A critical study of the principles and practice of magnetic tape recording

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A CRITICAL STUDY OF THE PRINCIPLES AND PRACTICE OF MAGNETIC TAPE RECORDING

A T h e s is P re s e n te d to T he F a c u l t y o f t h e D e p a r tm e n t o f C inem a The U n i v e r s i t y o f S o u t h e r n C a l i f o r n i a

In P a r t ia l F u lfillm e n t o f t h e R e q u ir e m e n t s f o r t h e D e g re e o f M a s te r o f A r ts

by A . V. B a l a k r i s h n a n Ju n e 1950

UMI Number: EP42698

All rights reserved INFO RM A TIO N 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

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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

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c.;^ *sf tin s' This thesis, written by

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A.V.Balakrishnan 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 -1 950__________

Faculty Committee

Chairman

......... __

;

3vT

TABLE OF CONTENTS CHAPTER X.

PAGE T he p r o b le m

1

S t a t e m e n t o f t h e p r o b le m Im p o rta n c e o f th e s tu d y D e fin itio n s

o f te rm s u s e d

M a g n e tic t a p e r e c o r d i n g R e v ie w o f t h e

lite ra tu re

. . . . . . . . .

1

....................................

2

s y s te m s

. . . .

2

.........................

2

O rg a n iz a tio n o f th e r e s t o f th e t h e s i s II.

1

.

6

MAGNETISM— A R E V I E W .........................................................

8

B a s ic phenom ena and d e f i n i t i o n s

. . . . .

8

M a g n e tiz a tio n an d h y s t e r e s i s

. . . . . .

16

...............................

. . . . . . .

20

H y s te re s is

D e m a g n e t i z a t i o n ..............................

24

D e m a g n e ti z i n g m e th o d s ' .................................................... 2 8 M a g n e tic m a t e r i a l s

....................................................

29

S o f t m a g n e t i c m a t e r i a l s ...................................

30

H a rd m a g n e t i c m a t e r i a l s

32

. . . . . . . .

M a g n e t o s t r i c t i o n ....................................................................3 3

III.

N a t u r e o f m a g n e tis m

.................................................... 3 5

GENERAL CONSIDERATIONS

.................................................... 4 0

H is to ry

o f m a g n e ti c t a p e r e c o r d i n g

E s s e n tia ls re c o rd in g

. . .

40

o f p r e s e n t d a y m a g n e ti c .

.................................................................... 5 4

A d v a n ta g e s o f m a g n e tic r e c o r d i n g

. . . .

\

55

iv CHAPTER

PAGE T y p e s o f m a g n e t i c r e c o r d i n g ..................................... 56 A d v a n ta g e s o f t a p e

o v e r w ire

................................ 57

P r i n c i p a l c o m p o n e n ts o f a t y p i c a l m a g n e t i c t a p e r e c o r d e r ....................................................................59 IV .

THEORY OF MAGNETIC TAPE R E C O R D IN G ........................... 62 T y p es o f m a g n e tic t a p e r e c o r d i n g a n d re c o rd in g h ead s

. . . . .

...............................

63

T he r e c o r d i n g p r o c e s s — t r a n s f e r c h a ra c te ris tic

.........................................................

69

R e c o r d i n g w i t h D-C b i a s ................................................72 T he A-C b i a s s y s t e m ......................................................... 7 6 The Toomim a n d W i l d f e u e r t h e o r y

. . . . .

81

T he H o lm e s a n d C l a r k t h e o r y ..................................... 85 G ra p h ic a l a n a ly s is

.....................

T h e o ry o f r e p r o d u c tio n

..........................................

...................................

E ra se th e o ry V.

. . . . . .

95 98 108

FREQUENCY RESPONSE AND DISTORTION IN MAGNETIC TAPE R E C O R D IN G .............................................1 1 2 F re q u e n c y re s p o n se

..............................................

112

D e m a g n e t i z a t i o n ............................................................ 1 1 5 P e n e tra tio n e f f e c t

.

.......................................... 1 1 7

S p e e d .............................. B ia s a m p litu d e

118

............................................................ 1 1 9

R e c o rd in g h e ad c h a r a c t e r i s t i c s

. . . .

121

V CHAPTER

PAGE R e p ro d u c in g h e a d c h a r a c t e r i s t i c s D is to rtio n

.

.

. 122

( G e n e r a l ) ....................................................124

N o i s e ......................................................................................1 2 4 M e a s u re m e n t o f d i s t o r t i o n V I.

...............................

129

DESIGN OF MAGNETIC TAPE R E C O R D E R S ........................ 13 6 C o m p o n en t d e s i g n

........................................................ 1 3 7

T he r e c o r d i n g t a p e

..................................................13 7

The r e c o r d i n g h e a d

. . . . . . . . . .

141

T he r e p r o d u c i n g h e a d

........................................

The e r a s i n g h e a d

............................................. 1 4 7

The d r i v e s y s t e m

.........................14 8

A c c e s s o r y e l e c t r o n i c e q u ip m e n t

. . . .

145

149

S y s te m d e s i g n ...................................................................1 5 0 E q u a liz a tio n ■V I I .

................................................................... 15 2

SUMMARY AND C O N C L U S IO N S .................................................. 1 5 6 Sum m ary

.

........................................................................1 5 6

C o n c l u s i o n s ........................................................................1 5 8 BIBLIGGRAFHY

............................................................................................ 1 5 9

L IST . OF FIGURES FIGURE 1. 2.

PAGE M a g n e t i z a t i o n C u r v e ......................................................... 1 8 H y s t e r e s i s C u rv e

............................ .

.................................. 18

3.

M in o r L o o p s .............................................................................

23

4.

D e m a g n e tiz a tio n

. . . . . . . . .

23

5.

S i m p l i f i e d B lo c k D ia g ra m

.........................................

61

6.

M a g n e tic H e a d s

7.

D-C B i a s T r a n s f e r C h a r a c t e r i s t i c

8.

D-C B i a s ;

9.

A-C B i a s T r a n s f e r C h a r a c t e r i s t i c

10.

......................

.......................................................................... 7 1

D is to rtio n

. . . . .

71

. . . . . . . . . . .

71

A c t i o n o f t h e A-C B i a s o n

t h e Toomim a n d

..........................................................8 0

V J ild fe u e r T h e o ry 11.

............................ 8 0

A c t i o n o f t h e A-C B i a s o n C la r k T h eo ry

t h e H o lm es a n d .............................................

12.

D e riv a tio n o f T ra n s fe r C h a r a c te r is tic

13.

F i e l d D i s t r i b u t i o n A b o u t A R e c o rd e d T ape

14.

D ia g ra m I l l u s t r a t i n g

15. 16.

S c a n n in g L o s s

87

. . .

87 .

102

. . . .

E f f e c t o f S p eed o n * F re q u en c y R esp o n se

.

102 . 114

D e m a g n e ti z i n g E f f e c t D u r in g R e c o r d i n g a n d P l a y - b a c k .........................

116

17.

E f f e c t o f B i a s on F r e q u e n c y R e s p o n s e

18.

