Magnetic Recording and Playback 1093
reading is the composite value of all flutter compo-
nents in the frequency band of 0.5–250 Hz,
including flutter due to not only the rotating capstan,
roller, and guides and their associated bearings but
also any ac-power-related motor torque pulsations.
- Select the Wtd. filter. The bandwidth is now reduced
to 0.5–20 Hz, to emphasize the once-around rates
due to eccentricities of the rotating components.
Capstans and rollers with diameters of ½–2 in
(12–50 mm) are major contributions in this band.
- Select the 250 Hz–5 kHz bandpass filter labeled l.
The dominant component in this range is scrape
flutter, which typically peaks at 3–4 kHz for most
recorders. Instabilities or oscillations of the capstan
or spooling servos, which tend to occur in the
100–500 Hz range, may also be evident.
- If the machine is equipped with a scrape flutter
idler, stall the idler by pressing the point of a pencil
against the top of the idler. The scrape flutter
component should typically rise to two or three
times the normal value. If little or no rise or even a
decrease is noted, the scrape flutter idler is not func-
tioning properly. Clean and lubricate the idler bear-
ings according to the manufacturer’s instructions.
Use the flutter meter to obtain optimum positioning
of the idler after cleaning.
- Select the 5 kHz filter. This overall reading covers
the entire range from 0.5 Hz–5 kHz.
28.7 Tape Testing
Contrary to popular belief, not all tape that reaches the
customer’s hands is fault free. Although the tape manu-
facturers are to be commended for the very high stan-
dards of excellence that are maintained, the customer
must be prepared to deal with the bad rolls of tape that
slip through the manufacturer’s quality control
screening. The problems that do arise can usually be
traced to one of the seven steps in the manufacturing
process:
- The basic recipe of approximately a dozen major
ingredients that form the oxide mixture must be
correctly formulated. Each ingredient must be pure
and must be measured correctly. Errors in mixing
and experimental formula modifications often lead
to nondurable oxides that shed debris onto the
guides and heads.
- The mixing of the ingredients must be thorough but
not excessive. Inadequate mixing leads to high
modulation noise and high background noise.
Excessive mixing reduces noise but increases print
through.
- The coating process must apply a uniform coating
across the width and length of the tape. The coating
is applied to jumbo rolls that range from 18–36 inch
(0.5–1 m) in width. To monitor the entire width of
one of these rolls fully would require over 400 chan-
nels of conventional record/reproduce circuits!
- The tape is baked to remove solvents by passing the
coated web through a multizone oven. Poor temper-
ature control can lead to either brittle or soft oxides.
- The jumbo roll is run through heated rollers that
make the oxide denser to increase output and
high-frequency response. This calendaring step is a
major factor in determining the modulation noise
content of the finished tape.
- The tape is slit to the final width by a set of rotary
shears. Poor slitting can produce ruffled edges,
wavy or crooked tape, and excessive oxide and
backing debris on the recording surface.
- The tape is rewound onto reels or hubs, tested, and
then packaged for sale. The tape cartons usually
pass through a very large degausser so that no
residual signals are left on the tape.
Mistakes during the manufacturing process create
four types of problems. The most common of these is
signal amplitude variations, which are due to either a
nonhomogeneous magnetic dispersion or erratic
tape-to-head contact due to physical distortions of the
tape. Other common problems include excessive noise
or distortion and high print through.
A common method of testing the signal instability
and dropouts is to observe the amplitude variations of a
sine-wave signal on either an oscilloscope or a VU
meter. While these techniques give some insight into the
performance of the tape, they do not yield a quantitative
value that can be used for determining acceptable limits
of performance.
A more informative method is to amplitude demodu-
late the test signal to remove the steady tone and
magnify the fluctuations. If the output of the demodu-
lator is properly filtered and fed to a metering circuit,
quantitative values for the fluctuations in various test
bandwidths can be read.
Unlike other flutter test instruments, the flutter
meter shown in Fig. 28-49 contains amplitude-demodu-
lating circuitry to be used for testing tape. The AM test
configuration is identical to the previous flutter setup,
except that the FM/AM selector is set for AM mode
testing to connect the phase-lock loop as a synchronous
amplitude demodulator. The AM meter ranges, which
