Ampex 351 - Page 43

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bump will occur when one-half wavelength of the
recorded signal equals the combined distance across
the two pole pieces, but there will be progressively
smaller bumps at IVz wavelengths, 21/2 wavelengths,
etc. Similarly the largest dip will occur when one
complete wavelength of the recorded signal equals
the distance across the pole pieces, and again there
will be progressively smaller dips at 2 wavelengths,
0068
The loss that occurs when the wavelength of the re 
corded signal approaches the length of the reproduce
head gap is indicated on this graph.
Low Frequency Response
Low frequency response is almost completely a
function of the effects generally known as “ head
bumps". This effect will occur in the reproduce mode
at the low frequencies, as the recorded wavelength
of the signal on the tape begins to approach the over 
all dimension of the two pole pieces on either side
of the head gap. In effect, the two pole pieces now
begin to act as a second gap, because they can pick
up magnetic flux on the tape quite efficiently.
As our frequency decreases we may start to notice
bumps and dips in the output of the head. The largest
Head Resonance
The coils of the heads are inductances which will
resonate with lumped or distributed capacity in the
circuit. At the resonant frequency of the reproduce
head there is an increased output, but a sharp drop
of approximately 12 db per octave occurs directly
after this point. Thus the resonant frequency must
normally be outside the pass band of the system,
or placed (in video and data recorders) at the ex 
treme upper limit so that it actually provides a shelf
at the point of resonance to extend the response.
As circuit capacitance is reduced to an absolute
minimum, only one way remains to place the point
of resonance at a higher frequency, and that is to
reduce the inductance of the head coil by employing
a lesser number of turns of wire. A reduction in the
number of turns, however, will reduce head output
over the entire frequency range, so a compromise
design must be provided.
3 wavelengths, etc. So as our frequency goes lower
and lower the bumps and dips will get bigger and
bigger. Below the largest bump, at I/2 wavelength,
the output rapidly falls to zero.
It is interesting to note the similarity between
the head bumps at the low frequencies and the gap
effect at the high frequencies. When the head gap
intercepts a complete wavelength we have no output;
when the pole pieces intercept a complete wavelength
we have a decline in output. The largest theoretical
output occurs when the head gap intercepts one-half
wavelength, there is an increase in output when the
pole pieces intercept one-half wavelength. There is of
course one great difference — increasing the tape
speed diminishes the gap affected by spreading the
signal over a greater length of tape, but decreasing
the tape speed dimishes the head bumps by shorten 
ing the wavelength on the tape. At 15 ips tape speed
the head bump is a rather serious problem, at 7¥2
ips the problem is reduced, and at 3% ips it has
practically disappeared.
Good engineering design is the only way to al 
leviate the head bump situation. The physical configu 
ration of the pole pieces and shields, and the angle
of wrap of the tape around the head, can be designed
so that the extremities of the pole pieces are farther
from the tape and cannot pickup the signal so readily.
An ideal solution, but rather impractical in today ’ s
compact equipments, would be to make the pole
pieces so large that no problem would exist down to
10 or 15 cps.
In any event, the head assembly must be designed
so that the head bumps occur at the lowest possible
frequency, so that if possible no more than one
.1 ,2 3 4 ,5 .6 7.8.91.0
RATIO OF GAP LENGTH TO WAVELENGTH
Uncorrected head bump curve produced artificially
by excessive tape wrap around an experimental re 
produce head.
DB — 10
The gap effect may be negligible when we are
dealing with audio frequencies at 71/2 or 15 ips tape
speeds. For instance, the A mpex reproduce heads
have a gap of 0.2 mil, and the gap loss is unimpor 
tant at the wavelengths involved. However, at lower
tape speeds, or for instrumentation or video appli 
cations where the high frequency requirements are
greatly extended, it becomes a serious limitation.