Handbook for Sound Engineers

Magnetic Recording and Playback 1045

from a crystal oscillator, any variations or errors in
speed are immediately detected. The control circuits use
this error to generate corrections in the voltage driving
the motor to cancel the speed error. The overall accuracy
of this closed-loop system is primarily dependent on the
accuracy of the tachometer and the reference clock.
The block diagram of a typical capstan speed control
is shown in Fig. 28-4. Commonly referred to as a
phase-lock servo, the system is, in essence, a clocked
position detector. The servo automatically adjusts the
motor voltage so that the tachometer will produce one
pulse for each pulse of the reference clock.
The crystal oscillator/counter provides a highly accu-
rate clock reference by dividing the frequency of the
crystal oscillator down to a convenient lower frequency.
The switching transition of the clock serves as a strobe
to sample the position of the tachometer. If the motor is
running exactly at the desired speed, each tachometer
transition will coincide exactly with a clock transition.
The phase comparator compares the tachometer and
clock signals to determine which signal arrives first and
the amount of timing error between the sources. The
error signal generated by the phase comparator is ampli-
fied and passed through a low-pass filter that smooths
the individual pulses into an average dc voltage that can
drive a dc motor. This smoothed voltage is applied to a
power amplifier, called a motor drive amplifier (MDA),
that drives the motor.
The phase-lock servo permits convenient speed
control at multiple tape speeds by selecting various
points along the divider chain. The minimum speed is
limited by the data rate from the tachometer and the
smoothing provided by the low-pass filter. The
maximum voltage and current available from the MDA

typically sets the maximum speed. Speed ranges of 2:1,

4:1, and 8:1 are common in audio recorders with

servo-controlled capstans.

Variable-speed operation for a servo system is much

simpler than for the hysteresis synchronous motor. A

simple variable-frequency oscillator can be substituted

for the fixed reference to provide infinitely variable

speeds!

The de facto standard for professional machines is

that an external VSO frequency of 9600 Hz from acces-

sories will drive a servo at nominal speed. This 9600 Hz

signal can be substituted for the crystal’s countdown

signal at an appropriate point in the countdown chain

before the final speed-determining dividers. The VSO

signal is thus able to control the machine at any of the

machine’s running speeds.

If the tachometer is accurately mounted, if the

tachometer samples occur frequently enough to provide

precise sensing, if the control circuit sends the correc-

tion signal to the motor quickly so that errors are sensed

as they start, and if the motor can respond swiftly to

corrections in its control voltage, then the motor will

turn at a constant speed. The string of “ifs” in the

previous sentence is a clue to the complexity of this

servo design. The results, however, of a good design are

very impressive, with professional recorders being able

to suppress mechanically induced speed variations to

below 0.05% rms at 15 in/s (38 cm/s) on a routine basis.

The speed-sensing device need not be attached to the

driving capstan for phase-lock operation. The tachom-

eter can be mounted on a free-running idler that is

driven by the tape, but the extra time delay introduced

into the error signal renders the system more difficult to

control. This delay usually requires a reduction in the

Figure 28-4. Capstan speed control block diagram.

Crystal

oscillator

Dividers

Internal 9600 Hz

External VSO

External

Speed dividers 3 3 / 4

7 1 / 2

15

30

Speed

select

Ref

Tach

Phase

comparator

Error

Filter

Nominal

Motor

drive

amplifier

Sensor

Tachometer disk

Motor

(^1) / 2 1 / 2 1 / 2

• Nominal 9600 Hz