When converting from analog to digital, the analog signal must usually be sampled  that is, measured or read  at discrete intervals of time. The length of the interval depends on the application, but is limited by the NyquistShannon sampling theorem. The sampling frequency is the inverse of this number: the smaller the interval, the higher the frequency, and, in general, higher frequencies imply higherquality sampling.
In digital audio, common sampling rates are:
In digital video, which uses a CCD as the sensor, the sampling rate is defined the frame/field rate, rather than the notional pixel clock. All modern TV cameras use CCDs, and the image sampling frequency is the repetition rate of the CCD integration period.
When analogue video is converted to digital video, a different sampling process occurs, this time at the pixel frequency. Some common pixel sampling rates are:
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