N o is e S p e c t r a

19.

O u tp u t v s . B ia s

.

.

•

12 0

. 127 ......................................... 1 2 7

FIGURE 20.

PAGE D i s t o r t i o n A s A F u n c t i o n o f O u tp u t F o r V a rio u s B ia s C u r r e n ts

..........................................132

21.

O u tp u t v s . B i a s a t C o n s t a n t I n p u t

22.

A m p l i f i e r F r e q u e n c y R e s p o n s e o f M a g n e tic T ape R e c o rd e rs

23.

. . . . .

.............................................. .....

13 3

154

O v e r a l l F r e q u e n c y R e s p o n s e o f M a g n e tic T ap e R e c o r d e r s

..............................................................1 5 5

CHAPTER I I .

THE PROBLEM.

M a g n e tic T a p e R e c o r d i n g S y s te m s o f t h e p r e s e n t d a y d e riv e

t h e i r s u p e r i o r p e r f o r m a n c e f r o m d e v e l o p m e n t s m ade

d u rin g th e w a r, a lth o u g h th e b a s ic p r i n c i p l e s o f re c o r d ­ i n g s o u n d o n f e r r o m a g n e t i c m e d ia h a v e b e e n kno w n f o r m are th a n a h a lf - c e n tu r y .

T h e s e d e v e l o p m e n t s w e re t h e r e s u l t

m o s t l y o f a d v a n c e d e n g i n e e r i n g m e th o d s e v o l v e d u n d e r t h e d u r e s s o f w a r, and r a is e d th e s t a t u s

o f M a g n e tic T ap e

R e c o rd in g fro m r e l a t i v e

one o f p re -e m in e n c e

in th e re c o rd in g f i e l d .

o b s c u rity to

C o m b in ed w i t h t h e i r i n h e r e n t

a d v a n ta g e s , th e h ig h e r q u a l i t y o b ta in a b le

f r o m M a g n e tic

T a p e R e c o r d i n g S y s te m s h a s r a p i d l y l e d t o t h e i r e v e r in c r e a s in g a d o p tio n . S ta te m e n t o f th e th is

p ro b le m .

It

is

th e p u rp o s e o f

t h e s i s to p r e s e n t a c r i t i c a l s tu d y o f th e p r i n c i p l e s

and p r a c t ic e

o f M a g n e tic T ap e R e c o r d i n g S y s te m s o f t h e

p re se n t day. Im p o rta n c e o f t h e s t u d y .

As a p o t e n t i a l r e c o r d i n g

m e t h o d , w h ic h a c c o r d i n g t o som e m ay e v e n t u a l l y e v e n d i s p l a c e o t h e r fo rm s o f r e c o r d i n g , th e n e e d f o r a s tu d y o f M a g n e tic T ape R e c o rd in g c a n n o t be m in im iz e d .

O w in g , h o w e v e r , t o

its

r e c e n t d e v e l o p m e n t , n o c o m p r e h e n s iv e s t u d y o f i t s

th e o r y and p r a c tic e tim e .

i s r e a d i l y a v a i la b l e a t th e p r e s e n t

D a ta on M a g n e tic T ape R e c o r d i n g a r e s p r e a d o v e r a

re la tiv e ly

l a r g e s p a n o f tim e

(fro m 1900 to th e p r e s e n t ) ,

and a re n o t c o -o r d in a te d to any d e s ir a b le

e x te n t.

The

p r e s e n t s t u d y was u n d e r t a k e n i n a n a t t e m p t t o f u l f i l l t h a t need.

A s y s t e m a t i c a n a l y s i s o f t h e p r i n c i p l e s o f M a g n e tic

T ap e R e c o r d i n g h a s b e e n d e v e l o p e d , a s s u m in g l i t t l e

o r no

p r e v i o u s k n o w le d g e o f t h e s u b j e c t on t h e p a r t o f t h e re a d e r;

in a d d itio n , c u r r e n t d e s ig n tr e n d s have b een

in d ic a te d . II.

DBFIKITIOKS CF TERMS USED

M a g n e tic T a p e R e c o r d i n g S y s te m s ;

B y M a g n e tic T ap e

R e c o r d e r s i s m e a n t a r e c o r d i n g s y s t e m w h ic h e m p lo y s a f e r r o m a g n e tic ta p e o r r ib b o n , h a v in g a w id th g r e a t e r th a n a p p ro x im a te ly t e n tim e s th e t h i c k n e s s .

T he t a p e m ay b e

hom ogeneous o r c o a te d . O n ly p r o f e s s i o n a l e q u ip m e n t — w h e re q u a l i t y i s t h e p r im e c r i t e r i o n a n d c o s t i s

o f s e c o n d a ry im p o rta n c e — i s

c o n s id e r e d , an d a l l s ta te m e n ts m ade, u n le s s o th e rw is e s p e c i f i e d , a r e b a se d on t h i s v ie w p o in t. III.

REVIEW OP THE LITERATURE

T he e a r l i e s t know n r e f e r e n c e t o t h e p o s s i b i l i t y

of

m a g n e tic r e c o r d i n g o c c u r s i n t h e S e p te m b e r, 1 8 8 8 , i s s u e th e

S c i e n t i f i c A m e ric a n ♦

c lo s e d i n th e

of

The o r i g i n a l i n v e n t i o n i s d i s ­

f i r s t A m e ric a n p a t e n t o f t h e s a m e , a n d h a s

b e e n d e s c r i b e d b y P o u ls e n h im s e lf i n t h e A n n a le n d e r P h y s ik . 2

F o r d a ta

e a rly t h i r t i e s , P a te n t F i l e s .

th e

on t h e

s u b s e q u e n t d e v e lo p m e n ts u p t o

o n ly s o u r c e o f in f o r m a tio n i s

I m p o r t a n t among t h e s e a r e

th e

th e U. S.

th e l o n g i t u d i n a l

r e c o r d i n g m e t h o d ,2 t h e a - c b i a s s y s t e m , ^ a n d p o w d e r - c o a t e d ta p e s . 5 M o s t o f t h e p r e - w a r w o r k on M a g n e tic T ap e R e c o r d i n g w a s d o n e i n G e rm a n y , a n d t h e f i r s t p u b l i s h e d t h e o r e t i c a l a n a l y s i s o f M a g n e t i c T ap e R e c o r d i n g u s i n g r i n g h e a d s i s d u e t o H . v o n L t i b e c k .2

A M a g n e tic T ap e R e c o r d e r u s i n g

v i c a l l o y t a p e , d e v e l o p e d b y t h e B e l l T e le p h o n e L a b o r a t o r i e s

