Sampling frequency

From Free net encyclopedia

(Redirected from Sample time)

The sampling frequency or sampling rate 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.

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 <math>f_s</math> which stands for frequency (subscript) sampled.

1 Sampling theorem
2 Oversampling
3 Audio
4 Video systems
5 See also
6 External link

[edit]

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.

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.

[edit]

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.

[edit]

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
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 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 192,400 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

[edit]

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.

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: