Ampex 351 - Page 34

SUPPLY TURNTABLE
TAKEUP TURNTABLE
loss in frequency re 
Tape Transport — Detailed Discussion*
5
Supply and Takeup Systems
Usually, the tape supply and tape takeup systems
can be considered as identical assemblies, with the
only probably differences being in the brake configur 
ation and the connection to the power source. Torque
motors (or their equivalent) arc used to drive the
turntables directly. These motors are connected to
rotate in opposite directions when power is applied
— the supply motor opposing and the takeup motor
supporting the normal direction of tape motion.
the head gap. This means that the gap must always
intercept less than one complete wavelength of the
signal recorded on the tape (see High Frequency Re 
sponse). However, if the gap is too small the flux will
not be forced through the core to the coil, and signal
level will be reduced. An optimum design, tailored
to specific requirements of frequency response and
level is thus necessary.
Typical professional quality tape transport showing
the top components on an Ampex Model 300.
*From “ Multichannel Recording For Mastering Pur 
poses ” , Journal of the A.E.S., October 1960.
© !
The Drive System
The drive system utilizes a synchronous motor
coupled either directly or through a pulley arrange 
ment to the capstan. The circumference of the cap 
stan and its rotational velocity determine the speed
of the tape in the record and reproduce modes.
While tape speed is a function only of the cap 
stan. tape motion in record and reproduce is insti 
gated when a capstan idler (sometimes called a pres 
sure roller) clamps the tape between the capstan and
itself, thus providing a surface against which the
capstan can drive the tape. The capstan idler is nor 
mally coupled to a solenoid, which in turn is actuated
by the play switch. This arrangement allows a “ fast
start ” condition in which the capstan motor is operat 
ing whenever power is applied to the equipment, and
tape can be quickly brought to full speed whenever
the play switch is pressed.
Tape Transport — General
The function of the tape transport is to move the
tape accurately across the heads at a precisely con 
stant rate of speed. We can consider that all tape
transports consist basically of three major divisions
— first a tape supply system, then a tape drive sys 
tem, and finally a tape takeup system. These divisions
can be likened to two reservoirs with a pumping sta 
tion between them that removes material from one
reservoir and adds it to the other. Most professional
quality equipment employs three motors (or their
equivalents), one each for the supply system, drive
system, and takeup system; however, if weight or
volume is important (such as in portable machines)
high quality results can be obtained by using one
motor to drive the tape and employing mechanical
coupling to the supply and takeup turntables.
In the record and reproduce modes these motors
act simply to maintain proper tape tension and have
no influence on tape motion, which is controlled en 
tirely by the drive system. During this operation the
supply motor imparts tension by opposing tape mo 
tion, while the takeup motor attempts to turn slightly
faster than necessary to wind in the tape from the
drive system.
In the fast winding modes of tape travel, the reel
motors do control the tape motion. Here one motor
is operated under full power and the other with re 
duced power; the greater torque of the motor under
full power overcomes the lesser opposing torque and
tape is simply pulled from one reel to the other, again
under correct tension.
Head-to-Tape Contact
Good head-to-tape contact and proper placement
of the tape on the heads is extremely important. An
inherent characteristic of magnetic tape recording is
that the effective recording or reproducing of a signal
on magnetic tape deteriorates with any spacing be-
Flutter and Wow
Flutter (or wow) is the amount of deviation from
a mean frequency, caused by anything in the system
that will affect tape motion.
tween the tape and heads. Thus, any loss in good
head-to-tape contact will result in impaired perform 
ance — in recording there will be signal drop outs,
in reproducing there will be a '
sponse.
Tape Tracking
If the tape does not track correctly across the
heads, frequency response, phasing, and level will be
affected. Two guides will thus bridge the head assem 
bly. In professional quality equipment the positioning
of the guides will ensure good head-to-tape contact
and the accurate placement of the tape.
TENSION ARM
CAPSTAN IDLER
CAPSTAN 0139