514 BELL SYSTEM TECHNICAL JOURNAL 



The Recorder 



One of the early practical phonographic applications of electric filter 

 design to mechanical problems was the development of an electromag- 

 netic recorder. The instrument as finally constructed is essentially a 

 properly terminated three-section mechanical filter in which the re- 

 cording stylus and its holder constitute the series mass in the second 

 section. Since a filter of this type appears at its input end as approx- 

 imately a pure resistance within the transmission band, the current in 

 the series inductances, that is, in the mechanical case, the velocity of 

 the series masses is proportional to the driving force. 



Figs. 10, 11 and 12 show respectively, a complete recorder, a drawing 

 of the mechanical filter of such a recorder and a diagram of the equiva- 

 lent electric circuit. The armature acts as the series mass Wi in the 

 first section; the magnetic field as the series negative compliance, — Co; 

 the shaft between the armature and the stylus holder as the shunt 

 compliance Ci; the balancing springs as the series compliance c-i; the 

 stylus holder and the stylus as the series mass m^; the shaft between 

 the stylus holder and the disk, coupling the system to the terminating 

 resistance, as the compliance c^; the coupling disk as the series mass 

 mz and the terminating line as approximately a mechanical resistance. 



All of these equivalents are seen from the simple analog previ- 

 ously outlined with the exception of the terminating resistance and 

 the negative compliance, — fo- The terminating resistance was origi- 

 nally made up of a series of filter sections of lumped series masses 

 and shunt compliances with a small amount of damping added to the 

 motion of each of the series masses. Fig. 13 shows one of the early 

 recorders equipped with this type of resistance termination. The 

 reason for using such a complicated termination lies in the fact that 

 most of the known mechanical resistances have values which are 

 functions of frequency or of amplitude or both. Also in most cases, 

 the mechanical resistance is accompanied by either a mass or com- 

 pliance reactance. By using a multi-section filter which is suffi- 

 ciently long so that a wave entering it will be essentially absorbed 

 before it has reached the far end, been reflected and returned to the 

 entering end, it has been possible to use imperfect types of damping 

 for this line and still obtain over the desired band, an essentially pure 

 resistance at the input end. 



More recently a continuous line has been developed which is much 

 easier of practical attainment than the complicated lumji-loaded 

 filter. The recorder shown in Mg. 10 is so equipped. 



Fig. 14 shows calibration curves of three types of recorders. The 