O b e r l i n S m i t h , “ Some P o s s i b l e Form o f P h o n o g r a p h 11, E l e c t r i c a l W o rld , S e p t e m b e r 8 , 1 8 8 8 , p p . 1 1 6 - 1 1 7 . 2 V. P o u l s e n , “D a s T e l e g r a p h a n " , A n n a l e n d e r P h y s i k .1 V o l. 3 , 1 9 0 0 , p p . 7 5 4 -7 6 0 . 2 P . O. P e d e r s o n a n d V . P o u l s e n , U . S . P a t e n t H o. 8 7 3 , 0 7 8 , D e c e m b e r 1 0 , -1907. ^ W. L . C a r l s o n a n d G. W. C a r p e n t e r , U . S . P a t e n t H o. 1 , 6 4 0 , 8 8 1 , A u g u s t , 1 9 2 7 . 2 P f a n h a u s e r , U . S . P a t e n t H o . 1 , 7 5 8 , 5 3 1 , M ay, 1 9 3 0 . 2 H . v o n L tib e c k , “ M a g n e tic S o u n d R e c o r d i n g W ith F i l m a n d R i n g - s h a p e d H e a d s 11, A k u s t i s c h e Z i e t a c h r i f t . V o l . 2 , N o v e m b e r, 1 9 3 7 , p p . 2 7 3 - 2 9 5 .

is

d e s c r i b e d b y C . N. H ic k m a n ,1? a n d a B r i t i s h d e s i g n e d

s y s te m u s e d b y th e B r i t i s h B r o a d c a s tin g C o r p o r a tio n h a s b e e n d e s c r i b e d i n d e t a i l b y B a r r e t a n d T w e e d .^ Of t h e w o rk d o n e d u r i n g t h e w a r , r e l a t i v e l y fe w a c c o u n ts a r e a v a i l a b l e , and th e s e h av e b e e n p u b lis h e d a s O ffic e

o f S c i e n t i f i c R e s e a r c h a n d D e v e lo p m e n t R e p o r t s

t h e U . S . D e p a rtm e n t o f

C om m erce.®

W h ile t h e

by

m a jo r p a r t

o f t h e d e v e l o p m e n t a l w o rk o n m o d e rn M a g n e t i c T ap e R e c o r d ­ i n g S y s te m s w a s d o n e i n

G e rm a n y , t h e o n l y e x t a n t d a t a o n

it

th o s e re p o rte d by th e B r i t i s h

o f a n y ' m a g n itu d e a r e

In te llig e n c e p a te n t on th e

O b je c tiv e s S u b - c o m m i t t e e .A l t h o u g h a a - c b i a s a n d e r a s e s y s te m s f o r u s e w i t h a

T a p e R e c o r d e r w a s g r a n t e d t o D . E . W o o ld r id g e i n 1 9 4 1 , ^ th e g r e a te r p a r t o f th e a c t i v i t i e s

in th is

c o u n tr y w as

^ C . N . H ic k m a n , " S o u n d R e c o r d i n g o n M a g n e t i c 'T a p e " , B e l l S y s te m T e c h n i c a l J o u r n a l . V o l . 1 6 , A p r i l , 1 9 3 7 , p p . 1 6 5 -1 7 7 . “ ® A . E . B a r r e t t a n d T w ee d , "Some A s p e c t s o f M a g n e tic R e c o rd in g an d I t s A p p l i c a ti o n t o B r o a d c a s tin g " , J o u r n a l o f t h e I n s t i t u t e o f E l e c t r i c a l E n g i n e e r s . ( L o n d o n ;, V o l . 8 2 , M a rch , 1 9 3 8 , p p . 2 6 5 -2 8 8 . 9 " B i b l i o g r a p h y o f R e p o r t s o n M a g n e tic W ire a n d T ap e R e c o r d e r s , " O f f i c e o f T e c h n i c a l S e r v i c e s , U . S . D e p a r t ­ m e n t o f C om m erce ( W a s h i n g t o n , D . C . ) , O c t o b e r , 1 9 4 7 . M. J . L . P u l l i n g , E . M. P a y n e a n d H . E . P a r k e r , ( B r i t i s h I n t e l l i g e n c e O b j e c t i v e s S u b - c o m m itte e R e p o r t ) , " T h e M a g n e to p h o n S o u n d R e c o r d i n g a n d R e p r o d u c i n g S y s t e m " , (PB H o . 6 0 8 9 9 , D e p a r t m e n t o f C o m m e rc e ). 11 D . E . W o o ld rid g e , U . S . P a t e n t H o. 2 ,2 3 5 ,1 3 2 , M a rch , 1 9 4 1 .

d ire c te d

t o d e v e l o p i n g w i r e r e c o r d e r s . 12

W ith t h e e n d o f t h e w a r a n d w i t h t h e

in c re a s e d

i n t e r e s t i n t h e u s e o f T ape R e c o r d e rs , s e v e r a l a r t i c l e s on th e

t h e o r y o f T ap e R e c o r d i n g w i t h t h e a - c b i a s s y s t e m

a p p e a re d .

Among t h e s e a r e t h e c l a s s i c

p a p e r s o f Toomim

a n d W i l d f e u e r , 1 ** H o lm e s a n d C l a r k , 1 4 a n d W. W. W e t z e l . 16 The s u b j e c t o f n o i s e h a s b e e n c o n s id e r e d i n d e t a i l b y L . C . H o lm e s ,1 6 a n d J . W. G r a t i a n , 1? w h i l e t h e f a c t o r s a f f e c tin g fre q u e n c y re sp o n se and d i s t o r t i o n have b e en

12 “ I n v a s i o n R e c o r d e r 11, G e n e r a l E l e c t r i c R e v ie w * V o l . 4 7 , H o. 7 , J u l y , 1 9 4 4 , p p . 4 4 - 4 5 . 12 H . Toomim a n d D . W i l d f e u e r , “ T he M e c h a n ism o f S u p e r s o n i c F r e q u e n c i e s a s A p p l i e d t o M a g n e tic R e c o rd in g * * , P r o c e e d i n e s o f t h e I . R . E . . V o l . 3 2 . N o. 1 1 , N o v e m b e r, 1944,

PP-

6 6 5 = 6 5 5 7 --------------

14 L . C . H o lm e s a n d D . L . C l a r k , " S u p e r s o n i c B i a s f o r M a g n e tic R e c o r d i n g 1*, E l e c t r o n i c s , V o l . 1 8 , J u l y , 1 9 4 5 , p p . 1 2 6 -1 3 6 . 15 W. W. W e t z e l , “ R e v ie w o f t h e P r e s e n t S t a t u s o f M a g n e t i c R e c o r d i n g T h e o r y 1' , P a r t I , A u d io E n g i n e e r i n g , V o l . 3 1 , N o v em b er 1 9 4 7 , p . 14 e t s e q . ; P a r t I I , A u d io E n g i n e e r i n g . V o l. 3 1 , D e c e m b e r, 1 9 4 7 , p . 12 e t s e q . ; P a r t I I I , A u d io E n g i n e e r i n g . V o l . 3 2 , J a n u a r y , 1 9 4 8 , p . 26 e t s e q . 1 6 L . C . H o lm e s . "Som e F a c t o r s A f f e c t i n g t h e C h o ic e o f a M edium f o r M a g n e tic R e c o r d i n g " , J o u r n a l o f t h e A c o u s t i c S o c i e t y o f A m e r i c a , V o l . 1 9 , N o . 3 , M ay, 1 9 4 7 . P p . 3 9 5 - 4 0 2 . ^ J . W. G r a t i a n , " N o is e i n M a g n e tic R e c o r d i n g S y s te m s " , J o u r n a l o f th e A c o u s tic S o c i e t y o f A m e ric a , V o l . 2 1 , N o. 2 , M a r c h , 1 9 4 9 . P p . 7 4 - 8 0 .

d i s c u s s e d b y 0 . K o m e i , 18 L . C . H o lm e s19 a n d o t h e r s . D e s i g n d a t a h a v e b e e n g i v e n b y S . J . B e g u n ,2 0 H . L i n d s a y a n d M. S t o l a r o f f , 2 1 a n d M. R e t t i n g e r , 22 am ong o t h e r s . IV .

