Ampex 351 - Page 32

COILS
GAP
0016
3
' — POLE PIECES
Construction of a typical magnetic head.
ate heads — erase, record, and reproduce — each
especially designed to perform its specific function.
Heads
No assembly in a magnetic recording system is
more important than the heads, which convert the
electrical cun-ent to a magnetizing force during the
recording operation, then reconvert that magnetism
to an electrical cun-ent during the reproduce mode.
Professional quality equipment employs three separ-
the clearance on guides is limited to minimum figures
to obtain extremely accurate guiding. If the width of
the tape then exceeds tolerances, the guides will bow
the tape, and it will again be lifted from contact with
the heads. Slitting the tape must, therefore, be rigidly
controlled.
The binder material must be wear resistant. This
is not primarily a matter of ensuring the durability
of the recording, but rather is to minimize oxide de 
posits on components in the tape threading path (see
Cleaning). Of course, if the binder breaks down suffi 
ciently to cause signal drop-outs it would affect the
durability, but this will be encountered normally only
after prolonged use at high tape speeds not usually
employed in audio work.
There are several considerations concerning the
iron oxide particles which affect tape characteristics.
These include the size and shape of the particles, and
their physical orientation so that the axes of easy
magnetization are longitudinal to the direction of
recording.
In addition to all the above, tape must be strong
enough to withstand the stresses it will undergo in
normal operation, and pliable enough to follow the
required turns in the tape threading path.
Recognizing that the quality of magnetic record 
ing is today limited by the properties of the tape, not
the equipment, A mpex recently entered the tape man 
ufacturing field. It is felt that the association of A m 
pex and its subsidiary — Orr Industries Co. — will
result in definite improvements in the art of magnetic
recording.
AT J ■ ’ -' vS
Recording
The operation of the record head is essentially the
same as that of an electro-magnet. If we insert a core
of permeable material within a coil of wire, then run
a direct current through that wire, we can set up an
intense magnetic field that will attract any nearby
material that is capable of being magnetized. If in 
stead of the direct cun-ent, we use an alternating cur 
rent, we would first attract then repel that material
(at a rate controlled by the frequency of our a-c)
until it assumed a position that was neutral in respect
to the alternating field.
In a magnetic recording head the core is shaped
like an incomplete ring — the discontinuity forms the
head “ gap ” — which is inserted within a coil of wire.
When the signal to be recorded is converted to an
electric current and passed through the coil, a strong
magnetic field is created across the gap. If we now
pass our magnetic tape across the gap, the iron oxide
particles in the tape will be magnetized in a pattern
which is a function of the instantaneous magnitude
and polarity of the original signal. Understand here
that these particles do not physically move, but are
simply magnetized by the flux at the head gap so that
each individual particle contributes to an overall
magnetic pattern.
The wavelength of the signal recorded on the tape
depends upon how far the tape moves during each
complete alternation of the signal cun-ent. For ex 
ample, if we were recording 60 cycles at 15 inches
per second, each cycle would be recorded on a 0.25
inch segment of the tape; if our frequency were 6000
cycles and our tape speed 7 ’ /2 inches per second, each
cycle would be recorded on a 0.00125 inch segment
of the tape. Such computations may be continued for
any frequency at any tape speed by simply dividing
the tape speed (in inches per second) by the fre 
quency (in cycles per second).
This brings up a point that sometimes confuses
individuals accustomed to considering wavelength
and frequency as being practically synonomous terms
— that a certain wavelength can denote only one fre 
quency or vice versa. This cannot hold true on any
device which employs a moving medium to store the
information. For example, say we record a frequency
of 10,000 cycles at a tape speed of 15 ips. If we re 
produce that tape at the same speed we will re-create
our original signal; but if we reproduce the tape at
71/2 ips the same wavelength on the tape will result
in a signal of only 5,000 cps, if our reproduce speed
is 3% ips our signal will be 2,500 cps. Similarly, if
we record 10,000 cps at 15 ips the wavelength is 1.5
mils, if we record the same signal at 7 ’ /2 ips the wave 
length is .75 mil, at 3% ips the wavelength is .375
mil. Thus, wavelength may vary for a constant fre 
quency and frequency may vary for a constant wave 
length, dependent on the speed of our medium.