1096 Chapter 28
of bias for all tracks simultaneously. The bias level can
be increased and decreased on all tracks with this single
control to verify that the proper level of overbias is
achieved without resorting to unnecessary adjustments
on each track.
The following sequence of steps represents a compre-
hensive alignment procedure that would be appropriate
whenever the proper performance of a recorder must be
verified. Since the details of each step vary with
machine type, the operator should consult the operator’s
manual published by the manufacturer of the recorder.:
- Clean and inspect the tape transport. (Refer to
Section 28.8). - Degauss the heads and guides. (Refer to Section
28.3.8). Before using a degausser, always verify that
the tips of the unit are covered with a soft material
such as plastic or tape that will not scratch the faces
of the magnetic heads. - Calibrate the reproduce section of the recorder with
a test tape of known accuracy. Several brands of
standard alignment tapes are available for this
purpose. Remember that the final results will be no
better than the measurement standard that is being
used as a reference.
First, verify the perpendicular alignment of the
reproduce head with the short-wavelength azimuth
test tone on the test tape. The azimuth and/or phase
alignment of the head can be measured with an
oscilloscope using either a Lissajous pattern or a
dual-trace display or with a phase meter that reads
phase error directly. If no specialized equipment is
available, invert one channel and sum the inverted
output with another channel that is not inverted.
Phase alignment produces a deep null in the
summed output. Since phase alignment at one
frequency does not eliminate the possibility of a
360 qerror, check the phase for several lower
frequencies. The voice announcements on the align-
ment tapes provide a convenient multifrequency
sample for this purpose.
Next, establish a convenient reference level for
making playback frequency-response measure-
ments. Check and adjust the high-frequency repro-
duce equalizer at 10 kHz to match this reference
level. Once the equalizer has been set, sweep
through the tones on the tape, noting the maximum
deviations from the reference value. Readjust the
equalizer and the reference level as necessary to
obtain the desired degree of flatness.
When the results are satisfactory, write down the
results for later comparison. Having a record of
correct performance makes troubleshooting much
easier.
Two pitfalls exist when making the previously
discussed adjustments: one affects the reference
level and the other affects the frequency- response
and reference level. Some recorders use different
track widths for the record and playback heads. For
machines that have wider playback heads, the
full-track test tapes used for most of the wide-tape
formats will produce an enhanced output during
testing. The reference level from the tape must be
set above the 0 VU reference by the amount of this
extra pickup due to the wider head when using the
playback head. When setting the reference level for
sync/overdub playback, the track width is correct,
yielding a true 0 VU level that requires no
correction.
If the record head has a wider track, then the
normal playback level will be correct and the error
will occur on the sync/overdub level.
The second problem is created by the fringing
effect of long wavelengths that produces a rise in
playback response at low frequencies whenever
additional flux is present beyond the area being
scanned by the reproduce head. Such a condition
exists for playback of a full-track alignment tape
and for test and alignment procedures that apply the
same low-frequency signal to all tracks of the
recorder simultaneously.
The fringing effect will first create a problem in
establishing the correct reference level for the
midband-level set tone. At 15 in/s and 30 in/s
(38 cm/s and 76 cm/s) tape speeds, sufficient
fringing may exist to create an error of approxi-
mately 0.5–1 dB, depending on the track format,
tape speed, and geometry of the head cores and
shielding. This extra fringing contribution in the
reference tone also makes the high-frequency
response appear to be deficient, tempting the oper-
ator to raise the equalizer adjustment. Consult the
operator’s manual for the correct procedure and
correction factors for a given model of recorder.
The final step in the reproduce alignment proce-
dure is to set the level and equalization of the
sync/overdub circuit. The operator may choose to
defer the azimuth alignment of the record head until
the following record alignment procedure if the
heads have not been disturbed.- The record alignment begins with the verification
and/or adjustment of the azimuth setting of the
record head. Using the playback head as a standard,
set the record head alignment while recording a