DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

SPECTRAL ANALYSIS 233TABLE 5.4 Some Center Frequencies and FrequencyDeviations Specified by IRIG to Yield Constant-BandwidthChannels for Acquisition through FM SubcarriersFrequency Deviation and NominalFrequency Response per Channel2 kHz 4 kHz 8 kHz 16 kHz 32 kHz400 Hz 800 Hz 1.6 kHz 3.2 kHz 6.4 kHzChannel Center Frequency (kHz)8 16 32 64 12816 32 64 128 25624 48 96 192 38432 64 128 256 512TABLE 5.5 Many Channels of Differing Bandwidth Can Be Accommodated by Selecting Channel BandwidthsProportional to Their Carrier Frequencies aIRIG Proportional Center Lower Deviation Upper Deviation Channel’s NominalBandwidth Channel Frequency Limit Limit Bandwidth Frequency Response7.5% Deviation Channels1 400 370 430 60 62 560 518 602 84 83 730 675 785 110 114 960 888 1,032 144 145 1,300 1,202 1,398 196 206 1,700 1,572 1,828 256 257 2,300 2,127 2,473 346 358 3,000 2,775 3,225 450 459 3,900 3,607 4,193 586 5910 5,400 4,995 5,805 810 8111 7,350 6,799 7,901 1,102 11012 10,500 9,712 11,288 1,576 16013 14,500 13,412 15,588 2,176 220aThese IRIG channels are useful for acquiring multiple physiological signals with different bandwidth requirements. For example, the subject’sskin conductance signal fits well within the 11-Hz bandwidth of channel 3, the 45-Hz bandwidth of channel 8 could accommodate anEEG channel, and a three-lead wideband ECG can be acquired through channels 11, 12, and 13.SPECTRAL ANALYSISThe analysis of a signal based on its frequency content is commonly referred to as spectralanalysis. Because of the rhythmic nature of practically every phenomenon in a livingbody, frequency-domain analysis is one of the most powerful tools in the examination ofphysiological signals. The mathematical basis for this operation, the Fourier transform, hasbeen known for many years, but it was the introduction of the fast Fourier transform (FFT)algorithm that made spectral analysis a practical reality. Implementing the FFT in personalcomputers and embedded DSP systems has allowed efficient and economical applicationof Fourier techniques to a wide variety of measurement and analysis tasks. Moreover,because the FFT has been found to be so valuable in applications such as medical signalprocessing, radar, and telecommunications, DSP chips are often designed to implement itwith the greatest efficiency.