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B.1
APPENDIX B
EFFECT OF CHANGES IN SLIT HEIGHT ON SLIT LOSSES
The slit has a finite height that cannot be reduced without a simultaneous reduction in the light
output and, thus, the electrical output of the system. The exciter lamp supply output could be
increased in an effort to compensate, but this would shorten the life of the lamp. Equally, the
cell preamplifier gain could be increased, but this could cause unwanted hum and noise. The
slit acts essentially as a high-frequency filter that has a sharp roll-off to a null at the specific
frequency at which the slit height is equal to a recorded wavelength. High-frequency roll-off
of the optical cell output is dictated by this slit loss that is, in turn, a function of the wavelength
at which these physical factors are equal. The resulting cancellation frequency varies almost
exclusively with the height of the slit. The shape of the roll-off curve is essentially independent
of the height of the slit; only the cancellation frequency depends on it.
Unfortunately, a conventional treble control cannot compensate for the slit loss characteristic
because of its fixed turnover frequency and the gradual slope of the curve. What is needed
is a curve that precisely complements the slit loss function by the provision of a boost that can
be shifted in frequency to compensate for various slit heights. This is provided by the Dolby
optical preamplifiers contained in the Cat. No. 240A.
The figures below show the slit losses at the indicated frequencies and the equalization circuit
characteristic for slit heights from 0.00075 to 0.00175 inch (0.018375 mm to 0.042875 mm).
Frequency in Hz
5,000 10,000 20,000
Increasing
slit height
0dB
–4
–10
+10
0dB
+4
Slit height
in inches
Slit Loss
Slit Loss Equalization