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SELECTED BIBLIOGRAPHY 201
distortion added by the extra transfer steps the following procedures will be necessary.
(1) Equalize only partially for the high-frequency losses introduced by re-recording with response-frequency characteristics that are "boosted" at the high-frequency end of the spectrum. The frequency of maximum amplification will customarily lie located somewhat lower than the upper frequency cutoff ; the practicable amount is limited to some 12 db. It is not practicable ordinarily to equalize more than this amount because of the excessive noise and excessive distortion that would occur as a result.
(2) When re-recording, reduce the frequency range by the use of appropriate band-pass filters. The range should be reduced in full steps in accordance with Table XIV. If excellent control of processing is present, a reduction of one full step may be sufficient; for more conventional control, a reduction of two or three steps will be needed. (For example, if the original was recorded with range F — 110-5300 cps — the range would be limited to range G — 130-4400 cps — for excellent control, to range H — 160-3600 cps — or ranges / or J for more conventional control.) Much depends upon the intermodulation distortion present.
(•>» Limit the working signal-to-noise ratio by introducing compression of the volume range. Assuming a 25-db signal-to-noise ratio for an original 16-mm speech sound record, a compression of some 5 db will usually be sufficient for excellent process control with projection of the print under excellent acoustical conditions, with an excellent sound projector ; and some 10 db for more conventional control with projection of the print under more conventional acoustical conditions with more conventional sound projectors.
In considering the corrections to be applied, it is reasonable to ask how the optimal amounts of each of the above corrections may be determined. Unfortunately, no simple rule-of-thumb is available, since the relationships between the objective factors and the subjective factors have not been determined statistically. For this reason the criterion must be subjective; it is merely "the method that produces the most pleasing print for the reference projector used under the reference projection conditions." Even if this criterion were satisfied accurately, the variation in performance of commercial projectors and the variations in auditorium characteristics (such as size, noise, reverberation, etc.) are often large enough to preclude satisfactory operation over such wide ranges.* Satisfactory operation presumes modern, well-maintained projectors together with an auditorium where ordinary conversation can be maintained at typical loudspeaker-to-listener distances.
Selected Bibliography Fletcher, H., Speech and Hearing. Van Nostrand, Xew York, 1929. Pender-Mcllwain, Electrical Engineers' Handbook — I', Electrical Communication and
Electronics. Wiley, New York, 1936.
*The acoustic performance characteristics of 6 commercial projectors are shown at the end of Chapter XIII.