Back in the Sailing days and early days of steam, ships inbound had to rely on "coast watchers" to observe the ship on its inward voyage and then would notify the line handlers and ship owners of the presence of the vessel. Signal fires would be placed on the ship to announce the arrival, and sometimes with unexpected results. Signal lights next, and as they got closer then the signal flags would communicate with shore facilities. Radio then came on the scene and the "spark" transmitter was the latest method of advance notice. The spark transmitter was usually noted by the crew as the transmitter actually generated a spark, much like the spark plug on a car, and the energy would be sent to the antenna and in hopes the shore station could copy the dit-dah's of the transmitter. It was no wonder why the Radio Officers were called "Sparky" and until lately, the name was carried on with the RO's.
Times, of course, have changed and now we have modern up-to date radio equipment that does not need all that "fine" tuning that the Radio Officer was able to perform. Much was said about not having a Radio Officer on board but as equipment became more reliable and there was no need for the "fine tuning" nor the knowledge of the "code". We now have Single Side Band, Narrow Band Direct Printing, and Digital Selective Calling.
In the old days we turned off the radio transmitter and turned it back on to send a signal that was on for a short time, we called a "dot" and a longer time we called a "dash" we could provide a code for these on and offs. As an example a short and a long was a "A". And so we had "code" or CW (continuous wave).The human ear could tell the difference in the ons and offs but when we went to computers the computer had a hard time to tell the difference when there was a log of interference, static, fading, etc. By changing the frequency slightly instead of the on and offs the computers could follow along. This method is called Frequency Shift Keying. FSK is used for data and telexes.
The carrier wave of the transmitter, is at a fixed frequency, which is called Radio Frequency (RF). By using the small current from our microphone became Audio Frequency. We then combined these two and we have four signals.
The original RF signal
The original AF signal
The Upper Sideband
The Lower Sideband
The radio now has two sidebands and one is mirror of the other. But we must not let the sum or difference be more than 3 kilohertz (the technical people set this standard) and we call this Amplitude Modulation and because we are transmitting both sidebands we call this double sideband. Your AM radio station used this kind of "modulation" but uses more than the 3 Khz.
