Modulation is the means whereby a signal of some type is entered into and carried by an electronic signal carrier. Within the scope of modulation, the type of signal or information that is introduced into and carried by the electric or optical signal carrier may vary, depending on the configuration of the carrier and the source. Different types are used with various types of broadcast and communication mediums today.
The use of modulation has been part of technology ever since electricity was harnessed and used for communication purposes. An excellent elementary example has to do with early telegraph technology. Using what was essentially binary code, telegraphs would forward a message that could be interpreted at the receiving end, decoded, and presented to the recipient. This form relied upon a more or less stable type of modulation that did not increase or decrease during the signaling process.
Another common example has to do with the reception of radio transmissions. Modulation helps to define the type of transmissions that are in use for general broadcast purposes, as well as more focused applications such as ham radios. Amplitude modulation (AM) describes a broadcast situation in which the level of voltage that is carried over the medium will vary noticeably over time. Anyone who listens to AM radio stations is usually acquainted with the way signals seem to weaken at night, and a clearer signal is harder to achieve.
Frequency modulation (FM) is also commonly used for radio transmissions. The difference is that the amount of modulation does not vary as dramatically as with AM broadcasts, although there is a small amount taking place. Phase modulation is a third configuration that sometimes will involve delays in the transmission and reception process. Ham radios are a good example of this short but noticeable delay in transmission.
As technology has continued to advance, and communications have become more comprehensive and varied, other constructs of modulation have appeared. Wireless communications and the use of the Internet have resulted in such important signaling tools as multiplexing and modem modulation. Along with the more common forms that are represented by AM and FM broadcasts, there are also some devices that make use of what is known as pulse code modulation, which can be utilized to encode analog and digital signals in a binary pattern.