Magnetic Recording and Playback 1097
short-wavelength signal such as a 10 kHz or 15 kHz
signal to give minimum azimuth or phase error
using whatever method was used for the reproduce
alignment procedure. This alignment should be
rechecked after the bias and record equalization
settings are made, since these adjustments can intro-
duce varying amounts of phase delay.
The bias should be set by adjusting for the
desired amount of overbias as recommended by the
tape and machine manufacturer for the appropriate
type of tape, record head gap width, and tape speed.
Note that a 10 kHz signal at 30 in/s (76 cm/s) does
not achieve the desired wavelength of 1.5 mils
(38μm) that is typically specified for bias adjust-
ment. The test frequency must be changed to match
the tape speed.
The bias should first be decreased to achieve
deliberate underbias, and then slowly increased to
the point at which a peak in the playback level is
observed. Continue to increase the bias until the
signal drops by the number of decibels desired.
Typical overbias settings range from 2–5 dB for
professional formats.
Once the bias is correctly adjusted, the input
signal should be set to the frequency used as a refer-
ence during the playback alignment. The record gain
control can then be set to produce the reference level
when driven with the appropriate 0 VU input level.
Adjust the high-frequency record equalizer to
match the record/play response as closely as
possible to the alignment tape response noted previ-
ously. Smoothness in the midband frequencies is
more important than trying to hold small errors at
15 kHz or 20 kHz.
Recheck the record head azimuth to verify that
changes in bias and equalization have not created
any phase differences. Readjust as necessary until
all parameters are optimized.
Set the record gain preset and the input monitor
gain calibration to achieve a 0 VU reading in all
monitor modes.
- After the record section has been aligned, a final test
and alignment of the low-frequency playback equal-
izers can be undertaken. To eliminate all the
fringing problems previously mentioned, the equal-
izers should be set in the record/play mode with
signal being applied to every other track. Make
small adjustments as required to optimize the
smoothness of the response.
If any large discrepancies are noticed, rerun the
alignment tape. Any failure in the low-frequency
record equalizer circuits, such as a faulty switching
component, will create an error that should be
obvious if a large correction is required. If any
doubt still exists, record a full-frequency sweep and
then flip the reels over to play the tape backward.
The alignment should be similar within a few tenths
of a dB to the values set in the forward direction.
- The alignment procedure is not completed until the
noise level and erasure have been checked. Record a
signal at +6 VU, rewind the tape, and then erase the
signal. Listen on the monitor speakers to the level of
the residual signal and to the subjective nature of the
tape noise. The tone should be either completely
eliminated or well buried in the tape noise. The
noise should be a smooth hiss without large or
frequent bursts or crackling. All tracks should be
similar in performance. Also, check for objectional
clicks and pops when changing modes.
Although these noise and erasure levels can be
read from instruments, the operator should take the
time to listen to the machine before issuing his or
her stamp of approval. Many sessions have died
aborning because the recorder was never given a
final listening test after alignment.
The previous procedure does not include several
steps that are more appropriately considered to be main-
tenance routines. Examples include tuning of the bias
and erase sources, tuning of bias traps, checking meter
calibration, and testing distortion levels. These tests are
not required on a day-to-day basis.
As a final note on alignment, never gloss over large
discrepancies. The corrections that should be required
for this alignment procedure should be on the order of a
small part of a dB, not several dB. Whenever a large
change seems required, stop long enough to determine
why such a large change is necessary. Look for faulty
components and recheck your own procedure. Recheck
the maintenance log to establish the proper level of
performance that should be expected. Heeding the small
symptoms may help you avoid a serious catastrophic
failure.
28.10 Automated Alignment
The onslaught of digital technology has provided the
tools to control the variable alignment adjustments of a
tape recorder with a microprocessor. Multiple sets of
calibration constants can be stored in nonvolatile
memory, permitting rapid changes of operating speeds,
equalization standards, reference flux levels, and tape
types.
