Methods of High Quality Recording and 



Reproducing of Music and Speech 



Based on Telephone Research ' 



By J. P. MAXFIELD and H. C. HARRISON 



SvNOl'sis: This paper deals wilh an analysis of the general requirements 

 of recording and reproducing sound without appreciable distortion. The 

 storing or recording of sound requires, first, a mechanical system which will 

 respond faithfully to the sound waves which are to be recorded. Then there 

 is required some material in or on which this sound may be recorded and an 

 intervening system which permits the sound waves to make the record in this 

 material. In the usual case, and in that which is particularly discussed, there 

 is a mechanical system which will vibrate in response to the sound which is 

 to be recorded and directly through some mechanical linkage, or less directly 

 through an electrical linkage, drives a cutting mechanism which will impress 

 a wax record. 



The amount of power available to operate the recorder directly from the 

 sound in the recording room is so small as to make the use of high quality 

 electrical apparatus with associated vacuum tube amplifiers of very distinct 

 advantage over the acoustic method. 



Where the question of reproduction is concerned, the same two alterna- 

 tives mentioned for recording present themselves, namely, direct use of 

 power derived from the record itself vs. the use of electro-mechanical equip- 

 ment with an amplifier. In this case, however, the situation is materially 

 different since the power which can be drawn directly from the record is more 

 than sufficient for many uses. It is, therefore, generally simpler to design 

 one single mechanical transmission system than it is to add the unnecessary 

 complications of amplifiers, power supply and associated circuits. In cases 

 where music is to be reproduced in large auditoriums, the power which can be 

 drawn from the record may be insufficient and some form of electrical repro- 

 duction using amplifiers becomes necessary. 



The paper points out, at length, how many of the heretofore unsolved 

 fundamental problems of sound recording and reproduction have been read- 

 ily solved by the application of a detailed knowledge of telephone transmis- 

 sion theorv. The advances which have been effected in telephone transmis- 

 sion theory and in related electrical measuring apparatus in the last few- 

 years, have been so great as to surpass previous knowledge of mechanical 

 wave transmission systems. The result is, therefore, that mechanical trans- 

 mission systems of the type here considered, and perhaps other types, can be 

 designed more successfully if they are viewed as the analogs of electric cir- 

 cuits. A detailed analysis is here made of the analogies between electrical 

 and mechanical systems in the voice frequency range and a discussion of the 

 resulting mechanical design is presented. 



Introduction 



THE problem with which this paper is concerned, in its broadest 

 sense, may be stated as that of taking sound from the air, storing 

 it in some permanent way and reproducing it again without appre- 

 ciable distortion. It is immaterial from the general standpoint whether 

 the means used are mechanical or electrical or a combination of the 

 two. The choice of which method to use will depend largely upon the 

 commercial requirements accompanying the specific purpose for which 

 the reproduction is being made. For instance, it is quite probable that 

 1 As printed here this paper is essentially as read before the A.I.E.E. Fel). 8-11, 1926. 



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