- Analog signals vary continuously (the human voice, radio waves).
- Digital signals use discrete levels, typically 0 and 1 (computer data).
Digital signals resist noise better because a receiver only needs to decide between two levels. Modulation converts digital data into an analog carrier for transmission over radio or telephone lines; a modem (modulator-demodulator) does this.
Analog signals in more detail
An analog signal is a smooth wave that can take any value within a range. Three properties describe a wave:
- Amplitude: the height (strength) of the wave.
- Frequency: how many complete waves pass per second, measured in hertz (Hz). Wi-Fi uses radio waves at about 2.4 billion (2.4 GHz) or 5 billion (5 GHz) waves per second.
- Phase: the position of the wave in its cycle at a given moment.
The problem with analog signals is noise: any interference adds itself to the wave, and the receiver cannot tell which part is the real signal and which part is noise. Each time an analog signal is amplified, the noise is amplified too.
Digital signals in more detail
A digital signal has only a few fixed levels, usually two: one for 0 and one for 1. Noise still affects it, but as long as the receiver can still tell "high" from "low", it recovers the exact original bits. A device called a repeater can then send out a perfectly clean copy (it regenerates the signal rather than just amplifying it). This is why digital communication can cross very long distances without any loss of quality.
Modulation: carrying digital data on an analog wave
Radio waves and old telephone lines can only carry analog waves, so digital data must be modulated: the bits are used to change one property of a carrier wave.
| Method | What changes | Simple description |
|---|---|---|
| Amplitude modulation (ASK) | The height of the wave | A tall wave means 1, a short wave means 0 |
| Frequency modulation (FSK) | The frequency | A faster wave means 1, a slower wave means 0 |
| Phase modulation (PSK) | The phase | The wave is shifted to show a change of bit |
Modern systems such as Wi-Fi and 4G combine amplitude and phase changes (QAM), so each change of the wave carries several bits at once. That is how they reach such high speeds.
At the receiver, demodulation turns the wave back into bits. A device that does both jobs is a modem (*mo*dulator-*dem*odulator). Your home router usually contains one for the DSL, cable or fibre line.
Analogy: think of a torch flashing Morse code at night. The light itself is the carrier. Switching it on and off (changing its amplitude) carries the message.
Remember
- Analog: continuous values, easily affected by noise. Digital: a few fixed levels, can be regenerated perfectly.
- Modulation puts digital data onto an analog carrier wave; a modem modulates and demodulates.