AfterDawn: Glossary

Samplerate

Samplerate refers to the samping Frequency of an analog to digital conversion (or analog capture). This literally means how frequently the analog source is sampled. Samplerate is measured in samples per second, where 1 Sample per second is equivalent to a sampling frequency of 1Hz.

Nyquist-Shannon Sampling Theorem
The basis for analog to digital conversions, the Nyquist-Shannon Sampling Theorem tells us that the maximum analog frequency which can be reliably reproduced is slightly less than 1/2 the sampling frequency (Samplerate). For example, audio CDs use a Samplerate of 44,100 samples per second, which equates to a sampling frequency of 44.1kHz (kiloHertz). A 44.1kHz signal can reproduce frequencies up to 22.049kHz ( (44100 - 1) / 2).

Oversampling
Although the Sampling Theorem only requires two digital samples per analog wave (2MHz sampling frequency per 1MHz source frequency), since the hardware and software used for capturing isn't capable of Nyquist-Shannon precision it can be useful to oversample. Oversampling produces greater accuracy by providing more than two digital samples per analog waveform. For example, although DVD-Video's 48kHz sampling can reproduce all the frequencies humans are capable of hearing, sampling at DVD-Audio's 96kHz will provide more accurate reproduction because of the increased Samplerate.

Standard Digital Audio Sampler Frequencies
Audio CD, VCD, SVCD - 44.1kHz
DVD-Video - 48kHz
DVD-Audio - 96kHz, 192kHz
Digital TV - 48kHz
HD DVD, Blu-ray - 48kHz, 96kHz

Video Sampling
When capturing video you also need to consider the sampling theorem. By calculating the Nyquist Frequency, which is the lowest frequency required to Capture all information in an analog signal, you can determine the minimum capture Resolution required to retain all detail. This varies from one analog source to another because the Bandwidth (range of frequencies) is variable. The minimum capture ersolutions lised below aren't actually recommended for use. Instead you should consider them a lower boundary to use when selecting the resolution for your own captures. Your actual selection will depend on your hardware and software. For Video Capture in particular, a higher samplerate wil almost always produce better quality captures.

Minimum Capture Resolution

SourceBandwidthCapture Res
PALBroadcast5MHz521x576
VHS3MHz313x576
S-VHS5MHz521x576
NTSCBroadcast4.2MHz443x480
VHS3MHz317x480
S-VHS5MHz527x480


Samplerate Conversions
Although it's possible to convert from one samplerate to another, nearly every conversion is a lossy process. You can never add detail simply by upsampling to a higher Samplerate. Detail is dependent on the Samplerate used for capture. Unless its necessary you should avoid upsampling or downsampling.

Additional Reading
Sampling Theory For Digital Audio

Related Guides
Digital Video Fundamentals - Resolution and Aspect Ratio

Digital Video Fundamentals - Color Formats




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