Handbook for Sound Engineers

1020 Chapter 27

and the bars are observed under a fluorescent or neon
light source fed from the normal ac lighting circuits.
When the speed of the turntable is correct, the black
bars appear to stand still. If the table is turning too fast,
the bars speed up and drift in the direction of rotation.
When running slow, the reverse takes place. Strobo-
scopic bars may be painted around the rim of a turntable
and illuminated by a 115 Vac neon light mounted close
to the table edge for constant observation. The equation
for calculating the number of bars on a 60 Hz strobo-
scopic disc is

(27-3)

where,

f is the frequency of the strobe light used to observe the
bars,

rpm is the speed of the turntable in revolutions per
minute.

Starting Time. Starting time is the time it takes for the
platter to reach its operating speed from a complete
stop. This time period is important to know for profes-
sionals who have to begin playing the song or selection

at the exact moment. To check the starting time requires

either a stop watch or timing device and a strobe disc or

the test record. As soon as the lines on the strobe disc

appear stationary, the turntable has reached its operating

speed. In playing the record test tone, the pitch changes

as the correct speed is attained. Starting time may vary

anywhere from a fraction of a second to two or more

seconds, depending on the construction of the turntable.

Turntables used by disc jockeys have to start as fast as

possible without overshoot, which means that the speed

should not, even for a moment, exceed the desired

speed. If this overshoot occurs as the program material

is already being transmitted, the variations of the speed

will be most objectionable.

Acoustical Noise. The third test concerns the acoustical

noise the motor and the turntable are producing.

Normally, this test can be easily performed in a quiet

listening room when everything is turned off and only

the turntable is energized. If the turntable noise is

clearly heard and it overshadows the normal room

noise, turntable drive is below an acceptable perfor-

mance level. A second part of the same test is conducted

when the turntable is turned off and the system is

bars^2 f^60

rpm

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Figure 27-5. A stroboscopic disc used for checking the rotational speed of a turntable. Courtesy Fairchild Recording
Equipment Corp.