Dolby Laboratories CP65 manual Appendix B Effect of Changes in Slit Height on Slit Losses

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
	+10
	–4
Slit height	+4
in inches
	–10
	0dB
Slit Loss	Slit Loss Equalization