Introduction to Sound Synthesis Signals
What a signal is
A signal is a level that changes over time. In analogue hardware the level is a voltage, and in software it is a stream of numbers. Every signal in a synthesiser, whether you hear it or use it to control something, is described by the same three properties: amplitude, frequency and waveshape.
The tool below draws one repeating signal and lets you change each of the three properties separately. The vertical scale is marked in volts from -5 to +5, the usual range for audio on a Eurorack modular synthesiser. A software synthesiser uses -1 to +1 for the same job, and the shape of the signal is the same either way.
A signal
A repeating signal is drawn against time. Its amplitude, frequency and shape are set separately, and the view can be zoomed out to two seconds or in to twenty milliseconds.
Listen for Amplitude changes how loud the signal is, frequency changes its pitch, and shape changes its tone colour. Below about 20 Hz the sine becomes inaudible, and the square and sawtooth turn into separate clicks: the jumps in the waveform are still heard when the repetition is too slow to be a pitch.
Amplitude
Amplitude is how far the signal swings either side of its centre line. It can be measured from the centre to the highest point, which is the peak, or from the lowest point to the highest, which is peak-to-peak. At 100% on the tool above the signal swings from -5 to +5 volts, which is 5 volts peak and 10 volts peak-to-peak.
A larger amplitude sounds louder, but the two are not proportional. People hear loudness on a roughly logarithmic scale, so doubling the amplitude raises the level by about 6 decibels, and about ten times the amplitude sounds around four times as loud. That is why volume controls on most equipment use a logarithmic response rather than a linear one, and why the VCA chapter distinguishes linear from exponential response.
Frequency
Frequency is how many times the signal repeats its shape each second, measured in hertz after the physicist Heinrich Hertz. A signal at 220 Hz repeats 220 times a second, so one cycle lasts 1/220 of a second, about 4.5 milliseconds. That length of time is the period, and it is the reciprocal of the frequency: halve one and the other doubles.
People hear repeating signals between roughly 20 Hz and 20 kHz as pitch, with the upper limit falling with age. Below about 20 Hz the repetition is too slow to fuse into a tone. Most control signals run in that range, and the chapter on control signals and audio signals explains why the same signal can do either job. Switch the view to two seconds and lower the frequency to see a slow signal as a shape you could follow by hand.
Waveshape
The waveshape is the shape of one cycle. Two signals with the same amplitude and frequency can still sound different if their shapes differ, because the shape determines which harmonics the signal contains. That is the subject of waveforms and harmonics and overtones.
Not every signal repeats. Noise has an amplitude but no single frequency, and an envelope rises and falls once for each note. Both are still signals, because each is a level changing over time, and both can be sent to the same inputs as a repeating signal.
Common questions
What is a signal in a synthesiser?
A signal is a level that changes over time. In analogue hardware the level is a voltage, and in software it is a stream of numbers. Every signal in a synthesiser, whether you hear it or use it to control something, can be described by its amplitude, its frequency and its waveshape.
What is amplitude?
Amplitude is how far a signal swings from its centre. A larger amplitude sounds louder, although loudness is heard on a roughly logarithmic scale: doubling the amplitude raises the level by about 6 decibels rather than doubling how loud it seems.
What is the difference between peak and peak-to-peak?
Peak is the distance from the centre line to the highest point of the signal. Peak-to-peak is the distance from the lowest point to the highest. Audio on a Eurorack modular synthesiser typically swings between -5 and +5 volts, which is 5 volts peak and 10 volts peak-to-peak.
What frequency range can people hear?
People hear roughly 20 Hz to 20 kHz, with the upper limit falling with age. Below about 20 Hz a repeating signal is no longer heard as a pitch, although sharp jumps in it, like the edges of a square wave, can still be heard as separate clicks.
What is a hertz?
A hertz is one cycle per second. A signal at 440 Hz repeats its shape 440 times every second, so each cycle lasts 1/440 of a second, about 2.3 milliseconds. The unit is named after the physicist Heinrich Hertz.
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