Your voice is now a river of ones and zeros. So the question waiting at the end of the last chapter is beautifully blunt: how do you throw a one or a zero across a room?
Here is the surprise: this is the easy part. For half the book we've been carrying voices, and a voice is a demanding passenger. Every subtle wiggle of it matters, so the receiver had to reproduce the whole curving shape faithfully, and every scratch the channel added stayed in the sound forever. A bit asks for none of that. To carry bits, the receiver only has to answer one crude question, over and over: was that a one, or a zero?
Crude questions are easy to answer, even through a lot of noise. A whisper can be hard to make out across a windy field, but "is that person shouting or silent?" you can answer from astonishingly far away. That toughness, more than anything else, is why the world went digital. Don't take our word for it: the figure below has a noise slider, so crank up the hiss yourself and watch which of the two signals gives up first.
the receiver reads back 1 0 1 1 0: all five bits correct
Three knobs, two positions each#
And what do we actually wiggle to send those ones and zeros? Nothing new at all. Back in chapter 1 we found that a wave has exactly three knobs: height, frequency, and phase. AM, FM, and phase modulation each grabbed one and swept it smoothly, painting a whole voice into it. Digital radio grabs the very same knobs and does something far lazier: it clicks each one between just two positions. Position one means 1; position two means 0. Three knobs, so three schemes, and you already know all three by their analog names.
Flip the height and you get amplitude-shift keying, ASK for short: loud means one, quiet means zero. It's the simplest scheme there is, and its extreme version (where "quiet" is turned all the way down to nothing) is the oldest digital radio of all: on-off keying, the carrier simply there or not there. That is exactly how the very first radio operators sent Morse code across the Atlantic, a century before Wi-Fi. And it never went away: the remote control for your TV blinks a beam of invisible infrared light on and off the same way, dozens of times per button press.
Flip the frequency and you get frequency-shift keying, FSK: two pitches, one for each bit, like a song that only ever uses two notes. If you're old enough to remember a dial-up modem warbling and screeching down a phone line, or a home computer loading a game from a cassette tape, you have literally heard FSK with your own ears. That strange two-note chatter was bits, sung.
Flip the phase and you get phase-shift keying, PSK, the machine's favorite from chapter 7: for a zero, flip the wave exactly upside down; for a one, don't. To any ear the two versions are the same steady tone, utterly indistinguishable. But a receiver holding a steady reference copy of the carrier just compares the incoming wave against it, and "upright or flipped?" is about the easiest question electronics can answer. Quietest to the ear, clearest to the chip.
Key in the bits yourself#
Here are all three schemes with their hands on the same message. Tap the eight bits to spell anything you like, pick a scheme, and watch the carrier redraw. In every cell the amber marks show the one thing the receiver is watching: the height of the envelope for ASK, the pitch for FSK, the flip for PSK.
tap the bits to change the message
amplitude-shift keying. The height knob: a one is loud, a zero is quiet. Turned all the way down, a zero becomes pure silence: on-off keying, the Morse-code way.
A name for the little slice#
Look once more at the playground and notice how naturally the wave divides into slices, one per bit. Each slice holds the carrier in one chosen position, each answers one crude question, and each has a name worth keeping: a symbol . A symbol is one little stretch of wave carrying one answer.
Symbols also set the speed limit. A radio's pace is measured in symbols per second: send more slices each second and the bits flow faster. So there are exactly two roads to a faster link. Wiggle faster, squeezing in more symbols per second. Or make each symbol cleverer, so one slice carries more than one bit. Notice that so far every symbol of ours answers a single yes-or-no question — one measly bit per slice. Remember that, because in chapter 20 we are going to get greedy about it.
But greed can wait, because two dangers come first. However tough our crude questions are, a big enough noise spike can still turn a one into a zero. And long runs of identical bits hide a second, sneakier trap: stare at that unchanging stretch in the playground and ask yourself how a receiver counts what it cannot see changing. Keeping bits honest is a craft of its own, and it's next.