Magnetic Recording and Playback 104728.2.2 FlutterRegardless of the passive and/or active contact
enhancements, servo design, and workmanship stan-
dards employed in a given transport, some residual
amount of tape speed variation will still be present. The
long-term or fixed component of this speed error is
denoted as speed accuracy, timing accuracy, or drift.
The small, rapid changes in instantaneous speed are
referred to as flutter.
Flutter is further broken down into three frequency
bands, based on perceptibility, Fig. 28-6.
Speed variations at rates up to a few cycles per
second are termed wow, with the listener perceiving a
cyclic pitch variation in music. The most common
source of wow is eccentric rotating parts.
Faster flutter rates due to motor torque pulsations
and rattling bearings add a fluttering sound to the music.
As the flutter rate increases beyond a few hundred hertz,
the listener no longer distinguishes the flutter compo-
nents from the music. Instead, the listener notices a loss
of crispness and clarity, with high frequencies created
by percussion, strings, and brass sounding dull or
mushy. These high-frequency scrape flutter compo-
nents are generated as the surface of the tape scrapes
over stationary elements such as fixed guides and heads,
creating vibrations in the tape similar to the plucking of
a stringed instrument.
Historically, wow and mechanical flutter have
received much more attention than scrape flutter. In
fact, tape recorders were used for music recording for
nearly 20 years before the first transport with low scrape
flutter was introduced. Even today designers of both
transports and tapes treat scrape flutter more as an after-
thought than as a primary problem, failing to quote any
specifications for scrape flutter performance. Unfortu-
nately for the user, the subjective evaluation of theclarity of a recording is very dependent on the amount
of flutter in all three flutter bands.
Weighted peak flutter is an attempt to characterize a
human listener’s perception of flutter. Many years ago,
numerous tests showed that the test subjects most
readily identified flutter disturbances that occur at a rate
of approximately 4 Hz. Furthermore, the tests indicated
that the listener responded to the peak levels of flutter,
even though the peaks may have been infrequent. Based
on these test results, flutter meters now include band-
pass filters peaked at 4 Hz and quasi-peak metering.
Today, every professional tape recorder produced in
the past 35 years includes components to reduce scrape
flutter, but the typical weighted peak flutter meter is
totally incapable of measuring these components to
verify proper performance!
In addition, misbehaving servo-controlled transports
can generate flutter frequencies at virtually any
frequency. Unlike the older machines with all their
mechanical resonances below 100 Hz, newer machines
can have servo oscillations well beyond 1 kHz.
The entire flutter spectrum should be measured,
especially when performing maintenance testing of
professional audio recorders.28.2.3 Tape TensioningMagnetic recording tape, like all elastic media, must be
stretched slightly to produce tension within the tape. For
normal recording applications, the tape is stretched
approximately 0.1% to achieve a typical tension of 4 oz
per ¼ inch of tape width. Since this small amount of
stretch is less than one tenth the level of stress required
to permanently deform the tape, no permanent deforma-
tion results.
Four separate and often conflicting functions are
performed by tape tension on a tape recorder:- Tape tension holds the moving tape firmly against
the record and playback heads to achieve good
high-frequency performance. - Tension stiffens the tape on the tape guides so that
the tape position will remain constant. - Tension controls the stacking of the layers of tape
on the takeup reel. - On machines without pinch rollers, the tension
holds the tape against the capstan to create enough
drive traction for proper tape speed control.
The classic tape transport of Fig. 28-3 utilizes the
supply reel spooling motor to generate tape tension over
the heads in the Play mode. The supply motor is ener-
gized in the clockwise (rewind) direction with a reducedFigure 28-6. Flutter bandwidths.10.000.010.0020.0030.0040.00ABC DEF0.2 1 10 100 1K 5K
“B-C” is weighted, “A–E” is unweighted, “D-F” is scrape,
and “A–F” is wideband. (Ideal response data graphed
using Audio Precision System One Software.)