688 



lIANDBOf)K OF PH'iSIOLOGV 



CIRCULATrON I 



ELECTRONIC DEVICES CONSTITUTING 

 IN EFFECT A FILTER OF AOJUSTABLE 

 PASS- BAND WIDTH AND VARIABLE 

 CENTER FREQUENCY 



MAGNETIC TAPE RECORDING 

 FOR SUBSEQUENT EDITING 



o 



f>^ 



^^ 



.\ 



STYLUS RECORDS FILTER OUTPUT ON 

 ELECTRO-SENSITIVE PAPER AS 

 EFFECTIVF TUNING OF FILTER IS 

 CHANGED 



^ 



FREQU£NCT SCALE . 

 TUNING CHANGES WITH MOVEMENT OF 

 ARM UP SCREW WIDTH OF ADJUSTABLE 



FILTER PASS-BAND REPRESENTED BY 

 WIDTH OF CONTACT 



-^, 



^ 



-^ 



T MAGNETIC DISC ON -EDGE OF 

 J WHICH SEGMENT OF SOUND 



IS RECORDED FOR REPEATED 



PLAYBACK 



FIG. 6. Schematic representation of the sound spectrograph used in making most of the spectral 

 phonocardiograms presented in this chapter. The intermediate step of tape recording and the seria- 

 tim rather than simultaneous frequency analysis are shown. [Courtesy of Williams & VVilkins (12).! 



monies of musical murmurs are displayed as nar- 

 rower bands, more nearly approaching what must be 

 their true state, than in the records made by the usual 

 method. However, no particular value of this more 

 precise definition is evident, and the transients and 

 noisy murmurs which constitute the great bulk of 

 cardiovascular sound are unsatisfactorily displayed. 

 Hence, the phase filter has nothing to recommend it 

 for routine use. 



The general principle and mechanical design of the 

 spectral phonocardiograph is shown in figure 6. As 

 now practiced, the sounds first recorded on magnetic 

 tape are audited and sections selected for analysis. 

 One such section is played over onto the magnetizable 

 margin of a disk mounted on the same axis as the 

 kymograph drum on which the records are made. As 

 explained above, the analyzer is in essence a single 

 pass-band filter, the tuning of which is changed 

 progressively as the segment of sound is played back 

 repetitively through it. For example, during the first 

 rotation of the magnetic disk and the coaxial drum, 

 the information in the 15 to 20 cps band might be 

 inscribed on the record, with the second rotation the 

 information in the 20 to 25 cps band, and .so on from 

 bottom to top of the record until the entirety of the 

 frequency scale desired has been scanned. There is, of 

 course, much overlap of the individual frequency 

 bands. In making an analysis from o to 750 cps the 

 soimd must l)e passed through the equivalent of as 



many as 425 individual filters, each with its pass-band 

 overlapping others but with its center frequency 

 about 1.8 cps removed from the center frequency of 

 the filters next above and below it. 



A frequency .scale from o to 720 cps encompasses 

 most of the information in cardiovascular sound, as 

 recorded at the chest surface. 



THE DISPL.AY 



Oscillography 



Display of the oscillogram is accomplished by 

 means of a cathode-ray tube or a galvanometer.'' The 

 cathode-ray oscillograph has the advantage of total 

 lack of inertia .so that its frequency response is un- 

 limited. In phonocardiography, necessitating photog- 

 raphy of a fluorescent screen, difficulties arise as a 

 result of limitations on the brightness of the spot 

 where vibrations of high frequency are to be recorded. 

 The faster the spot moves the fainter becomes the 

 trace. Several methods exist for circumventing this 



■■ Yet a third method for display of the oscillogram is the 

 jet-ink writer (Mingograph) devised by Elmqvist and produced 

 by the Elcma Co., Stockholm. The pressure of the ink jet is 

 modulated as a function of the amplitude of the vibrations 

 being recorded. The system has little inertia and consequently 

 an ad\'antageously high-frequency response. 



