Ampex 351 - Page 45

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Underlying each of these adjustments is at least one
of the principles of magnetic recording we have been
discussing.
Tape Tension
As indicated in our discussion of Tape Transport
Design, the tension of the tape as it winds through the
system is very important. Proper tape guiding is, to a
large degree, dependent on correct tensions. A good
tape pack on the takeup reel is also determined by this
function. And very importantly, if tape is stored
under excessive tension, it will tend to stretch; also
the phenomenon known as “ print through ” (where
the magnetic signal on one layer of tape on the reel is
transfered to adjacent layers) will be accentuated.
Tape tension control in professional quality equip 
ment is normally adjusted by varying the resistance
The record pre-emphasis is then adjusted for a
flat overall frequency response through the previously
standardized reproduce system.
which, as we have already stated, are not fully under 
stood. Therefore, a certain variable pre-emphasis is
employed in the recording process, which is adjusted
to achieve a flat response when the reproduce ampli 
fier is set on a standard curve.
Equalization Adjustment
A series of tones will be recorded on the standard
alignment tape so that the reproduce amplifier re 
sponse can be set on curve.
The rising characteristic of the reproduce head
is not only the consideration in achieving an overall
flat response; there are certain wavelength losses
Record Bias Adjustment
We make the high frequency bias adjustment
using a signal of specific wavelength (normally 15
mils — 1000 cycles at 15 ips, 500 cycles at 7 ’ /2 ips,
etc.) at the normal tape operating level. The bias is
set, while recording this signal, to achieve a maxi 
mum output.
Because the output vs bias current is very broad
near the peak bias current setting, the adjustment
is simplified by increasing the bias current until the
output drops Vz db then decreasing the bias until the
output again drops Vz db; the correct setting is the
average of the over- and under-bias.
The maximum amplitude point at the given wave 
length will give low distortion and reasonable short
wavelength losses. It is also comparatively easy to
adjust and can be consistently repeated using simple
test equipment.
Because the magnetization curve varies with
different tapes, the bias voltage ideally should be
adjusted each time the tape is changed — particu 
larly if the change is to a tape from a different manu 
facturer. However, this would normally be done only
when extreme fidelity was required, such as when re 
cording a master tape for a commercial recording com 
pany. Usually, a carefully adjusted “ average ” bias set 
ting will produce excellent results with a wide variety
of tapes.
Level Adjustments
The volume level in reproduction is strictly a
matter of personal preference, but the record level
must be accurately calibrated if optimum noise and
distortion are to be maintained. This is again most
easily accomplished by using a standard alignment
tape to set the reproduce level to a reference amp 
litude. The record level is then calibrated to produce
this reference playback level.
The record calibration can be set by using a
distortion meter to measure the third harmonic con 
tent. Normal record level is usually at a 1% harmonic
distortion level, so it can be adjusted to that value.
However, distortion meters are seldom available in
practice, the record level is nominal, and different
tapes may vary by ±1 (or even ±2) db. Therefore the
standard alignment tape procedure is certainly ade 
quate.
The easiest way to set the playback response on
curve is to play a standard alignment tape, and
adjust the variable equalizing components for a flat
response as the precisely recorded tones are repro 
duced. Another widely used method is to use an audio
oscillator and a vtvm to actually follow the response
curve provided with the equipment; this, however,
does not allow for variations in head characteristics.
Head Azimuth Adjustment
It is important that the heads be aligned so that
the gaps are exactly perpendicular to the top and
bottom edges of the moving tape. If the gaps are
slanted across the width of the tape we have created
a situation where the signal reproduced from the
upper part of the tape is out of phase with the signal
from the lower part of the tape. This phasing con 
dition causes a cancellation of signal, accentuated
at the higher frequencies. Of course, if the record
and reproduce head gaps on an individual single
channel recorder were exactly parallel, it would make
little difference if they were slanted slightly, as long
as the equipment played only those tapes it had re 
corded and os long as those tapes were not to be re 
produced on other equipment. But as soon as we want
interchangeability of tapes from machine to machine
we must establish a universal head alignment. Also,
as we have seen, we cannot tolerate phasing prob 
lems in stereophonic equipment.
The best method in procuring this alignment is
to use a standard alignment tape, produced under
stringent laboratory conditions. This tape will be
recorded with a head alignment signal, and the
reproduce head is adjusted to give a maximum
output of this signal. The standard tape is then re 
moved, and the record head is aligned so the its
recordings result in a maximum output on the pre 
viously aligned reproduce head. Both heads are thus
set to a universal standard.