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The sampling rate, sample rate, or sampling frequency defines the number of samples per second taken from a continuous signal to make a discrete signal. It is measured in hertz (Hz). The inverse of the sampling frequency is the sampling period or sampling time, which is the time between samples. Sampling rate should not be confused with bitrate.
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.
The common notation for sampling frequency is which stands for frequency (subscript) sampled.
Sampling theorem
The Nyquist-Shannon sampling theorem states that the sampling frequency has to be greater than twice the bandwidth of the signal being sampled, or equivalently that the Nyquist frequency must exceed 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.
In other words, the sampling frequency must be at least twice the maximum frequency component of the signal. Otherwise, the original signal cannot be recovered from the sampled signal.
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
- 32,000 Hz - miniDV digital video camcorder, DAT (LP mode)
- 44,100 Hz - audio CD, also most commonly used with MPEG-1 audio (VCD, SVCD, MP3)
- 47,250 Hz - world's first commercial PCM sound recorder by Nippon Columbia (Denon)
- 48,000 Hz - digital sound used for miniDV, digital TV, DVD, DAT, films and professional audio
- 50,000 Hz - first commercial digital audio recorders from the late 70's from 3M and Soundstream
- 50,400 Hz - sampling rate used by the Mitsubishi X-80 digital audio recorder
- 96,000 or 192,000 Hz - DVD-Audio, some LPCM DVD tracks, BD-ROM (Blu-ray Disc) audio tracks, and HD-DVD (High-Definition DVD) audio tracks
- 2.8224 MHz - SACD, 1-bit sigma-delta modulation process known as Direct Stream Digital, co-developed by Sony and Philips
Video systems
In digital video, the sampling rate is defined the frame/field rate, rather than the notional pixel clock. The image sampling frequency is the repetition rate of the sensor integration period. Since the integration period may be significantly shorter than the time between repetitions, the sampling frequency can be different from the inverse of the sample time.
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:
Aliasing of high-frequency luminance components shows up as a moiré pattern.See also
- Continuous signal vs. Discrete signal
- MP3
- Digital control
- Sample and hold
- Sample (signal)
- Sampling (signal processing)
- Signal (information theory)
External link
Digital signal processing | Signal processing
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