A wave can carry information: a radio song, a phone call, a photo. There are two ways to code it. An analog signal copies the original smoothly, with every in-between value. A digital signal sends only a few separate values, usually just two: 0 and 1.
Key idea
A digital signal sends information as distinct values (bits, 0 or 1). Because there are only a few allowed values, the receiver can still read every bit exactly after some noise is added, and copies can be perfect. With $n$ bits you can send $2^n$ different values.
Why it works
Suppose 0 is sent as 0 volts and 1 as 5 volts. Noise on the way might turn 5 V into 4.3 V or 0 V into 0.6 V, but the receiver only asks "closer to 0 or to 5?" and recovers the exact bits. An analog signal has no such rule: every bit of noise becomes part of the signal, and copying a copy makes it worse each time.
Worked example: Count the values
How many different values can a 4-bit signal send?
- Each bit has 2 choices, so $2 \times 2 \times 2 \times 2 = 2^4$.
- $2^4 = 16$ values (0000 up to 1111).
Answer: 16
Worked example: Read a noisy signal
A receiver uses 0 V for 0 and 5 V for 1. It gets 4.6 V, 0.8 V, 4.1 V, 0.3 V. What bits were sent?
- Anything closer to 5 V than to 0 V (above 2.5 V) is a 1; below is a 0.
- 4.6 → 1, 0.8 → 0, 4.1 → 1, 0.3 → 0.
Answer: 1010
Common mistake
Thinking "digital" just means electronic. An old analog radio is electronic too. Digital means the information is sent as separate, countable values rather than a smoothly varying wave. Check: can the signal only take a few set values? Then it is digital.