1098 Chapter 28
Once the provisions for automated adjustment are
made available, three methods of alignment are
possible. Under the simplest mode, the operator
performs a manual alignment with the calibration
constants being stored for later use. This method
permits rapids changeovers, but does not simplify bias
and equalization adjustments to optimize a specific roll
of tape.
If the microprocessor can be provided with input
information from the metering devices on the individual
tracks, then calibration programs can be automatically
executed without operator intervention. The program
contains the “strategy” for alignment, including desired
amounts of overbias, equalization adjustment frequen-
cies, and operating levels. Beware that such systems use
an inferred adjustment technique which does not actu-
ally test many of the critical parameters. For example,
the recorder will set the bias level for minimum distor-
tion based on an overbias criterion at a specified
frequency. In reality, the machine doesn’t have the
ability to measure distortion. The strategy only infers
that overbiasing by the desired amount corresponds to
minimum distortion. Unfortunately, if a malfunction
exists that causes abnormal operation, the adjustment
routine may not detect the symptoms.
Nearly automatic calibration can be implemented by
connecting external automated test equipment such as
an Audio Precision System One test set to the machine
through an external intelligent controller such as an
IBM-compatible computer. A remotely controlled
input/output switching matrix will also be necessary for
multitrack machines. An operator is still required to
adjust nonautomated devices such as head azimuth and
to change tape reels for calibration tapes and sample
stock. The calibration program of the intelligent
controller sequences through a comprehensive set of
tests which rigorously exercise the machine. Parame-
ters such as harmonic and intermodulation distortions,
crosstalk, erasure, flutter, speed, noise, and phase can be
tested against absolute standards of acceptance.
Hopefully, the advent of inexpensive DSP (digital
signal processor) chips will allow manufacturers to
include the diagnostic equipment as a part of the built-in
calibration hardware.
A final word of caution is appropriate at this point.
Many operators and test technicians ignore symptoms
that indicate problems are developing in a tape recorder.
A good example is the frequent need to boost the
high-frequency equalization adjustments of a recorder.
A properly operating machine should not show such
trends, but a gradually deteriorating head would create
just such a problem. Simply readjusting without deter-
mining the cause of the change wastes an opportunity to
fix a problem at an early stage before it grows to cata-
strophic consequences. Try to avoid problems by fixing
things before they break completely.