User`s manual

A-1
APPENDIX A
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. 510.
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 10,00020,000
Increasing
slit height
0dB
–4
–10
+10
0dB
+4
Slit height
in inches
Slit Loss Cat. No. 510 Equalization