ORGANIZATION OF THE REST CF THE THESIS

S i n c e a k n o w le d g e o f t h e b a s i c a re

in d is p e n s a b le

re v ie w

p h e n o m e n a o f m a g n e tis m

i n a S t u d y o f M a g n e tic R e c o r d i n g , a

o f t h o s e a s p e c t s o f m a g n e tis m p e r t i n e n t t o m a g n e t i c

re c o rd in g th e o ry i s

p re s e n te d

g e n e r a l c o n s id e ra tio n s

i n C h a p te r I I .

in tr o d u c to r y to th e

T ap e R e c o r d i n g , s u c h a s i t s

h is to ry ,

its

C e rta in

s t u d y o f M a g n e tic

a d v a n ta g e s , t o ­

g e t h e r w ith a b re a k -d o w n i n b lo c k s c h e m a tic fo rm o f p r e s e n t d a y M a g n e tic T a p e R e c o r d e r s a r e n e x t o u t l i n e d .

T he t h e o r y

p r o p e r o f M a g n e t i c T ap e R e c o r d i n g i s t a k e n u p i n C h a p t e r IV , fo llo w e d b y a c o n s i d e r a t i o n o f th e f a c t o r s a f f e c t i n g

18 0 . K o r n e i , ‘’F r e q u e n c y R e s p o n s e o f M a g n e t i c R e c o r d ­ i n g ” , E l e c t r o n i c s . V o l. 2 0 , A u g u s t, 1 9 4 7 , p . 124 e t s e q . 1 9 L . C . H o lm e s , ’’T e c h n iq u e s f o r I m p r o v e d M a g n e tic R e c o r d i n g " , E l e c t r i c a l E n g i n e e r i n g . V o l . 6 8 , N o. 1 0 , O c to b e r, 1 9 4 9 , p p . 8 3 6 -8 4 1 . 2 0 S . J . B e g u n , M a g n e tic R e c o r d i n g (New Y o rk : M u rra y H i l l B o o k s, I n c . , 1 9 4 9 ). 2 1 H . L i n d s a y a n d M. S t o l a r o f f , " M a g n e tic T a p e R e c o r d e r o f B r o a d c a s t Q u a l i t y " , A u d io E n g i n e e r i n g . V o l . 3 2 , N o. 1 0 , O c to b e r, 1 9 4 8 , p p . 1 3 -1 6 . 2 2 M. R e t t i n g e r , " M a g n e tic R e c o r d i n g i n M o tio n P i c t u r e s " , A u d io E n g i n e e r i n g . V o l . 3 4 , N o. 3 , M a rc h , 1 9 5 0 , p . 9 e t seq.

q u a lity

i n C h a p te r V.

p r i n c i p l e s g o v e rn in g

C h a p te r VI i s d e v o te d t o th e t h e d e s i g n o f M a g n e tic T a p e R e c o r d e r s ,

t o g e t h e r w i t h s a m p le d e s i g n d a t a . ta in s

t h e su m m ary a n d c o n c l u s i o n s .

The f i n a l c h a p t e r c o n ­

CHAPTER I I MAGNETISM--A REVIEW I.

BASIC PHENOMENA AND D EFIN ITIO N S1

M a g n e tic p r o p e r t i e s o f c e r t a i n m a t e r i a l s h a v e b e e n o b s e r v e d f o r c e n t u r i e s , c h i e f am ong w h ic h i s

th e a t t r a c t i v e

f o r c e t h a t e x i s t s b e tw e e n a n a t u r a l m ag n e t ( l o d e s t o n e — m a g n e t i t e , FegO^j.) , o r a n a r t i f i c i a l m a g n e t ( a " p e r m a n e n t m a g n e t" ) a n d a p i e c e o f s o f t i r o n . a ttra c te d

Iro n filin g s

to w a r d a m a g n e t , m o s t s t r o n g l y i n t h e

a re re g io n

n e a r e i t h e r e n d j t h e s e a r e a s a lw a y s o c c u r i n p a i r s a n d a re re fe rre d

to a s th e " p o le s " .

T he f o l l o w i n g b o o k s h a v e s e r v e d a s g e n e r a l re fe re n c e s f o r t h i s c h a p te r: L . F . B a t e s , M o d e rn M a g n e tis m (L o n d o n : U n iv e r s ity P r e s s , 1 9 3 9 ). L td .,

E . C . S t o n e r , M a g n e tis m (L o n d o n ; 1 9 4 6 ).

F . B r a i l s f o r d , M a g n e tic M a t e r i a l s M e th u e n a n d C o . , L t d . , 1 9 4 8 ) .

C a m b rid g e

M e th u e n a n d C o . , (L o n d o n ;

S. R . W i l l i a m s , M a g n e tic P h e n o m en a (New Y o r k : M c G r a w s H ill B ook C o . , I n c . , 1 9 3 lJ"" T . H . S p o o n e r , P r o p e r t i e s a n d T e s t i n g o f M a g n e tic M a t e r i a l s (New Y o r k : M c G ra w -H ill B o o k C o . , I n c . , 1 9 2 7 ) . H . C. R o t e r s , E l e c t r o m a g n e t i c D e v i c e s J o h n ¥ /ile y an d S o n s, I n c . , 1941) .

(New Y o rk ;

W hen a p e r m a n e n t m a g n e t ( e . g . , s o t h a t i t m ay f r e e l y r o t a t e

a b a r m a g n e t)

i s su sp en d ed

in a h o r iz o n ta l p la n e ,

its

a x i s co m e s t o r e s t r o u g h l y a l o n g a n o r t h - s o u t h l i n e , a n d we s p e a k o f t h e n o r t h - s e e k i n g s e e k in g o r s o u th p o le T he e a r t h tw o s u c h p o l e s ,

o r n o r th p o le , and s o u th -

o f t h e m a g n e t.

its e lf

is

in e f f e c t a la r g e m agnet w ith

one i n th e

r e g io n o f e a c h end o f th e a x is

o n w h ic h th e e a r t h r o t a t e s . It

i s a n e x p e rim e n ta l f a c t t h a t lik e p o le s r e p e l

a n d u n l i k e p o l e s a t t r a c t e a c h o t h e r w i t h f o r c e s w h ic h obey a n in v e r s e

s q u a re la w .

