A CRITICAL STUDY OF THE PRINCIPLES AND PRACTICE OF MAGNETIC TAPE RECORDING A T hesis Presented to The F aculty of the Departm ent of Cinema The U n iv ersity of Southern C alifo rn ia In P a rtia l F u lfillm en t of the Requirem ents fo r the Degree of M aster of A rts by A. V. B alakrishnan June 1950 UMI Number: EP42698 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. U M T Dissertation: PwMisMng UMI EP42698 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 c.;^ *sf tins' This thesis, written by A.V.Balakrishnan rttd 3vT under the guidance of h~?..... Faculty Committee, and approved by all its members, has been presented to and accepted by the Council on Graduate Study and Research in partial fulfill­ ment of the requirements for the degree of Master of Arts Z - .................. n * .___ M -1950__________ Faculty Committee Chairman ......... __ ; TABLE OF CONTENTS CHAPTER PAGE X. The problem 1 Statem ent of the problem . 1 Im portance of the study . . . . . . . . 1 D efin itio n s of term s used .................................... 2 M agnetic tape recording system s . . . . 2 Review of the lite ra tu re ......................... 2 O rganization of the re s t of the th e sis . 6 I I . MAGNETISM—A REV IEW ......................................................... 8 B asic phenomena and d e fin itio n s . . . . . 8 M agnetization and h y ste re sis . . . . . . 16 H y steresis ............................... . . . . . . . 20 D e m a g n e tiz a tio n .............................. 24 D em agnetizing methods ' ....................................................28 M agnetic m a te ria ls .................................................... 29 S o ft m agnetic m a t e r i a l s ................................... 30 Hard m agnetic m aterials . . . . . . . . 32 M a g n e to stric tio n ....................................................................33 N ature of m agnetism ....................................................35 I I I . GENERAL CONSIDERATIONS ....................................................40 H isto ry of m agnetic tape recording . . . 40 E sse n tia ls of p resen t day m agnetic recording . ....................................................................54 A dvantages of m agnetic recording . . . . 55 \ iv CHAPTER PAGE Types of m agnetic r e c o r d in g .....................................56 A dvantages of tape over w ire ................................57 P rin c ip a l components of a ty p ic a l m agnetic tape r e c o r d e r ....................................................................59 IV. THEORY OF MAGNETIC TAPE RECORDING...........................62 Types of m agnetic tape recording and recording heads . . . . . ............................... 63 The recording process— tra n s fe r c h a ra c te ristic ......................................................... 69 Recording w ith D-C b i a s ................................................72 The A-C b ias s y s te m .........................................................76 The Toomim and W ildfeuer theory . . . . . 81 The Holmes and C lark t h e o r y .....................................85 G raphical an aly sis ..................... . . . . . . 95 Theory of reproduction .......................................... 98 E rase theory ................................... 108 V. FREQUENCY RESPONSE AND DISTORTION IN MAGNETIC TAPE RECORDING.............................................112 Frequency response .............................................. 112 D e m a g n e tiz a tio n ............................................................115 P en etratio n e ffe c t . .......................................... 117 S p e e d .............................. 118 B ias am plitude ............................................................119 R ecording head c h a ra c te ris tic s . . . . 121 V CHAPTER PAGE Reproducing head c h a ra c te ris tic s . . . 122 D isto rtio n (G e n e ra l)....................................................124 N o i s e ......................................................................................124 M easurement o f d is to rtio n ............................... 129 VI. DESIGN OF MAGNETIC TAPE RECORDERS........................136 Component design ........................................................137 The recording tape ..................................................137 The recording head . . . . . . . . . . 141 The reproducing head ........................................ 145 The erasin g head .............................................147 The d riv e system .........................148 A ccessory e le c tro n ic equipm ent . . . . 149 System d e s i g n ...................................................................150 E q u alizatio n ................................................................... 