S-TRIG Mode Selection:

Moog Synthesizers activate their envelope generators using a negative going strobe called the Shorting Trigger (S-TRIG).  Typically, this S-TRIG is an inverted GATE signal. The MDD allows the selection of either the GATE or Velocity Trigger as the source of the S-TRIG signal.   This results is greater flexibility for Moog envelope generator contours.   A front panel switch selects between the two options.   There is a board option which selects whether the S-TRIG has a pull-up resistor or not.   For Moog and Moog work-alikes this should not be closed.   Envelope generators which need a negative going strobe which "swings" from +5 Vdc to 0 should close this jumper.