What Is Frequency-Shift Keying (FSK)?

In digital communication, Frequency-Shift Keying (FSK) is like a party. There are two kinds of people at the party: those who are having fun and those who are watching. And then there's you, asking everyone to tell you what they're doing so you can go home and write it down. The fun-havers are sending each other FSK signals. They're dancing around in their little worlds and not even thinking about the people who aren't dancing with them—or even paying attention to them. Then there are those people watching and taking notes on everything that happens at the party. They write every move everyone makes and then send it home as an FSK signal so that another machine can read it. A modem receives digital information and converts it into an FSK signal transmitted over telephone lines. An analog carrier sine wave (or square wave) sends the data. Two states, zero and one, are represented by analog waveforms, each of which is a binary signal. If you're trying to send a text message in Morse code, you can't just throw letters into the air and hope they land on their intended target—you need to use a tool that converts your message into dots and dashes. The same goes for digital communications. To transmit data through a computer, you need to transform those bits into something the people on the other end of the line can understand. Enter FSK modulation or frequency-shift keying. FSK uses two different frequencies to represent 0s and 1s, which is why it's also known as bi-phase modulation. The most common application of FSK is sending text messages over a modem: each character is encoded using one of two frequencies (one higher than average, one lower than average), so when a modem receives an encoded series of characters from another modem, it knows precisely what was sent without having to decode anything first.