152 ■ V II. SUMMARY AND CONCLUSIONS..................................................156 Summary . ........................................................................156 C o n c lu s io n s ........................................................................158 BIBLIGGRAFHY ............................................................................................159 LIST. OF FIGURES FIGURE PAGE 1. M agnetization C u r v e .........................................................18 2. H y steresis Curve ............................. ..................................18 3. M inor L o o p s ............................................................................. 23 4. D em agnetization ...................... . . . . . . . . . 23 5. S im p lified Block Diagram ......................................... 61 6. M agnetic Heads ..........................................................................71 7. D-C B ias T ran sfer C h a ra c te ristic . . . . . 71 8 . D-C B ias; D isto rtio n . . . . . . . . . . . 71 9. A-C B ias T ran sfer C h a ra c te ristic ............................80 10. A ction of the A-C B ias on the Toomim and V Jildfeuer Theory ..........................................................80 11. A ction of th e A-C B ias on the Holmes and C lark Theory ............................................. 87 12. D eriv atio n of T ran sfer C h a ra c te ristic . . . 87 13. F ield D istrib u tio n About A Recorded Tape . 102 14. Diagram Illu s tra tin g Scanning Loss . . . . 102 15. E ffe c t of Speed on*Frequency Response . . 114 16. D em agnetizing E ffe ct During Recording and P la y - b a c k ......................... 116 17. E ffe c t of B ias on Frequency Response . . • 120 18. Noise S pectra . 127 19. O utput vs. B ias .........................................127 FIGURE PAGE 20. D isto rtio n As A Function of Output For V arious B ias C urrents ..........................................132 21. Output v s. B ias a t C onstant Input . . . . . 133 22. A m plifier Frequency Response of M agnetic Tape R ecorders ................................................... 154 23. O verall Frequency Response of M agnetic Tape R ecorders ..............................................................155 CHAPTER I I . THE PROBLEM. M agnetic Tape Recording Systems of the p resen t day d eriv e th e ir su p erio r perform ance from developm ents made during the w ar, although the basic p rin c ip le s of record­ ing sound on ferrom agnetic media have been known fo r mare than a h a lf-c e n tu ry . These developm ents were th e re s u lt m ostly of advanced engineering methods evolved under the d u ress of w ar, and raised the sta tu s of M agnetic Tape R ecording from re la tiv e o b scu rity to one of pre-em inence in th e recording fie ld . Combined w ith th e ir in h eren t advantages, the higher q u a lity obtainable from M agnetic Tape R ecording Systems has ra p id ly led to th e ir ever in creasin g adoption. Statem ent of the problem . I t is the purpose of th is th e s is to p resen t a c r itic a l study o f the p rin c ip le s and p ra c tic e of M agnetic Tape Recording System s of the p resen t day. Im portance of the stu d y . As a p o te n tia l recording m ethod, which according to some may ev en tu ally even d isp lace o th er form s of reco rd in g , the need fo r a study of M agnetic Tape Recording cannot be m inim ized. Owing, however, to its rec en t developm ent, no com prehensive study of i t s th eo ry and p ra c tic e is re a d ily a v aila b le a t the p resen t tim e. Data on M agnetic Tape Recording are spread over a re la tiv e ly larg e span of tim e (from 1900 to the p re s e n t), and are not co -o rd in ated to any d e sira b le e x te n t. The p resen t study was undertaken in an attem pt to f u l f i l l th a t need. A system atic a n aly sis of the p rin c ip le s of M agnetic Tape Recording has been developed, assum ing l i t t l e or no previous knowledge of the su b ject on th e p a rt of the rea d er; in a d d itio n , c u rren t design tren d s have been in d ic ate d . I I . DBFIKITIOKS CF TERMS USED M agnetic Tape R ecording System s; By M agnetic Tape R ecorders is meant a recording system which employs a ferrom agnetic tape or ribbon, having a w idth g re a te r than approxim ately te n tim es the th ick n ess. The tape may be homogeneous or coated. Only p ro fessio n al equipm ent—where q u a lity is the prim e c rite rio n and co st is of secondary im portance— is considered, and a ll statem ents made, u nless otherw ise sp e c ifie d , are based on th is view point. I I I . REVIEW OP THE LITERATURE The e a rlie s t known referen ce to the p o s s ib ility of