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Figure 2. Minimum acceptable sampling rate for the data acquisition system on a particular experiment depends on how much information the researcher wants from an analog signal such as intensity as a function of time. If 20 points must be recorded in peaks as short as 1 s, the minimum data collection rate would be 20 points/s
completed. But data processing online during collection may be either necessary or desirable, and the required sampling rate may be low enough to make it feasible. Table I gives maximum throughput rate in samples per second for several common reduction/analysis subroutines running during data collection on a PDP-11/34 minicomputer (these rates would be substantially reduced in a MINC system supported by the smaller PDP-11/03 computer). These rates were determined by measuring the amount of time it takes for the subroutines to perform the calculations. In actual practice, data are collected in a buffer (a collection of samples) and then passed through to the subroutines. The relatively simple procedure of interval histogramming is fast enough not to restrict throughput. Real-time computing of phase angle and amplitude spectra, on the other hand, substantially reduces throughput and thus isn't feasible in
many types of experiments. Controlling an experiment in response to collected data is a processing function like reduction or analysis that requires CPU time and other computer resources. The question, again, is whether or not the sampling rate is so high that there's no time for control. Collection rates as low as 20 samples/s would normally leave more than adequate time for control functions, whereas a rate of 20 kHz permits no control at all. Throughput in data collection alone and with real-time processing can be substantially improved by upgrading the computer to a more powerful model having a larger main memory. Another approach is to use parallel computers: One machine is dedicated to collecting and storing data on a shared mass storage device, and the second machine devotes all its capabilities to processing the data and controlling the experiment. Types of Wiring. There are two
Table 1. Effect of On-the-Fly Data Reduction or Analysis on Sample Throughput Rate Subroutine
Peak processing subroutine Envelope processing subroutine Interval histogramming subroutine Interval histogramming with references FFT subroutine Phase angle and amplitude spectra Power spectra subroutine Auto-correlation subroutine Cross-correlation subroutine
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854 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
Throughput rate (samples/s)
1 4 20 14
000 400 000 000 800 180 5 400 420 280
