Ampex 351 - Page 42

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FREQUENCY RESPONSE
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Head-To-Tape Contact
A knowledge of the effects of losing good head-to-
tape contact will help us realize the importance of
would provide a system with a greater signal-to-noise
ratio.
Two methods may be employed to counteract
this “ gap" effect — either the gap can be made
smaller or the record-reproduce tape speed can be
increased. We can reduce the dimension of the gap
only so far and retain adequate signal levels and
realistic manufacturing tolerances; as this point is
reached any further extension of high frequency re 
sponse must be accompanied by a corresponding in 
crease in tape speed.
maintaining good contact. The predicted loss in sepa 
rating the reproduce head from the surface of the
medium is 54.6 db per wavelength separation. Thus
at short wavelengths, say '/z mil (15,000 cps at 7 ¥2
ips), it takes very little space to result in a 5 db loss
in signal strength. When we remember that commen 
surate losses also could occur in the record mode,
it becomes evident why good contact is a major con 
sideration in achieving top performance in a magnetic
tape recorder.
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This curve indicates the result of poor head-to-tape
contact as a function of the amount of separation and
the signal wavelength.
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ratio or stfAOAtiON io wavuingth
In this illustration sinusoidal waveforms are used to
denote the average state of tape magnetization and
to indicate how the reproduce head gap derives a large
output from a medium wavelength signal (A), a small
output from a long wavelength signal (B ), or no output
when the wavelength equals the gap length (C).
High Frequency Response
In audio applications, and at tape speeds normally
used in professional work, the high frequency re 
sponse is almost entirely limited by the tape and mag 
netic heads, in what arc referred to as “ wavelength
losses". Despite numerous tomes attempting to ex 
plain these losses they are as yet not fully understood,
and we would be presumptuous if we attempted any
explanation on this plane
As our high frequency requirement rises — in
video or instrumentation applications — or as our
tape speed is lowered, we enter a region where the
dimensions of the reproduce head gap. and the reso 
nant frequency of the heads become important factors.
Gap Effect
As shown on the accompanying diagram, when
the recorded frequency rises to a degree where the
reproduce head gap intercepts a complete wave 
length of the signal on the tape, there can be no
difference in flux magnitude across the gap, and the
head output will be reduced to zero. Practically, this
will occur at the ‘ 'effective ” gap length, which is
slightly longer than the physical length. For all prac 
ticable purposes this effect causes the head output
at this frequency and above to be useless.
A mpex engineers therefore devised a 15 ips equal 
ization known as A mpex Master Equalization (AME)
wherein a post-emphasis is designed to minimize
audible noise, and then the pre-emphasis is employed
to make the overall system flat. AME admittedly
trades overload margin for a lower noise level, and
must be properly used to obtain the intended results
without distortion. It is intended for professional
use, such as the recording industry, and is not to be
considered as supplanting the NAB standard for pub 
licly released tapes.
FREQUENCY IN CYCIES PEP SECOND 0018
This graph shows how a flat overall frequency re 
sponse is achieved Curve A is an "ideal" record-re 
produce response. Curve B is the result of adding the
standard NAB post-emphasis to the ideal response.
Curve C indicates the amount of record pre-emphasis
needed to achieve flat response. As the post-emphasis
curve is established as a standard, any deviation from
the ideal response must be accompanied by a change
in pre-emphasis.
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