"... we may acquire a natural image, process it to enhance the picture, compress it for transmission, and then encode and transmit it in some fashion over a digital network. On the other end, the image is decoded, decompressed, and displayed to create another signal (the visible light of the display). But that isn’t the end of the story: the signal is then received by the eyes, processed further, and interpreted in some fashion by our brain. From acquisition to interpretation, the initial signal may be transformed, modified, and retransmitted numerous times. In this example, the signal underwent more than 10 transformations ..."
When a signal has continuous domain and range, we usually call it analog. When a signal has discrete domain and range, we call it digital.
"... The spacing of discrete values in the domain of a signal is called the sampling of that signal. This is usually described in terms of some sampling rate–how many samples are taken per unit of each dimension. Examples include “samples per second”, “frames per second”, etc.
The spacing of discrete values in the range of a signal is called the quantization of that signal. Quantization is usually thought of as the number of bits per sample of the signal. Examples include “black and white images” (1 bit per pixel), “16-bit audio”, “24-bit color images”, etc ..." - B. Morse, BYU, On Signals and Images
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