T h is l e a d s t o t h e d e f i n i t i o n

o f a u n i t p o l e a s o n e w h ic h w i l l r e p e l a n e q u a l a n d s i m i l a r p o l e p l a c e d 1 cm . aw ay i n v a c u o w i t h a f o r c e o f o n e d y n e . The '’p o l e s t r e n g t h "

o f a m agnet i s ,

t h e r e f o r e , m ea su re d

b y t h e n u m b e r o f u n i t p o l e s t o w h ic h e a c h p o l e i s e q u iv a le n t. A m ag n e t w i l l a t t r a c t a p ie c e o f i r o n e v e n th o u g h th e

tw o a r e n o t i n p h y s i c a l c o n t a c t , a n d t h i s a c t i o n - a t -

a -d is ta n c e th e

is

s a id to be cau sed by th e f i e l d

" m a g n e t i c " f i e l d — a r o u n d t h e m a g n e t.

of fo rc e —

I f we c o u l d p l a c e

a u n i t p o le a t a f i x e d p o i n t i n a m a g n e tic f i e l d

(in

v a c u o ) , i t w o u ld b e a c t e d

o n b y a f o r c e w h ic h w o u ld b e a

m ea su re o f th e

o r " in te n s ity "

" s tre n g th "

o f th e f i e l d ,

a n d u n i t i n t e n s i t y e x i s t s a t t h e p o i n t w h en t h e f o r c e th e u n i t p o le

is

one d y n e .

T h is u n i t i s

c a lle d

th e

on

10 “ o e rs te d " . A m a g n e tic f i e l d

c a n a l s o be d e s c r i b e d i n te rm s o f

th e f a m ilia r co n cep t o f " p o te n tia l" .

The w o rk d o n e i n

t a k i n g a u n i t n o r t h p o le fro m a f i x e d

p o in t B to a fix e d

p o in t A i s A and B .

know n a s t h e When B i s

" p o t e n t i a l d i f f e r e n c e " b e tw e e n

lo c a te d a t i n f i n i t y ,

b r i n g in g th e u n i t n o r t h p o le t o A i s " p o te n tia l"

te r m e d

s i m p ly t h e

a t t h e p o i n t A , f o r we m ay n e g l e c t m a g n e t i c

fo rc e s a t an in f in ite

d i s t a n c e fro m m a g n e tiz e d b o d i e s .

T he d i r e c t i o n i n w h ic h a s i r g l e a t a p o i n t i n a m a g n e tic f i e l d know n a s t h e d i r e c t i o n p o in t.

t h e w o rk d o n e i n

U n it i n t e n s i t y

is

s a id

w o u ld b e u r g e d t o move i s

o f th e " l i n e

I n a " u n ifo rm " f i e l d ,

n o r th p o le p la c e d

of fo rc e " a t th a t

su ch lin e s a re

p a ra lle l.

t o e x i s t a t a p o i n t w h en o n e l i n e

o f f o r c e p a s s e s th ro u g h a u n i t a r e a p la c e d p e r p e n d ic u la r to th e

lin e s

o f f o r c e a t t h e p o i n t , a n d t h u s t h e m a g n e ti c

i n te n s ity a t a p o in t in any f i e l d of lin e s cm .

c a n b e d e f i n e d i n te rm s

o f f o r c e p e r s q . c m ., o r s im p ly l i n e s

T hese l i n e s

p er sq .

o f f o r c e m ay b e d r a w n w i t h t h e a i d

s m a ll co m p ass n e e d le , th e m s e lv e s a lo n g th e

of a

o r b y u s i n g i r o n f i l i n g s w h ic h l i n e lin e s

of fo rc e .

o f one cm . f r o m a u n i t p o l e t h e

S in c e a t a d i s t a n c e

in te n s ity

is

one o e r s te d ,

o v e r a s u r f a c e o f a r e a 4 T T e m s .2 , t h e t o t a l n u m b e r o f l i n e s o f f o r c e t h a t l e a v e a u n i t n o r t h p o l e i s 4TT . I f a m ag n et o f p o le s t r e n g t h m i s p la c e d i n a

11 u n ifo rm f i e l d p o le s

o f s t r e n g th H w ith th e l in e

jo in in g

th e

( t h e ’’m a g n e t i c a x i s ” ) m a k in g a n a n g l e ® w i t h t h e

f i e l d , t h e m om ent o f t h e c o u p l e a c t i n g mL s i n e ® , w h e re L i s t h e

on th e m ag n et i s

d i s t a n c e b e tw e e n t h e p o l e s .

The m axim um c o u p l e i s t h u s mL a n d i s c a l l e d t h e m a g n e t i c ‘’m o m en t” o f t h e m a g n e t , a n d i s m e a s u r e d i n e r g s p e r o e r s t e d . I n o r d e r t o d e s c r i b e th e m a g n e tic p r o p e r t i e s m a te ria ls ,

of

one m u s t h a v e a q u a n t i t a t i v e m e a s u r e o f t h e

e x te n t o f m a g n e tiz a tio n . o f m a g n e tiz a tio n ” .

S u ch a m e a su re i s t h e " i n t e n s i t y

T h is i s d e f i n e d a s

p e r u n i t v o lu m e , a n d i s

t h e m a g n e t i c m om ent

a v e c to r q u a n tity .

In a v e ry

lo n g , u n if o r m ly m a g n e tiz e d r o d , th e p o le s c a n b e a p p r o x i ­ m a te d t o b e a t t h e e n d s , e a c h e n d b e i n g c o v e r e d w i t h m u n it p o le s .

The i n t e n s i t y

d i v i d e d b y LA , w h e re L i s th e is

o f m a g n e tiz a tio n i s th e

c r o s s - s e c tio n j h e n c e , th e th e p o le

le n g th o f th e m agnet and A in te n s ity

s tre n g th p e r u n it a re a .

th e p o le s a re n e v e r lo c a te d a t

t h e n mL

o f m a g n e tiz a tio n

I n p r a c t i c e , h o w e v er,

th e e n d s , an d th e e q u i v a l e n t

m a g n e t i c l e n g t h i s a lw a y s l e s s t h a n i t s g e o m e t r i c l e n g t h . A " m a g n e tic s h e l l "

is a th in sh e e t of m a te ria l

u n if o r m ly m a g n e tiz e d i n a d i r e c t i o n n o rm a l t o i t s

su rfa c e ,

and th e s t r e n g t h o f th e s h e l l a t a n y p o i n t on i t s

su rfa c e

i s d e f in e d a s th e p r o d u c t o f th e i n t e n s i t y and th e s h e ll is

o f m a g n e tiz a tio n

th ic k n e s s o f th e s h e l l a t t h a t p o i n t . one i n w h ic h t h i s

p ro d u c t i s th e

A u n ifo rm

sam e a l l o v e r t h e

12 s h e ll. The m a g n e t i c f i e l d

fo r its

d e f i n i ti o n re q u ire d an

i s o l a t e d p o le to he p la c e d i n v a c u o . th e

fie ld

I n o rd e r to

i n a n y m a t e r i a l m e d iu m , a n e n c l o s u r e

h a s t o b e m ade t o h o u s e t h e p o l e . h o w e v er, b r in g s

d e te rm in e

o f som e t y p e

Such an e n c lo s u re ,

in f u r t h e r c o m p lic a tio n s .

