Communication consists of an information-bearing message that originates at a message source and is transmitted to a message sink. The source can be a person or a machine. The message at the source can take various forms such as voice, images, audio, video, digital data, etc. In modern digital communication systems, all messages are transmitted in digital form. This means that even if the source produces analog data, it is first converted to digital form. This is typically done through a device called an analog-to-digital converter (ADC). Oftentimes the digital data generated by this process are "compressed" to reduce the number of bits used to represent the data. This process is called source coding.
The medium separating the transmitter and the receiver is called a communication channel. Some examples of communication channels are air (free space), telephone wires, coaxial cable, and optical fiber. These media essentially carry electromagnetic (EM) waves. The electric and magnetic field strengths of these waves vary in time as analog waveforms. Thus, the digital data to be transmitted must be mapped to analog signaling waveforms before they can be sent over a channel. This process is called digital modulation.
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