Technical Editor S.K. Foss

Associate Technical Editors R.H. Marloff B.C. Dykes

Publisher K.A. Galione

Managing Editor M.E. Yergin

Editorial Assistant H.A. Molinaro

Advertising Manager D.L. Vidmark

Circulation Manager D.R. Coelho

E/T Editorial Committee S.K. Foss, chairman M.L. Basehore D.A. Dillard B.C. Dykes G.W. Eggeman D.R. Harting R.L. Johnson 4.F. Lawrence L . J . Lazarus R.H. Marloff 3.E. Swartz Z.E. Warren 3.L. Willis

SEM Executive Board R.J. Rinn, president 1.M. Allison, president-elect C.A. Calder, vice-president D.L. Willis, treasurer K.A. Galione, managing

W.L . Fourney A.S. Kobayashi I.B. Ligon D.H. Morris W.N. Sharpe, Jr. R.F. Sullivan W . M . Murray, honorary

president

director

$EM Editorial Council :.A. Calder, chairman L A . Galione, secretary M.L. Basehore 2 .L . Cloud 1.F. Doyle S.K. Foss a.L. Foumey Z.E. Passerello w. Phillips

M . E . Tuttle 3 . L . Willis <.I. Rinn, ex officio

Better The Imperfect Truth than Perfectly Accurate Garbage or The Real-Time On-Line Interactive Super-sophisticated Data-Acquisition System vs. The 'Old' Analog Recorder

At a recent short course on instrumentation and measurement engineering, tour the speakers emphasized the same message. record ALL dynamic data on analog recordcrs, regardless ot any other method used tor data acquisition, either in parallel or subse- quently The speakers' backgrounds ranged trom confirmed analog to confirmed digital. The audience was shocked! Several members even requested a written explanation since their managements wcrc about to discard all analog recorders in favor ot new, real- time, on-line, computer-interactive, super-sophisticated digital data-acquisition systems.

Among the analog recorders eligible for consideration here are the direct-record magnetic tape, the 'old' cathode-ray oscilloscope, galvanometer recorders and strip- chart recorders, depending on the trequcncy range of the signal and the capability ot the recorder WHY???!!!

ANY conversion from analog to anything else, even AM (amplitude modulation) or FM (frequency modulation), but especially the process of digitization, ettectively requires a time sampling of the data. INEVITABLY some data are lost, or, what's worse, are 'created'. Our profession needs creative measurement engineers, not creative measurement systems!

The problem of mtruchannel 'aliasing' appears tor single-data channels In this process, high signal frequencies which exceed one halt the sampling rate (or carrier frequency) are converted to lower frequencies which appear in the data, never to he recognized, separated out or eliminated forever atter. The data have been distorted, and that tact is totally hidden from the experimenter's or analyst's view. In AM or FM systems this condition may occur during the demodulation process.

The use of anti-aliasing pretiltering at the analog data, to avoid this condition, may result in tiltering out trequencies a t which signiticant data m a y lie. rhc cxpcriincnicr is thus caught in a vicious circle. use anti-aliasing pretiltering to t i t the sampling rate ot the data-acquisition system and possibly lose signiticant high-trcquenry in lormat ion , o r do not use these filters and possibly distort the data. In neither case will the experimenter EVER KNOW WHAT REALLY HAPPENED! The only sateguard is to record the analog data in analog torm, frequency analyze i t , determine the sampling rate required t o reproduce that data in another torm, and select the data-acquisition system accordingly. Looking at or listening to the analog-wave shapes is also highly recommended It the analog recording is in reproducible form, then the original record can always be used tor redesign of a digital (or other] data-handling system which was used but did not tultill the requirements.

In multichannel systems operating on the trequency-divislc)n Inultiplexinl: \ystein w t up by the Inter-Kange 1nStrumentation Group IIKIG standards), each channel need\ ai l t l -

premodulation tilters to prevent trcqucncics in one channel trom appearing in the bandwidth assigned to another channel. This is the prohlem c i l iritcrchanncl alln5lIlg or cross talk, which IS not dirrcrly related t c ~ the samplir~i: rate o r carrier trequcncy used to convey the intormation in each channel. But these prei11cidulation tilters will a l w limit the bandwidth of the channel. This eftect is recognized and speciticd in the IKIG standards. again, [he only sate way is to hase thc chuice u t lKlG chdnnci on .I

the ANALOG data, so that a11 signiticiint trcquency contcnt in thdi data 1s reproduced attcr modulating or saniplini:

SO-SAVE YOUR 'OLL)' ANALOG KECORUEKS, and USE them at led\( in pardlie1 all dynamic data channels which arc being processed hy AM, Fhl, digital, o r any

"[her nonanalog nleans * These 'old' systems will sonied.iy \avc your neck whcn Illexplicab~e plague The ,malog recorders may he Inipcrlcct, but better the lnlpcrfect truth, than pertectly accuratc ga1-bF"

Peter li Stein Ste in €ngiricering SCII . ICL' \ . IIIL'

~~rl,ornldllon ill nltzdi,\urin,~ \v>rt'n> I \ j ~ e n , [ r < , ~ ~ c , t I 8 1 , OII [ I < I I I I ' ~ I I \ 1 8 1 P I ~ P C ~ I I ~ ' \ 0 1 w\i l lc ' \h+'<

s t ~ c ~ [ c ~ i r ~ . p c ~ r ~ ~ r h . hc.ii$or\ il, lriforrn'ilrori I J r i k t ' \ \ ( 1 r ~ , R ~ \ C d r i l i L ~ ~ ~ ~ ~ ~ ~ ~ T r l I ~ ~ r l l md nor

.M-JO (lunc I Y 7 0 /

I,lt. u.\u,i/ ~ l i l c l )~Js ~ ~ ~ ~ f , ~ / L i i i l l ~ i ~ , , ~ i ~ r , ~ ; I I ~ / I I I I , I A ~ ~ ~ 110 \ c n v n j i ~ . r i Wl~i i t '~; 1~~ rrii'J~LlrlriS

Experimental Techniques 3