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M, *i vs M2
^M^HP-J^/VvYAA^/^
M3
C5
C4
R2'
C7
C9
C8
C6
MI & M2, MS & M4,
& Ga,
G4,
& Ce,
INTERNAL FILTER CIRCUIT Fig. 3. Film-drive electrical analogue.
C? & Cg, Compliance
Inertia of flywheels
Inertia of upper & lower filter
arms Compliance of spring common
to both arms Compliance of arms when mov
ing together Compliance of arm-positioning
spring Compliance of film between
flywheels & sprockets
unchanged from a single-flywheel type. This is because the elements C7, C8 & C9 and R2 & R3 represent relatively small magnitudes, and MI & M2 tend to become a single flywheel as these elements decrease in value. C7, C8 & C9 are short, stiff lengths of film and, therefore, constitute small values of compliance. Likewise, R2 & RS, representing the effective damping resistance of the film friction over the heads, have relatively small values. This results from the well-known characteristic of solid or sliding friction acting at considerable velocity, as compared to viscous friction, because of their differences in force-velocity characteristics.3 This may be illustrated by removing the dashpot, Rb whereby the small remaining amount of damping caused by film friction permits highly undamped oscillation of the filter arms following a disturbance.
of film between flywheels & heads Cg, Compliance of film between
heads Si & 82, Upper & lower film drive
sprockets
RI, Resistance of dashpot damper R2 & RS, Resistance of film over heads Vi & V2, Reference film velocity at heads
The displacing force created by film friction over the magnetic heads is offset by an adjustable spring, C4. By this means the filter arms can be maintained in their correct operating positions in spite of large differences in the frictional coefficient of various film samples.
In the present design, forward-running speed of 90 ft/min is provided and the customary high-speed rewind is retained. For special applications where reverse operation at standard speed is required, a double torque-motor drive will be furnished for each film-spool spindle. A footage counter located in the central angle bracket is an added feature of the triple-track recorder.
The associated transmission equipment providing for three complete recording channels and three complete reproducing channels, operating at a nominal recording input level of —30 dbm and reproducing an output level
108
February 1952 Journal of the SMPTE Vol. 58