What Is a Signal?
A signal is just a number changing over time. That's it. Every wire in a synthesizer carries a stream of numbers, and every module reads those numbers, transforms them, and passes the result along. Understanding this single idea unlocks everything that follows.
Numbers all the way down
When an oscillator produces a sound wave, it isn't creating air vibrations directly. It's outputting a stream of numbers, one per audio sample, tens of thousands per second. At any single instant, the output is just one number: maybe 0.73, or −0.41, or 0.00.
A filter takes that number, does some math, and outputs a different number. An amplifier multiplies the number by another number. At the end of the chain, the final number moves your speaker cone in or out. That's the entire story of digital audio.
The signal chain
The demo below shows the simplest possible synthesizer: an oscillator generates a signal, a filter shapes its brightness, and a VCA (amplifier) controls its volume. At each point in the chain, you can see the live value of the signal: the actual number flowing through the wire at this instant.
Press Play, then watch the numbers update in real time. Try changing the waveform, filter cutoff, or gain, and notice how each stage transforms the same stream of numbers in a different way.
Post-Oscillator
Post-Filter
Post-VCA
Each number is the signal's instantaneous value at that point in the chain. Values range from −1 to +1.
Frequency sets the pitch. Filter cutoff removes high frequencies. VCA gain controls volume.