Sampling frequency

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 Nyquist-Shannon 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 higher-quality sampling.

In digital audio, common sampling rates are:

• 8000 Hz - telephone
• 44100 Hz - compact disc
• 48000 Hz - digital sound used for films

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.

• 50 Hz - PAL video
• 60 / 1.001 Hz - NTSC video

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:

• 13.5 MHz - CCIR 601, D1 video[?]

• Data compression/MP3