Handbook for Sound Engineers

Magnetic Recording and Playback 1049

of the tape to give us the desired 4 ounces of tape
tension, then the outgoing capstan must have a surface
velocity 0.1% higher than the incoming capstan. This
can be achieved by using two hysteresis synchronous
motors with slightly different capstan diameters. The
Gauss high-speed tape duplicators used this technique
with great success. A very similar technique is to use a
nonstretching plastic belt to couple the drive motor to
both capstans. If both capstan shafts are identical, but
the pulley on the outgoing capstan is 0.1% smaller than
the other pulley, the desired speed differential will be
realized.
Many dual capstan cassette decks provide bidirec-
tional operation. A simple trick is to use an elastic
rubber belt to drive both capstans, Fig. 28-9. Since the
belt is elastic, it will stretch slightly whenever it delivers
a pulling force to a load. It must pull on the incoming
capstan’s pulley with sufficient force to overcome the
holdback tension, the friction due to the incoming pinch
roller and capstan’s bearings, and the load caused by the
elastic deformation of the pinch roller at the point of
contact with the tape and capstan. As a result, the
stretched belt leaving the incoming pulley will have a
slightly higher linear velocity due to the stretching. As
this stretched belt passes around the outgoing pulley, the
higher linear velocity will turn the outgoing pulley
slightly faster than the incoming pulley. The difference
in speed generates the desired tape tension. Everything
is symmetric, so if the motor is reversed, both the tape
and the tape tensioning will reverse.

These dual-motor and single-motor designs are both
classified as tight loop or closed loop tape drives.
Closed tape loops were first developed for the very
high-performance recorders used to gather telemetry
information from rocket testing. If the capstans are free
of flutter, these designs can yield low mechanical flutter
and wow since the heads are isolated from spooling
disturbances. As a bonus, a closed loop tape path yields
low-scrape flutter because only a short span of tape is
free to vibrate at the heads.

Yet another closed loop design, the 3M Isoloop™,

achieves the effect of two capstan diameters by using a

single capstan with multiple alternating rings of large

and small diameters. The step between rings is so small,

on the order of 0.1%, that specially contoured pinch

rollers can press the tape against the smaller-diameter

rings on the incoming side and against the larger-diam-

eter rings on the outgoing side of the capstan, Fig. 28-10.

Unlike recorders that derive tape tension by control-

ling torque on the spooling motors, the tension of the

closed loop drives varies slightly with tape thickness.

Since the change in tape length is always constant,

lower tensions are generated in thin tapes that stretch

more easily. This decrease in tension is generally unno-

ticed since the thinner tape conforms more readily to the

face of the heads, offsetting any pressure reduction.

28.2.3.2 Spooling-Motor-Derived Tensioning

The classic tape transport of Fig. 28-3 experiences a 2:1

tension change from beginning to end of reel. For nearly

25 years the recording industry was forced to struggle

with recorders that had this doubling of tape tension,

with attendant speed variations, splicing problems, and

tape guiding variations. The advent of economical inte-

grated circuits has led to more sophisticated designs that

replace the constant holdback torque with an active

tension servo control.

Tension servos fall into two categories—closed loop

and open loop, Fig. 28-11. The closed loop servos

directly sense the tape tension with a spring-loaded

surface in contact with the tape. The tape pushes against

the spring, causing a displacement of the sensing

Figure 28-9. Bidirectional dual capstan drive.

Supply

reel

Takeup

reel

Capstan

motor

Outgoing

capstan and

pinch roller

Ingoing

capstan &

pinch roller

Tension ingoing

Tension ingoing +

Tension outgoing

Figure 28-10. 3M Isoloop¥ drive.

Incoming

capstan idler

Capstan

Tape

Outgoing

capstan idler

Reversing idler

R P