The signal that is transmitted over the air is at a particular frequency. This signal is called the carrier. Modulation is the method of putting information on the carrier signal. It is sometimes helpful to visualize modulation as a moving truck and cargo to be moved. The carrier is the moving truck that moves from point A to point B. The signal or information to be transmitted is the cargo. Looking at the Morse code figure it can be seen that by turning on and off the carrier to represent the intelligent content of a message, the operator of the telegraph key is serving as the modulation source. This type of On Off Key modulation is very simple and inexpensive, but there are many other forms of modulation. Each has advantages and disadvantages, so there is not a universal type of modulation.