I t is

a know n

f a c t t h a t o n c u t t i n g a m a g n e t, new m a g n e t i c p o l e s a p p e a r on th e s u r f a c e s c r e a te d b y c u t t i h g . fie ld

In o rd e r to

d e fin e th e

i n s i d e a m a t e r i a l m e d iu m , c o n s i d e r a l o n g c y l i n d e r

o f m a g n e tic m a t e r i a l , s o lo n g t h a t a n y f r e e p o l e s g e n e r a t e d a t t h e e n d s d o n o t p ro d u c e a n y m e a s u ra b le c h a n g e i n th e f i e l d i n t h e m id d l e p o r t i o n s fie ld

a c tin g

o f th e c y li n d e r .

i n t h e a b s e n c e o f th e m a g n e tic m a t e r i a l .

a th in d is c -lik e

c u t s u r f a c e s , and th e f i e l d

b e a u g m e n te d b y t h e f i e l d I p e r u n it a re a

p o le s w i l l be c r e a te d a t

in s id e th e c re v a s s e w i l l

a r i s i n g fro m t h e m a g n e tic p o l e s

on e i t h e r s i d e , I b e in g th e

m a g n e ti z a tio n o f th e m a t e r i a l . o f fo rc e

If

c r e v a s s e be c u t th ro u g h th e c y li n d e r

p e rp e n d ic u la r to th e a x i s , u n lik e th e

L e t H b e th e a x i a l

jo in in g

It

is

in te n s ity

of

c le a r t h a t th e l i n e s

th e s e p o le s w i l l t r a v e r s e th e g a p n o rm a lly ,

a n d , h e n c e , 4*TT l i n e s

o f f o r c e m u st p a s s th ro u g h e a c h

s q . cm . o f t h e g a p c r o s s - s e c t i o n .

The t o t a l n u m b e r o f

lin e s p e r u n it a re a i s , th e r e f o r e , B and t h i s

q u a n tity

(b )

-

H + 4TTI is

c a l l e d th e " m a g n e tic in d u c tio n "

13 " f lu x d e n s ity ” .

The u n i t o f f l u x d e n s i t y

o r “ m a x w e ll p e r s q . c m .” . m a te ria l is

c a lle d i t s

o f vacuum i s ,

The r a t i o

“p e rm e a b ility ”

is th e “g a u s s ”

of B to H in a CK-).

T he p e r m e a b i l i t y

by d e f in itio n , u n ity .

M a t e r i a l s f o r w h ic h p i s

le s s

t h a n on e a r e c a l l e d

“d ia m a g n e tic ” .

T hey a r e r e p e l l e d fro m th e

p o le s o f an

e le c tro m a g n e t:

t h e y move to w a r d a w eaker f i e l d .

M any o f

th e m e ta ls and m o st o f th e n o n -m e ta ls a r e d ia m a g n e tic , M a t e r i a l s w h ic h h a v e m a g n e t i c p r o p e r t i e s s i m i l a r to

iro n

( e . g . , n i c k e l a n d c o b a l t a n d m any a l l o y s o f t h e s e

t h r e e e le m e n ts ) a r e te rm e d “ f e r r o m a g n e tic " , an d a r e c h a r a c t e r i z e d b y p. s o f v a l u e c o n s i d e r a b l y g r e a t e r t h a n one.

The p e r m e a b i l i t y v a r i e s w i t h t h e f i e l d , a n d a l s o

d e p e n d s o n t h e p r e v io u s m a g n e tic h i s t o r y o f th e m a t e r i a l . T h e y l o s e t h e i r p r o p e r t i e s a b r u p t l y w h en h e a t e d t o te m p e ra tu re c a l l e d

a

th e " C u rie p o i n t ” o f t h e m a t e r i a l .

f e r r o m a g n e t i c s know n a r e

A ll

s o l i d s ; a l l m a g n e tic r e c o r d in g

m e d ia a r e f e r r o m a g n e t i c s . In a n o th e r c la s s th e , l a s t , a re c a lle d

th e

p -s a re

o n ly s l i g h t l y g r e a t e r th a n o n e , and

" p a ra m a g n e tic s " .

in d e p e n d e n t o f f i e l d h is to ry .

o f m a t e r i a l s , m o re n u m e ro u s t h a n

s t r e n g t h an d th e p r e v io u s m a g n e tic

T h e y m ay b e s o l i d s ,

p a ra m a g n e tic , and i t s

T he p e r m e a b i l i t y i s g e n e r a l l y

liq u id s

or g ases.

A ir i s

p e rm e a b ility can be ta k e n to be

u n ity f o r a l l p r a c tic a l p u rp o se s.

14 The t o t a l f l u x o v e r a s u r f a c e

is

d e fin e d b y th e

in te g ra l

0

. da

2

. . . . . .

d a b e in g th e e le m e n ta l a r e a . “ m a x w e ll” . v a n is h e s ;

a c lo s e d s u r f a c e i s flu x a re

is

th e

i n t e g r a l ^ 3 .d a

t o t a l n o rm a l in d u c tio n o v e r

z e r o ; and c o n s e q u e n tly , th e l i n e s

of

c o n tin u o u s e v e ry w h e re .

An e l e c t r i c m a g n e tic f i e l d ; p a th i s

The u n i t o f f l u x

O ver a c lo s e d a r e a , th e i n o th e r w o rd s , th e

2 .1

c u rre n t

an d th e

is

know n t o h a v e a n a s s o c i a t e d

i n t e g r a l .

= n ., o - i

S ysts

CHAPTER IV THEORY OF MAGNETIC TAPE RECORDING M any o f t h e p h e n o m e n a o f m a g n e t i c r e c o r d i n g a p p e a r to

h e c o m p le x o n c l o s e e x a m i n a t i o n .

i n p a r t fro m th e

f a c t t h a t th e b a s ic phenom ena o f f e r r o ­

m a g n e t i s m , o n w h ic h t h e w h o le e d i f i c e in g r e s t s , a re s to o d .

It

o f m a g n e tic r e c o r d ­

i n t h e m s e l v e s c o m p le x a n d n o t w e l l u n d e r ­

M o re o v e r, t h e a r t o f m a g n e tic r e c o r d i n g

i n t h e d e v e lo p m e n t s t a g e th e

T he c o m p l e x i t y a r i s e s

to a c o n s id e ra b le

is

s till

e x te n t, and

l a s t w o rd o n m any p r o c e s s e s h a s n o t y e t b e e n s a i d . is not s u rp ris in g ,

d iffe re n c e m an y o f t h e

th e re fo re ,

th a t th e re

o f o p in io n r e g a r d in g b a s ic

is

c o n s id e ra b le

e x p la n a tio n s o f

o b se rv e d phenom ena.

