A VCA, or Voltage-Controlled Amplifier module, lets you use a voltage to control the amount of an audio signal that is allowed to pass through from the input to the output of the module.
The higher the control voltage, the more signal is passed. In SynthEdit when the control voltage reaches 10V the entire signal is let through, and when the control voltage is 0V (or below), no signal is passed and the output is silent.
VCA or Level Adj. Which should I use?
While you could use the Level Adj module in place of the VCA, there are differences between the two modules. The VCA has a slightly faster response time to its Volume plug than Input2 on the Level Adj module. Also without conversion you’ll only get a linear response to the envelope, as opposed to the choice of curves for the VCA. Strictly speaking the Level Adj module is an audio voltage multiplier. When controlling audio volume or applying an audio envelope for best results the VCA should always be used.
The Level Adj module multiplies one input by the other. It can be used for ring modulation, or for amplitude modulation, or for scaling a signal/CV by a fixed amount. The two inputs are multiplied together, then normalised. (e.g. 5V multiplied by 2 V = 1V, (5 * 2 ) / 10).
Uses for a VCA module.
Volume Control
You can use your VCA to turn just about anything into a volume/level control.
Run your audio signal through it, then connect the CV input to a mod wheel, or any voltage source you want.
Envelope Shaping
One of the most common uses of a VCA is envelope shaping. Think about when you hit a key on a piano; the amplitude starts out pretty loud, then over time it fades away. If you let go of the key then the volume drops off pretty quickly. You can use a VCA in conjunction with an envelope generator to achieve the same effect with notes on your synthesizer.
An envelope generator (EG) is a module or circuit that generates a voltage that is triggered by something and changes over time. If you’re trying to mimic a piano, you can configure the EG so that it is triggered by a key being pressed on your keyboard, it sends out a strong voltage at first, then it fades down to 0 over time.
The voltage sent out by the EG matches the way you want your amplitude to change over time. Connect the output of the EG into the CV input of your VCA and it will cause the amplitude of your note to fade out like a piano note.
The structure shown below illustrates a typical ADSR/VCA combination to trigger an audio envelope from a MIDI input
The VCA response curve modes:
The VCA Module allows you to choose from 3 different response curves via a drop down list, or a selection in the VCA module properties:
1) Linear
2) Exponential
3) Decibel
4) Decibel (Old)
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 shows more accurately how loud the signal sounds in relation to the control, voltage (below).
This graph shows that volume plug input of 10 Volts produces full volume (or 0 Decibels), and an input of 0 volts effectively gives silence (-70 decibels, very quiet).
A full-scale audio input signal is -10 to +10 Volts.
The normal output range of SynthEdit’s Oscillators is -5 to +5 Volts (about -6dB).
Note: SynthEdit’s own VU Meter module displays an averaged signal. However you can switch it to peak mode.
What do the audio envelopes look like? All these sounds have the same ADSR envelope settings, but use different VCA modes.
1) Linear Mode.
This is useful for controlling the level of LFO’s or other modulation sources.
However for audio use such as a VCA this doesn’t sound like a natural audio decay to the human ear, as it seems to become faster as the level decreases.
2) Exponential Mode:
This emulates the discharge rate of a capacitor (which is how an analogue ADSR works) and so is the closest reproduction of the audio envelope produced by an analogue synthesizer.
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
3) Decibel (dB) Mode:
The human ear hears this as a constant, natural fade.
The Decibel curve drops by 35 dB between 10 – 1 Volt.
dB = (35/9) * ( volume – 1.f )
Volts = 10 * 10.f ^ ( dB * 0.5 )
Since a perfect dB curve can never reach zero volume in reality, the Synthedit VCA is designed so that below 1 Volt the VCA dB curve fades out to silence. This mode gives the most natural sounding VCA envelopes of all.
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, use the following formula:
volts = 10 × 10^ (dB ÷ 20)
Tremolo
Mix a slow sine wave with 8V DC from a Fixed Value(Volts) module (to make sure the whole sine wave stays above 0V), then feed this into the Volume Plug of your VCA. The audio signal will mostly come through to the output because of the DC bias, but you will hear the amplitude get louder and quieter in time with the sine wave you are using to modulate it.
This effect is called tremolo (Amplitude Modulation). In the screenshot below the Yellow waveform is the modulation sinewave and the green is our audio. You can see how the peaks and troughs in the audio level follow the modulating sinewave.
The slider control changes the level of the modulating sine wave, this works best with the maximum level set as 8V.
Note: For this effect to work correctly the response curve must be set as Linear.
Amplitude Modulation
Tremolo (shown above) uses a slow (say 3Hz for example) sinewave to modulate the amplitude of your audio signal, so you can actually hear the resulting loud/quiet cycles. If you increase the modulating frequency so that it gets up into the audio range, however, things start to get interesting.
The modulation has become so fast that you are now changing the shape of the original audio signal’s waveform, and new frequencies appear.
In the example below I have modulated a 7kHz sine wave with a 4kHz sine wave. As you can see in the Frequency analyser, not only do we have the 7kHz audio signal, but also two new frequencies have appeared at 3kHz and 11kHz. This is where the 4kHz signal has interacted with the 7kHz. Why 3 and 11 kHz? It’s because the frequencies are added and subtracted in the modulation process:
7kHz – 4kHz = 3kHz and 7kHz + 4kHz = 11kHz. This is similar to ring modulation, but there’s one key difference, with Amplitude Modulation the carrier frequency (the 7kHz signal) is still present at the output, whereas with a true balanced ring modulator only the new 3kHz and 11kHz frequencies would be present the 7kHz carrier having been suppressed.
Complex Amplitude modulation.
However it’s not always this simple to predict the results, if we modulate the 7kHz sine wave with a 4kHz sawtooth then the mathematics becomes more complex- we get many more frequencies added, (due to the more complex harmonic structure of the sawtooth)and would need to use Fourier analysis to predict the outcome.
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