REF ID:A105070 

WAR DEPARTMENT 

SIGNAL CORPS LABORATORIES IN REPLY REFER TO 

FM(SCL) 600 - Proj. 85 

EXPRESS AND FREIGHT ADDRESS 

LITTLE SILVER, NEW JERSEY 

FORT MONMOUTH 

SUBJECT: Magnetically Recorded Signals. 

RBC:N/F 

POST OFFICE ADDRESS 

OCEANPORT, NEW JERSEY 

March 19, 1937 

TO: Chief Signal Officer, War Department, Washington, D.c. 

1. There is inclosed herewith correspondence between Captain Harrod 

G. Miller, of your office, and Mr. w. D. Hershberger, o.f these Laboratories, 

now on leave attending the University o.f Pennsylvania, concerning an invention of Captain Miller and Mr. Friedman, of your office. This invention 

has been reviewed by these Laboratories and reported on by indorsement 

dated June 5, 1936 and December 28, 1936. Attention is invited to the 

attached copy of Engineering Report No. 20, dated February 9, 1932, which 

covers the test of nine types of magnetic material in an endeavor to secure 

suitable material for magnetic recording purposes. Attention is particularly invited to the high insertion loss reported at the bottom o.f page 4, and 

the masking of high frequencies reported upon in subparagraph 3, page 8, 

end the finding: 11 In short, the principle of superposition does not hold 

for the telegraphone as an element in a signal network. 11 and to the final 

conclusions, paragraph v, page 12. 

3 Incls. 

Incl. 1- Le.Capt.Miller,3-15-17, 

wi~h 2 photostat incls. 

Incl. 2- Memo.fzn.Mr.Hershberger, 

3-17-37. 

Inc1. 3- E.R.No. 20, 2-9-32 

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• REF ID:¥?-9.?r?1~937. 

Memorandum for Capt. Earrod q. Miller, Office of the Chief 

Signal Officer, Washiru;ton, D. C., 

ThrOUP,'h Director, Sio,-nal Corps Laboratories. 

Subject: System and Me.ans for encipi113rin~ and decipheringo 

f.~ar;netically-recordec Si~als. 

1. In essence the answer to both questions depends on one 

factor: the lack of linearity in the recordin~ (and reproducing) process. If the mag"1letic record were a faithful replica 

of the input signal as re~ards amplitude, we could superpose 

si~als without limit and these siP,'llalS would not ~et mixed 

up with each other irrevocahly. Unfortunately, this is not 

the case. Larqe signals will mask small signals previously 

' 

recorded due to an erasin~ effect similiar to that normally 

exerted by a large ma~net provided fc;>r that ver"<J purpose. 

The erasin~ process consists essentially of introducin~ a 

'( 

lartte si,~al of zero frequency and it would be ineffec.ive 

for th.is purpose if the recordin~ were linear. 

2. Py accident I have recorded signals on a steel wire when the 

erasing ma~et was not energized. The result was a garbled 

signal in which both old and new recordin~s could. be recognized distinctly, but the last sic;nal recorded was the predominant one. In short, I have f'ound experimentally that we 

have superposition, but of a non-linear variety. I have very 

scanty measurements on this effect, and they will 'be found .· .·· 

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A REF ID:A10507-

in my 1931 and 1932 notebooks. Put since we do not have 

strict linearity, the r·est thin~ to co is to attempt to 

minimize its effects. I would make the followin~ su~~stions: 

(1) use the mnallest possible amplitude of input si~al, so 

as to redu~e distortion o~ the non-linear variety, and (2) 

taper the power in the recorc.in~ ma~ets so the first ma~et 

to record carries the greatest amount of power anc. the last 

one· to record the srrallest ar.iount of power. This could be 

accomplishec. r-:y us in~ recordinrr rna.~ets with dift"ereht nwnbers 

of turns, or by varyi!l.fr the air-.ffap in an appropriate manner. 

These are merely tricks ~o minimize the.deleterious effects 

arising from the non-linear record which you will have in any 

3. To sum up, the resultant record is not the al~ebraic sum 

.of the component records but can be made to approach ·this sum 

by low amplitude recorcing. As to your seconc question, ! 

do not know how it could be answered.other tha~ experimentally. 

w . tIJ. /t .. JJ .. r 

W. D. Hersh'he~er 

(2)