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Sampling frequency
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.
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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.
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
See Also
- Continuous signal vs. Discrete signal
- MP3
- Digital control
- Sample and hold
- Sample (signal)
- Sampling (information theory)
- Signal (information theory)
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