Modulation

The idea

A pure radio wave says nothing — it is just a steady hum. Every radio signal has two ingredients: the carrier, a steady wave at the station's frequency (the delivery truck), and the information — voice, music, or data (the cargo). Modulation loads the cargo onto the truck by changing something about the carrier in rhythm with the information.

Imagine signaling a friend across a dark field with a flashlight. Make it brighter and dimmer in rhythm with your message — that is AM (amplitude modulation), which varies the wave's height. Flicker it faster and slower instead — that is FM (frequency modulation), which varies the wiggle rate. FM sounds cleaner because most noise (lightning, motors) disturbs amplitude, and FM receivers simply ignore amplitude.

Digital modulation sends bits by switching the carrier between distinct states — an alphabet. BPSK uses 2 phase states (0° and 180°) for 1 bit per symbol: a rugged two-letter alphabet that GPS relies on. QPSK uses 4 phase states for 2 bits per symbol — twice the data in the same bandwidth (satellite TV uses it). 16-QAM combines amplitude and phase for 16 states and 4 bits per symbol, and modern WiFi pushes to 1024-QAM — 10 bits per symbol at close range. Under the hood, receivers describe all of this with I/Q signals: two components 90° apart that together capture any amplitude and phase, like X and Y coordinates for a wave.

Plot the states as dots on an I/Q chart and you get a constellation diagram. A bigger alphabet means dots packed closer together, so noise can smudge one letter into another. That is why your WiFi silently drops from 1024-QAM to QPSK as you walk away from the router — trading speed for reliability, automatically.

The math

In general, 2ⁿ states carry n bits per symbol:

$$n = \log_2(M)$$

where M is the number of constellation points — QPSK (M = 4) carries 2 bits, 16-QAM carries 4, 1024-QAM carries 10. For analog FM, Carson's rule estimates the occupied bandwidth:

$$BW \approx 2(\Delta f + f_m)$$

where Δf is the peak frequency deviation and f_m is the highest message frequency.

Common misconceptions

• Myth: AM and FM are just two brands of radio station. Reality: They are two physically different encoding methods — AM varies the carrier's amplitude, FM its frequency — and the same techniques appear all over RF, not just broadcast.
• Myth: Higher-order modulation (like 1024-QAM) is simply better. Reality: It needs a very clean, strong signal. With tightly packed constellation points, a little noise causes misreads; fewer, widely spaced states survive weak links.
• Myth: Faster data means the wave travels faster. Reality: All carriers travel at the speed of light. Higher data rates come from packing more bits into each symbol or using more bandwidth.

Try it

Open the Modulation Explorer: start with AM and watch the carrier's height follow the message, switch to FM and watch the wiggle rate change instead. Then step through BPSK → QPSK → 16-QAM and count the constellation points multiplying — each doubling of the alphabet is one more bit per symbol.