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Digital Signal Processing
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Digital Signal Processing |
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Scanning Rates
As mentioned previously , the throughput rate of a digital data system was defined as the total number of useful data points that can be acquired, stored, reduced, or displayed by the system, per unit of time. While many hardware and software factors contribute to the throughput rate, one of the most important is the sampling rate, which can be defined as the total number of useful datum per unit of time for each signal channel . While the throughput and scanning rates may be the same for a system containing a single signal channel, the scanning rate is more commonly the throughput rate divided by the number of channels. For static signals, where the measurand does not vary during the measurement process, the sampling rate is of little consequence. For dynamic signals, unless the user has an understanding of how the system acquires data, how many measurement channels are being sampled, and how the data are to be used, the term can be particularly misleading, resulting in data that is somewhat inaccurate, or completely wrong. For example, consider a digital measurement system that acquires data sequentially, with a single analog-to-digital converter (ADC) running at 100 samples/second, and a single strain gage on a test part vibrating at 10Hz. This sampling rate of 100 samples/second/channel provides for 10 datum/cycle/channel, and should be adequate to reconstruct the signal in the time domain. But if the frequency of the signal increases to 100Hz, the system can provide for only 1 datum/cycle/channel -- clearly insufficient to reconstruct the signal. While both the throughput and sampling rates are unchanged, the data becomes meaningless when the frequency of the signal changes . (continued...) 
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