Edwin Howard Armstrong's best known invention is Frequency Modulation. He showed this invention to RCA in 1933. RCA got quite upset about FM, as they'd put a lot of resources into making AM something people bought based on price rather than quality. FM was a quality product. RCA petitioned the FCC to give Armstrong's allocations for FM, for which he'd invested building recievers, to their new television system. They also started selling FM products without paying any royalties to Armstrong. Armstrong took them to court. RCA managed to convince the court that they'd invented FM on their own. On January 31, 1954, Edwin Armstong jumped out of a 13th story window to his death on a third story overhang, where his body wasn't discovered until the next day.

[ rfc791.ORG : Ham Help : Modulation ]

Radio frequency electrical waves have different properties depending on frequency that we might want to take advantage of. Twenty-meter waves (which oscillate 14 million or so times per second), for example, can bounce off the ionosphere and travel around the world. Plus, by using radio frequencies, there are a lot more frequencies that are available so many people can be using the airwaves at once. Modulation is how we get information into these radio waves.

The oldest and simplest form of modulation is called OOK or On-Off Keying. OOK works by turning a radio frequency transmitter on and off. Different sequences of on and off time at the transmitter can be used to encode morse code onto the radio frequency. Essentially we are modulating a square wave onto a radio frequency sine wave. We call that RF sine wave our carrier. Transmitting OOK encoded with morse code is frequently known as CW in the ham radio world.

There are also various ways to modulate audio frequency sine waves onto an RF carrier.

Amplitude Modulation (AM)
Amplitude modulation is the simplest audio frequency modulation technique. It works by changing the amplitude of the carrier at the transmitter to be proportional to the amplitude of the audio frequency being modulated. The disadvantage in this is that as you move the receiver farther from the transmitter, the amplitude of the incoming signal is smaller. The percieved amplitude at the reciever will be proportional to the inverse of the square of the distance between the transmitter and the reciever (distance being along the path of propagation, not necessarily line-of-sight). The intuitive answer to this is turn up the volume if you're far away. This works for keeping the volume constant, but assuming the noise level is the same, the noise will eventually drown out the signal anyway. Am recievers of more than rudimentary design have Automatic Gain Control circuitry to adjust the volume for you when signal strengths change.
In the time domain, AM looks like a carrier whose envelope (the curve of where the peaks of the waves are) is shaped like the modulated audio. In the frequency domain, AM looks like a center carrier frequency with the sum of the carrier and the frequencies in the audio to its right and the difference of the carrier and the audio modulated to its left. In the frequency domain, the portions to the left and right of the carrier are called the lower and upper sidebands, respectively. The upper sideband is the sum of the carrier with the input audio, and the lower sideband is the difference.

Frequency Modulation (FM)
Frequency modulation works by slightly changing the frequency of the carrier in response to changes in the amplitude of the audio frequency being transmitted. The amplitude of the carrier stays constant. Now the amplitude of the received waves don't depend on the distance between the transmitter and reciever. And the signal strength can be greater than the noise level when the transmitter is transmitting silence.
FM has a curious phenomenon associated with it called the "capture effect". When two signals are recieved in an AM (or sideband, which we'll cover next) receiver, you can hear both signals as they essentially mix in the air. When a receiver picks up two FM signals, the strongest one wins. This makes interference less of a problem in FM, but it makes doubles (when two stations accidentally transmit at the same time) slightly harder to detect.
In the time domain, FM's peaks are closer together during peaks in the incoming audio being modulated, while the amplitude of the peaks remains constant. In the frequency domain, FM has a spike, the position of which is determined by the current amplitude of the audio being modulated.

Single Sideband Modulation
Single Sideband, also known as suppressed-carrier single sideband, SSB, or just sideband, has two types, upper sideband (USB) and lower sideband (LSB). Sideband is essentially AM that has had the center carrier frequency and the other sideband removed. In AM, we're using power unnecessarily to generate the carrier frequency, and the other sideband. We only need to really transmit one. When we transmit one sideband, we're using 100% of our output RF power to transmit information in the most efficient way possible. We're not duplicating any information, and we're using less than half the spectrum.
In the frequency domain, sideband looks like AM without the carrier and the opposite sideband. If you take upper sideband and shift it down the frequency domain such that where the carrier used to be is now at the zero Hz point, you end up with the original AF. If you shift lower sideband until where the carrier was hits the zero mark, you end up with the entire sideband in the negative, but if you mirror that on the other side of the y axis, essentially taking it's absolute power, you get the original audio frequencies.
In the time domain, upper sideband looks like the original audio except with the wave peaks closer together. The amplitude ofthe sideband waves mirror the amplitude of the original audio, thus sideband suffers the same gain problems as AM.

Here are some example waveforms in the time domain:
Modulating audio

The original modulated audio in the frequency domain

AM in the frequency domain (note the prominent carrier frequency)

FM in the frequency domain.

Upper sideband in the frequency domain (looks just like the original audio, but shifted up to RF).

Lower sideband in the frequency domain

Digital Modulation Types
The new big thing is no longer getting audio on the airwaves, but data. Data can be anything - pictures, video, anything you can store in a computer file. And there are new modulation techniques to get it on the air. Data modulation is usually done by modulating the data into audio frequencies in the computer through the sound card, or through a device called a TNC or Terminal Node Controller. The audio created is then modulated onto an RF frequency wave using existing audio modulation techniques (FM, AM or SSB). Using one of these techniques allows us to get data on the air on the cheap, using our preexisting radios and computers. There are a couple of other techniques, pulse modulation for example, but they're less frequently used.

Most of the first style of data modulation techniques I described really only make sense if you look at them in the frequency domain. Packet, for example, is composed of two frequencies, 1200Hz and 2200Hz. When one frequency is sent, it signifies a one, another signifies a zero. This is called Frequency Shift Keying, or FSK, as a shift in frequency means a change in the data. This is the digital equivalent of FM, essentially.

If you feed audio frequency FSK into an FM transmitter instead of a sideband transmitter, the RF frequency is no longer related to the audio frequency. We call this variation AFSK or Audio Frequency Shift Keying.

The simplest kind of digital mode is the oldest mode there is! OOK works, as described above, by turning the carrier on and off to represent ones and zeros (or the dits and dahs of CW). OOK can actually be used to transmit data between computers, and it's frequently used to transmit data between humans in the form of CW. Since clear RF patterns represent the symbols of the communication, OOK, and by extension, CW, is truly the world's oldest digital mode.

OOK is similar to ASK, or amplitude shift keying, which encodes data by shifting amplitude. This is a poor method for reasons discussed in the section on AM.

The new kid on the block is PSK, or phase shift keying. PSK works by changing the phase of the carrier to encode information. The simplest PSK is bi-phase PSK. This kind of PSK encodes ones and zeros by 0 and 180 degree shifts in carrier phase. This is like switching the wires to your antenna -- a 180 degree phase shift is the same as making everything backwards. There's also quartenary PSK, with 0, 90, 180, and 270 degree phase shifts to encode information. I believe there's also an 8-phase PSK. More phases means faster data transfer, but less ability to cope with noise.

There are some digital modes that use several of these techniques. QAM, or Quaternary Amplitude Modulation, is kind of mix of PSK and ASK. The new G4GUO protocol for digital voice uses 32 PSK carriers in parallel. More on this later.

Watch this space!

Coming soon - software to help you understand modulation.