A ny d i s c u s s i o n o f m a g n e t i c r e c o r d i n g t h e o r y i s , th u s ,

s u b je c t to

lim ita tio n s

im p o s e d b y t h e f o r e g o i n g

c o n s i d e r a t i o n s a n d s h o u l d b e v ie w e d i n t h a t l i g h t . t h e p r e s e n t s u r v e y , a n a t t e m p t h a s b e e n m ade t o a l l p o in ts

in c lu d e

o f v ie w on c o n t r o v e r s i a l t o p i c s , w i t h e m p h a s is

on t h e m o st w id e ly a c c e p te d th e o ry .

In

T he s p e c i f i c

(a s f a r a s is

a s c e rta in a b le )

a s s u m p t i o n i s m ade t h a t t h e

r e c o r d i n g m ed iu m i s a t a p e . An a d d i t i o n a l h a n d ic a p o f a d i f f e r e n t n a t u r e th e

la c k o f s ta n d a r d iz a tio n o f te rm s .

m a g n e tic r e c o r d i n g

is

is

N o m e n c la tu re i n

j u s t b e g in n in g to b e s t a n d a r d i z e d .

-63 T he te rra s u s e d i n t h e

p r e s e n t s tu d y a r e

e x p la in e d

in th e

b o d y o f t h e d i s c u s s i o n w h e re a p p r o p r i a t e ; f o r t h e r e s t , th e s e

te rm s h av e b e e n s i f t e d

fro m th o s e s u g g e s te d b y

v a r i o u s S t a n d a r d i z a t i o n C o m m itte e s a n d i n d i v i d u a l in v e s tig a to rs . I.

TYPES OF MAGNETIC TAFE RECORDING AND RECORDING HEADS

B a s i c a l l y , M a g n e tic T ape R e c o r d in g i s im p r e s s in g tim e v a r i a t i o n s a m o v in g f e r r o m a g n e t i c fie ld

s tre n g th

i n m a g n e tic f i e l d

ta p e .

is

s tr e n g th upon i n m a g n e tic

i n th e b o u n d a ry

o f a n e le c tro m a g n e t, r e f e r r e d

t o a s th e “R e c o rd in g H ead“ . tip s

The v a r i a t i o n

e x i s t s a t th e p o le t i p s ,

o f a n a ir - g a p i n th e c o re

a s y s te m f o r

T he m a g n e t i c f i e l d

p ro d u c e d b y th e a u d io s i g n a l c u r r e n t s

Among t h e s e

a t th e

in th e c o i l s

a re :

R e s e a r c h C o u n c i l B a s i c S o u n d C o m m itte e , “D i s c u s s i o n o n M a g n e t i c R e c o r d i n g 11, J o u r n a l o f t h e S . M . P . E . . V o l. 4 8 , N o. 1 , J a n u a r y , 1 9 4 7 , p p . 5 0 - 5 6 . P r o p o s e d A m e r ic a n S t a n d a r d A c o u s t i c a l T e r m i n o l o g y . (New Y o r k : A m e r ic a n S t a n d a r d s A s s o c i a t i o n , I n c . , F e b ru a ry , 1 9 4 9 ). B o o k o f t h e A m e r ic a n S o c i e t y o f T e s t i n g M a t e r i a l s . ( P h ila d e lp h ia , P a ., 1 9 3 6 ). L . C . H o lm e s , “ Some F a c t o r s I n f l u e n c i n g t h e C h o ic e o f a M edium f o r M a g n e t i c R e c o r d in g * 1, J o u r n a l o f A c o u s t i c a l S o c i e t y o f A m e ric a . V o l. 1 9 , N o. 3 , M ay, 1 9 4 7 , p . 3 9 6 .

64 in th e h e a d .

T h is f i e l d

p r o g r e s s i v e l y m a g n e tiz e s th e

m o v in g f e r r o m a g n e t i c t a p e . - D e p e n d in g o n t h e d i r e c t i o n o f m a g n e t i z a t i o n o f th e

ta p e , th re e

t y p e s o f r e c o r d i n g m ay h e d i s t i n g u i s h e d .

F o r t h i s p u r p o s e , c e r t a i n te rm s p e r t a i n i n g o f th e ta p e w i l l be d e fin e d f i r s t M a jo r p l a n e s

M a jo r a x i s :

t h e g e o m e try

. 2

The m a jo r p la n e o f t h e t a p e i s i t s la rg e r su rfa c e . I t s b o u n d a rie s a re d e te rm in e d b y i t s l e n g t h an d w id th . T he m a j o r a x i s o f t h e t a p e i s i t s l o n g e s t a x is on th e m a jo r p l a n e .

L o n g itu d in a l a x i s ;

T ra n sv e rse a x is :

L a te ra l a x is :

W ith t h i s

to

The l o n g i t u d i n a l a x i s i s i n t h e sam e d i r e c t i o n a s th e m a jo r a x is an d c o in c id e s w ith it. The t r a n s v e r s e a x i s i s t h e a x is n o rm a l to th e m a jo r p la n e and t h e m a jo r a x i s .

The l a t e r a l a x i s i s t h e a x i s n o rm a l to th e m a jo r a x is b u t l y i n g i n th e m a jo r p l a n e .

n o m e n c la tu re , th e v a r io u s

r e c o r d i n g m ay b e l i s t e d

a s fo llo w s i n th e

ty p e s o f o rd e r o f th e ir

im p o rta n c e : 1.

L o n g itu d in a l re c o rd in g .

In lo n g itu d in a l

r e c o r d in g , th e ta p e

i s m a g n e tiz e d i n i t s m a jo r p la n e i n a

d i r e c t i o n a lo n g th e

lo n g itu d in a l a x is .

2

H aynes, op. c i t . ,

p . 22.

65 2. ta p e

L a te r a l re c o rd in g . 3

i s m a g n e tiz e d i n

its

I n l a t e r a l r e c o rd in g , th e

m a jo r p la n e a lo n g i t s

la te ra l

a x is . 3. th e to

T ra n s v e rs e r e c o r d in g .^

ta p e i s m a g n e tiz e d a lo n g i t s

In tra n s v e rs e re c o rd in g , t r a n s v e r s e a x i s , n o rm a l

t h e m a jo r p l a n e . In

o rd e r to g e n e r a te th e n e c e s s a ry f i e l d

s e v e r a l ty p e s o f h ead s have b een u se d .

p a tte rn s ,

Of th e s e , th e th r e e

ty p e s a r e : 1. head

is

The r i n g h e a d .

sh ap ed l ik e

As t h e nam e i m p l i e s ,

a h o rse sh o e m agnet ( q u a s i - t o r o i d a l )

b u t w ith a n e x tre m e ly s m a ll a i r - g a p , p ra c tic a lly a rin g .

th e r i n g

It

so t h a t th e h e a d i s

i s u sed e x c lu s iv e ly

in lo n g itu d in a l

re c o rd in g . 2.

The d o u b l e - p o l e - p i e c e h e a d .

