measurement may be established as the zero dBr reference following
the test by locating the graphic cursor on this measurement and
pressing the “dBr ZERO” button.
Record-Reproduce Mode
For most three-head tape recorders at tape speeds of 7.5 inches per
second (19 cm/sec) and faster, the MED sweep speed selects sufficient
time delay for the tape to travel from record to reproduce heads for
proper measurements. At slower tape speeds or with recorders with
very widely-spaced heads, it may be necessary to select the SLOW
sweep speed. The record level controls on both channels may be set
for specified line output level while driving the line inputs at the
specified line level and measuring the outputs in LEVEL mode with
dBu units.
For azimuth alignment of the record head in record-reproduce mode
(assuming that the reproduce head has already been properly aligned),
a repeating three-step (four-point) sweep of phase versus frequency is
recommended. Select PHASE function, NO-AVERAGE, SWEEP
mode, resolution “3", with the sweep lower frequency limit at about 1
kHz and the upper limit at about 15 kHz. Select the FAST speed if
time delay between heads at the tape speed in use is less than about
200 milliseconds. Select MEDIUM speed for situations with delay
between about 200 and 500 milliseconds. Press the SWEEP button
and hold it until the first measurement is numerically displayed at the
left of the display. The sweep will now repeat continuously. Slowly
adjust the record head azimuth for the flattest possible graph (zero
phase shift at all frequencies). The graph will curve upwards at higher
frequencies with misadjustment in one direction and downwards with
misadjustment in the other direction. The advantage of the
frequency-sweeping technique is elimination of the ambiguity of phase
measurements at high frequencies, where a moderate misalignment
can produce an apparent zero degree reading which is really 360
degrees or 720 degrees away from proper alignment. At lower
frequencies, very large misalignment would be required for a complete
phase rotation. Thus, a flat phase-vs-frequency graph from 1 kHz to
high frequencies indicates proper alignment with no chance of
accidentally aligning for a complete rotation at 15 kHz.
5
Applications
Analog Tape Recorder Alignment and Performance Verification Applications
Portable One Plus Access User's Manual 5-3
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