Ampex 351 Series - Page 28

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At points near the ongm the c urv e is ex-
tr emely non-linear a nd , without some correc-
tive factor, the s ignal record ed on the tape
wo uld not be dir ec tly proporti onal to the signal
applied to the h ead, resulting in a high degr ee
of distortion when the tape was reproduced.
This distortion is grea tl y reduced by mixin g
a
high frequency , co n stant amplitude , bi as
signal with th e ac tual s ignal being record ed.
so
that ope ration is obtained on th e Linear por-
tion
of the c ur ve. Thi s m ay be liken ed to apply-
in g a d-c bia s to a tub e to force it to work on
the linear portion of it s c urv e. Th e bias signal
is ge ne rally se l ec ted to be at l eas t fi ve tim es
th e hi gh es t fr eq uency to be re corded so that no
beatin g will occur betw ee n the bias frequenc y
an d th e ha rmonic s of the r ecor ded signal.
While the tap e is in th e r ecord in g gap th e
bias causes th e ma gne tization c: h aradcr i st i cs
of the iro 11 ox ide to fo ll ow th e das h ed lin e loops
known as the ' 'm inor hyst eres is loops." As
th e tap e l eaves the gap th e influenc e of Lhe mag -
netic field
cr e at ed by th e bi as is reduc ed to 1/.ero
and th e tape ass um es a permanen t state o f
m ag neti zation ( known as '' re rnan e nt indu c-
ti
on" ) determined by th e gap influ x at th at tim e.
Aft er th e recor din g process ther e ex ists on
the tap e a flux patt e rn whic h is proport i ona l
in ma g nitude a nd dil' ec ti on to th e si gna l re-
co rd ed. If th e tape is th en moved past a repro-
du ce head - w hi ch is s imil ar in c onstruction
to the record h ead - th e ma gnetic flux on th e
moving tape will indu ce a vo lt age in the co il of
the reproduce h c::i d. This indu ce d vo lta ge js pro-
port i ona l to th e numb er of turns of wire on
th e h ea d a nd the rat e of c han ge of flux. Thi s ls
expressed by th e e quation E= N ( d cJ, / dt )
Wh e re
E= indu ce d vo lta ge
N = number of turns of wir e
d 4, / dt = rat e of chan ge of flux
ll is d es i rable that the ga p in th e re produc e
h
ea d be as s mall as possible so it will int ercep t
l
ess than one wave leng th of th e signa l on the
tape at th e high es t fr e qu en cy to be re produ ce d.
H
owever , as tbc ga p is made s mall er th e in -
duced vo lt age deCTeases, so the re is a practica l
lim it ation in d ec r eas in g th e gap a nd still main -
taining an ade quat e si gna l-to- nni se rat io.
Th e vol tage indu ce d in the r epro du ce h ea d
duri ng re produ c tion is comp ut ed by the eqw 1-
ti
on E = B" V SJ N,,-, .. /, \
4-3
Where
E = induced vo lt age
B M:::: m ax imum flux density of the record -
in g mat e ri al
V =velocity of tape over the h ea d
"'= width of th e gap
,\:::: wav elen g tb of the s ignal on th e tape
From this e qu at ion it ca n be see n tbat the
vo lt age acro ss th e co il increa ses di rectly as
the ve lo c it y in cre as es and as th e waveleng tl,
d
ecreases (-frequency in cr eases). ff the tap e
vel
ocity and gap widt h arc ass um ed to be co n-
sta nt , th e ou t put voltage from th e h ea d is
dir ec tly proportional to th e frequency as l ong
as th e wavelcll g th on the ta pe is la rge co mp a r ed
to th e ga p \IVidt h. This results in an o utput vs.
fr e qu en cy characteris t ic such as is sh own in
c ur ve A of t11c figure below.
OUTPUT - ------ - - ---~--------•-- -r- - -- --r ---r -- -- -
(d b) 1-25 _____ _ir_J ________ Jr __ j __ ___ j__J ___ j
I t I I
J : I : Al ! :
+-20 --- -- -.J- - ♦ - - ---- --1--- r--- -- - - - - - --,----- -_)
I I ' I I I I
: : j f I :
+15 -- -- - -r, --r---------r-- 1 ----- ---{--- 1 -- - ◄
I 1 l
1
--..L : I
•1 0 - -----~---;------1---r --r --.. -r----~
I ' ! <c., I I '\ :
+s -----r--',----- ~ ,---- ---~--- 1, --- -~
o _____ _ J _ _j ___ _
0
~"<"' i_ _____ L__~ ~ -~-1
- 5 -------l---~- ~\ --~ __ J _______ J,-_J ___ j
-10 __ _____ J ___ j_~ ____ J _L _ ___ L __ l __ __ !
: : : t I l l
-1 5 --- -- ---!--- -- -- --- ~----! --------}---- .J _____ ...
: : : : l ! l
-20 ------ -7---------- -{----- --- - -- --i----r- ---- ,
I I I I I I I
-
2 5
------ -r---r----- ---r--: ·----r-- :~-- --i
I t I I I I I
10
100
1000
IOKC
FREQUENCY
R eproduce head characteristics
The vo lt age do cs not co ntinu e to rise ind efi-
nit ely. As el ect ri ca l l osses in the c or C' m ate rial
in crease and as th e wavelength on tl1c tapf'
app r oac h es the same dim e nsi ons as the r cp 1 •0-
duc e h ead ga p, the actua l o utput resemb l es
c urv e Bin the sa me figure.
In or d er to provide an over-a ll frequ e nc y re-
spon
se th at is O at (sec the figure below ) an
e
qualiz a tion c ir c uit cons isting of a se ries resis-
tance and ca pa c it anc<' is in ser ted in one of th e
ea rl y stages of' thr r epro duce amplifier. Thi s
e
qu aliz in g c:lrcuit has a high-frcgucncy dro op
c haract eris ti c ( c ur ve B) wh ich is the inverse of