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Sampling rate)

The **sampling frequency** or **sampling rate** defines the number of samples per second taken from a continuous signal to make a discrete signal. The inverse of the sampling frequency is the **sampling period** or **sampling time**, which is the time between samples.

The sampling frequency can only be applied to samplers in which each sample is periodically taken. There is no rule that limits a sampler from taking a sample at a non-periodic rate.

## Sampling theorem

The Nyquist-Shannon sampling theorem states that the sampling frequency has to be greater than twice the Nyquist frequency or, equivalently, twice the bandwidth of the signal being sampled.

If a signal has a bandwidth of 100 Hz then to avoid aliasing the sampling frequency must be greater than 200 Hz.

## Oversampling

In some cases, it is desirable to have a sampling frequency more than twice the bandwidth so that a digital filter can be used in exchange for a weaker analog anti-aliasing filter. This process is known as oversampling.

## Audio

In digital audio, common sampling rates are:

- 8,000 Hz - telephone, adequate for human speech
- 11,025 Hz
- 22,050 Hz - radio
- 44,100 Hz - compact disc
- 48,000 Hz - digital sound used for films and professional audio
- 96,000 or 192,400 Hz - DVD-Audio, some LPCM DVD audio tracks, BD-ROM (Blu-ray Disc) audio tracks, and HD-DVD (High-Definition DVD) audio tracks

## Video

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:

## See Also