I n th e d o u b le -

p o l e - p i e c e h e a d , t h e e l e c t r o m a g n e t s t r u c t u r e h a s tw o p o l e p ie c e s ,

one on e i t h e r s i d e

o f th e ta p e .

T h is ty p e o f

h e a d h a s b e e n u s e d i n a l l t h r e e m o d es o f r e c o r d i n g — lo n g itu d in a l,

tr a n s v e r s e and l a t e r a l .

F o r lo n g itu d in a l

r e c o r d i n g , t h e tw o p o l e s a r e p a r a l l e l t o a x is and a r e d is p la c e d a s m a ll d is ta n c e

3

u sed h e re 4

u se d h e re

th e tr a n s v e r s e (th e

re c o rd in g

A l s o c a l l e d “ T r a n s v e r s e R e c o r d i n g 11. is f e l t to be s u p e rio r.

The te r m

A l s o c a l l e d “ P e r p e n d i c u l a r R e c o r d i n g ’*. i s c o n s id e re d to be s u p e r io r .

The te rm

66 g ap le n g th )

a lo n g

th e lo n g itu d in a l a x i s .

and l a t e r a l re c o rd in g , w ith th e

F o r tra n s v e rs e

th e y a re a c c u r a te ly a lig n e d

o th e r a lo n g th e

one

tr a n s v e r s e and l a t e r a l a x e s ,

re s p e c tiv e ly . 3.

T he s i n g l e - p o l e - p i e c e h e a d .

p ie c e head i s p o le -p ie c e .

th e s i m p l e s t o f th e t r e e , an d h a s b u t one I t h a s b e en u sed in b o th tr a n s v e r s e and

lo n g itu d in a l re c o rd in g , and in e ith e r c a se i s th e

s e t a lo n g

tra n s v e rs e a x is . D ia g r a m s o f t h e

F ig .

T he s i n g l e - p o l e -

6

, page 71.

th re e

ty p e s o f h e a d s a re g iv e n in

T he m a g n e t i c f i e l d

p a tte r n c re a te d a t

t h e g a p h a s a l s o b e e n sh o w n i n e a c h c a s e . re a liz e d

I t m u st be

t h a t th e a c t u a l s p a t i a l d i s t r i b u t i o n o f th e

m a g n e tic f i e l d

a t th e t i p

is

a n e x tre m e ly c o m p lic a te d

one

t o d e t e r m i n e e x a c t l y , a n d t h e d i a g r a m s sh o w o n l y f i r s t a p p r o x i m a t i o n s w h ic h a r e g o o d e n o u g h f o r p r a c t i c a l p u rp o se s.

T he p a t t e r n

flu e n c e d by th e n a tu re

is ,

o f c o u rs e , c o n s id e ra b ly in ­

o f th e

w h e th e r , f o r e x a m p le , i t

is

r e c o r d i n g m ed iu m p r e s e n t —

th ic k o r th in ,

it

has a

lo n g itu d in a l o r o th e r p re fe rre d a x is , e tc . P r a c t i c a l l y a l l p r e s e n t d a y m a g n e tic r e c o r d i n g lo n g itu d in a l; lim ite d

tra n s v e rs e

re c o rd in g

is

i s u se d to a v e ry

e x t e n t ; b u t l a t e r a l r e c o r d i n g a lm o s t n e v e r .®

5 S . J . B e g u n , M a g n e tic R e c o r d in g . M u rra y H i l l B o o k s, I n c . , 1 9 4 9 ) p . 5 0 .

(New Y o r k :

The

r e a s o n s f o r t h i s m ay b e b r i e f l y

o u tlin e d h e re .

tr a n s v e r s e a s w e ll a s l a t e r a l re c o rd in g , th e

In

ta p e

is

d ra w n b e tw e e n t h e p o l e - p i e c e s o f a d o u b l e - p o l e - p i e c e h e a d . I n t r a n s v e r s e r e c o r d in g , th e m a jo r p la n e o f th e ta p e i n c o n ta c t w ith

th e p o l e - t i p s ;

th e e d g e s o f th e ta p e a r e O b v io u s ly ,

in o rd e r to

an d , in l a t e r a l re c o rd in g ,

i n c o n ta c t w ith th e p o l e - t i p s .

s e c u r e a n e f f i c i e n t m a g n e tic f i e l d

p a t t e r n a t th e p o l e - t i p s ,

it

is

n e c e s s a ry t h a t th e g ap

b e t w e e n th e m m u s t b e a s s m a l l a s p o s s i b l e . th e ta p e f o r u se

T h is m eans t h a t

in tra n s v e rs e re c o rd in g h a s to be v e ry

t h i n , th u s s e r io u s ly d u ra b ility

is

lim itin g

o f th e ta p e .

In

sam e c o n s i d e r a t i o n * , t h e

th e t e n s i l e

s tre n g th and

l a t e r a l re c o rd in g ,

on th e

ta p e h a s t o b e e x c e e d in g ly n a rro w .

A g a in , f o r good h ig h f r e q u e n c y r e s p o n s e , th e p o l e - p i e c e s s h o u ld h av e th e d ire c tio n

s m a ll e s t p o s s ib l e d im e n s io n i n th e

o f m o tio n o f th e ta p e a n d , c o n s e q u e n tly , th e

p o l e - p i e c e s c a n n o t b e m ade a s s t u r d y a s i s

d e s ira b le .

L o n g itu d in a l r e c o r d in g ,

i s s u b je c t to

no su ch lim i ta t io n s . th e

on th e

o th e r hand,

G ap w i d t h c a n b e m ade s m a l l , a n d a t

sam e t i m e , w i d e , s t r o n g t a p e a n d

s tu d y p o le - p ie c e s c a n

be u sed . W h ile a l l t h r e e

t y p e s o f h e a d s c a n b e e m p lo y e d i n

lo n g itu d in a l re c o rd in g ,

o n ly th e r i n g h e a d i s

c o n s id e r a b le e x t e n t a t th e p r e s e n t tim e . ^

6

I b i d , p p . 5 1 -5 2

used to any

The s i n g l e -

68 p o le - p ie c e h e a d , w ith i t s la c k

o f sh a rp n ess o f th e

c o n s id e ra b le f l u x

le a k a g e and

f i e l d p a t t e r n p ro d u c e d a t th e

g a p , w as r e p l a c e d b y th e d o u b l e - p o l e - p i e c e h e a d a s e a r l y a s 1 9 0 7 .^

T he d o u b l e - p o l e - p i e c e , h o w e v e r , a l s o h a d m an y

d raw b ac k s.

As a s tu d y o f t h e f i e l d

sh o w ( F i g . s id e

),

th e re c o rd in g f i e l d

h a s c o m p o n e n ts o n e a c h

o f t h e g a p t h a t o p p o s e t h e m a in f i e l d .

a c tio n is is

6

d is trib u tio n w ill

T h is o p p o s in g

a d i s t i n c t d i s a d v a n t a g e a s t h e r e c o r d i n g m ed iu m

s u b je c te d to

s e v e ra l re v e rs a ls o f th e

p a ssa g e a c ro s s th e h e ad .

fie ld

d u rin g i t s

Fr