Signal level conversions
SynthEdit is modelled after real analog synthesizers. In a modular analog synth the various modules offer many features and functions and there are no rules as to how you connect them together (that’s the fun part!). As a result, all the modules must be compatible with each other. This was achieved in the real world with voltage control. The standard for most Synthesizers being 1 octave per volt.
All modules responded to the same voltage range in a consistent way.
SynthEdit uses the same principle of voltage control signals.
The control voltage plugs of SynthEdit’s modules generally have a useful range from 0.0 to 10.0 volts.
Voltage to Pitch conversion
Example of voltage to pitch conversion:
The pitch of SynthEdit’s Oscillator modules are calibrated in Volts per Octave.
A control voltage of 5 Volts sets the oscillator frequency to 440 Hertz or Middle A, the note in the center of a piano keyboard. Increasing the input by 1 Volt will cause the pitch to rise one octave (the frequency doubles to 880 Hz ).
Simplified: volts = 1.442695041 Log (0.07272727273 Hz)
To convert Volts to Frequency
Frequency = 440*2^(Volts-5)
To convert MIDI note number to Volts
There are 128 midi notes. There are 12 semitones on an Octave. 5.0 Volts is middle-A (MIDI note 69)
Volts = 5.0+(MIDI-note-number-69)/12
Voltage to volume conversion.
The VCA Module allows you to choose from 3 different response curves:
a) Linear
b) Exponential
c) Decibel
The following chart shows the relationship between input and output voltages.
A more useful graph is the output volume in decibels for a given input voltage. This more closely shows how loud the signal sounds.
This shows that an input of 10 Volts produces full volume ( 0 Decibels ), and a gain of 0 volts gives silence ( -70 decibels, very quiet).
A full-scale signal is -10 to +10 Volts. A 1kHz sine signal, amplified to full-scale (-10 to +10 on the Oscilloscope) shows as 0dB in Cubase, this is a peak reading. SynthEdit’s Oscillators normal output range is -5 to +5 Volts ( -6dB in Cubase). Unlike Cubase SynthEdit’s own VU Meter displays an averaged signal. However you can switch it to peak mode. In “dB Peak” mode SynthEdit’s VU meter reads 10dB above Cubase’s.
What do they look like?, all these sounds have the same envelope settings, but different VCA modes.
Decibel (dB) mode
The human ear hears this as a constant, natural fade.
The Decibel curve drops 35 dB between 10 – 1 Volt.
dB = (35/9) * (volume-1.f)
Volts = 10 * 10.f ^ ( dB * 0.5 )
Since a perfect dB curve never reaches zero volume, below 1 Volt the VCA dB curve is faded to silence.
Exponential mode
This scale imitates the voltage drop of a discharging capacitor. Many hardware synths generated their envelopes using this method, as it is the easiest to produce with an electronic circuit, and is similar to the decay curve of a natural sound.
Given a volume from 0 – 10, this formula gives the output level in volts.
volts = 10-c1(1-e^(3(volume/10-1)))
Where ‘c1’ is a constant that determines the amount of curve:
c1 = 10/(1-e^-3)
c1 =10.524
Linear mode VCA, the or Level Adj module.
This is the most direct method of controlling level. However to the human ear, this sound fades in an irregular way, appearing to go quiet too quickly at the end.
Converting Volts to dB
To convert a level in volts to dB, use the following formula:
dB =20*log10(volts/10)
To convert a level in dB to Volts
volts =10*10^(dB/20)
Voltage to Time conversions
Envelope generator module times are based on a Time Cent scale. This is a similar concept to the decibel scale, whereby you get finer control over the short envelope segments.
(Pictured – ADSR2 curve)
ADSR2
Due to popular request the ADSR2 is faster than it’s predecessor. It’s designed to range between 1ms to 10s.
Time = 10^(Volts0.4-3) Volts = (log10(time)+3)2.5
ADSR (Old deprecated module!)
The ADSR segments range from about 10ms at 0 Volts up to approx. 10s at 10 Volts
Time =2^(Volts-6.666666)
Volts =log(Time)/log(2)+6.66666
NOTE: Sometimes you may need even faster envelope times, for example when generating percussive sounds like drums. You can get shorter times by using negative voltages.
MIDI-CV Portamento time
Time = 2^(Volts-8.666666)
Volts = log(Time)/log(2)+8.66666
MIDI-CV2 and Keyboard2 Portamento time
Time = pow(10.0,Volts0.4-3) Volts = (log10(time)+3)2.5
In constant-rate mode, the formula is the same except the result is the glide time per octave