Using Waves to Send Information: Analog and Digital Signals

Core Idea

Waves can carry messages from one place to another when we put a pattern into them. A drum beat, a flashing light, and a radio wave can all carry a code. An analog signal changes smoothly, copying the shape of the original sound or picture. A digital signal uses only a few set values, usually two, such as on and off or 1 and 0, and it is easier to send and copy without errors.

How It's Best Learned

Have students invent a flashlight or tapping code and send a short word across the room; then add "noise" by having classmates tap randomly and see which codes still work. Compare a smooth wavy line (analog) with a line that jumps between two levels (digital). Look at everyday examples: a phone call, a Wi-Fi signal, a TV remote, and a fiber-optic cable. Ask why a digital message can be copied many times without getting worse.

Common Misconceptions

• Digital means "shown on a computer screen." (Digital means the information is coded as separate values, like 1s and 0s; a flashing light can be a digital signal.)
• A radio station sends sound waves through the air to your radio. (It sends radio waves, a kind of light we cannot see; the radio turns the pattern in those waves back into sound.)
• Analog signals are always worse than digital ones. (Analog signals can be very good, but small errors add up each time they are copied or sent far, while digital signals can be cleaned up and checked.)

Explainer

People have used waves to send messages for a very long time. Drums carried signals between villages, and ships used flashing lamps at night. In each case the sender puts a pattern into a wave, and the receiver reads the pattern. The wave carries energy, but the pattern carries the information.

There are two main ways to put a pattern into a wave. In an analog signal, the wave changes smoothly to copy the original. When you speak into an old telephone, the electric signal goes up and down just as the air pressure of your voice does. The groove of a vinyl record wiggles in the same shape as the music. Analog signals can sound very natural, but they have a weakness. Every time the signal is sent far or copied, a little noise sneaks in. The receiver cannot tell the noise from the real signal, so the errors build up.

In a digital signal, the message is turned into a code that uses only a few set values, usually two. Morse code is an early example. In the 1840s, Samuel Morse's telegraph sent messages as short and long pulses of electric current. Today computers use 1s and 0s. A light can be on or off, and a radio wave can be strong or weak, to stand for 1 or 0. The receiver only has to decide which of the two values each piece is. A little noise does not change that decision, so the message arrives correctly. Digital messages can also include extra check numbers that help the receiver find and fix mistakes.

Many waves can carry digital signals. Wi-Fi and cell phones use radio waves. A TV remote uses invisible infrared light. Fiber-optic cables send pulses of light through thin glass threads, carrying huge amounts of information across oceans. Even sound can carry digital codes, as in the beeps of old modems.

The rule for a good code is the same everywhere: make each message's pattern very different from the others. Then wind, static, or a flicker will not turn "yes" into "no." Engineers use this idea when they design the signals that connect phones, computers, and satellites.