Projection engineering (Sept 1929-Nov 1930)

Page 28 Projection Engineering, September, 1929 as the neon lamp was available. The decision finally made was to use another one of the noble gases — argon — which has a very considerable number of emission lines in the blue and green region of the spectrum. Two argon lamps are employed, one with a blue filter to transmit the blue lines and one with a green filter transparent to the green lines of its spectrum. These argon lamps unfortunately are not nearly so bright as neon lamps and it was, therefore, necessary to use various expedients to increase their effective brilliancy. Special lamps to work at high current densities were constructed with long narrow and hollow cathodes so that streams of cold water could cool them. The cathode is viewed end-on. This greatly foreshortens the thin glowing layer of gas and thus increases its apparent brightness. Even so it is necessary to operate these lamps from a special "I" tube amplifier to obtain currents as high as 200 milliamperes. The receiving apparatus at present consists of one of the 16 inch television discs used in our earlier experimental work. Behind it are the three special lamps and a lens system which focusses the light into a small aperture in front of the disc. The observer looking into this aperture receives, through each hole of the disc T7, " // «. V// V V X / / @ -J^ro. »rr„ « c.«s^ How the grouping of the colored filters before the color-sensitive photoelectric cells is arranged. as it passes by, light from the three lamps — each controlled by its appropriate signal from the sending end. When the intensities of the three images are properly adjusted he therefore sees an image in its true colors, and with the general appearance of a small colored motion picture. Difficulties Presented Satisfactory television in colors is a far more difficult task than is monochromatic television. Errors of quality which would pass unnoticed in an image of only one color may be fatal to true color reproduction where three such images are superimposed and viewed simultaneously. In three-color television any deviations from correct tone rendering throw out the balance of the colors so that while the three images might be adjusted to give certain colors properly, others would suffer from excess or deficiency of certain of the constituents. A further source of erroneous color exists at the scanning end. If the light from the object were not distributed equally to all the cells, the object would appear as if illuminated by lights of different, colors shining on it from different directions. Color television constitutes a definite further step in the solution of the many problems presented in the electrical communication of images. It is, however, obviously more expensive as well as more difficult than the earlier monochromatic form, involving extra communication channels as well as additional apparatus. SPEECH AND HEARING— By Harvey Fletcher, Ph.D., Director of Acoustical Research, Bell Telephone Laboratories. 331 pages. Illustrated. 6x9 inches. Stiff buckram binding. Published by D. Van Nostrand Co., Inc., Neiv York City. Price, $5.50. If the average engineer employed in the radio industry were asked something about sound he would very likely give you a more or less learned dissertation on the frequencies which he has encountered in his work and let it go at that. He would know, for example, that a loudspeaker, for effective operation, must reproduce a band of frequencies covering the musical range, with audibility. But, that is only the start of the story. Dr. Fletcher, who for thirteen years has been studying sounds, how they are made and heard, covers in this book many facts that the radio engineer should know, whether he designs sets, loudspeakers, or collects his paycheck from a broadcast station. Dr. Fletcher's presentation of his subject is most clear and interestingly written ; well illustrated by curves and tables, and where mathematical formulae are necessary for clarity, they are wisely included. "Speech and Hearing" is divided into four parts : Speech ; Music and Noise ; Hearing, and The Perception of Speech and Music. The opening chapter discusses the beginnings and the evolution of language, the functions of the lungs, vocal cords, and other organs of speech, the principal English speech sounds and their formations, artificial speech sounds — their production, and the artificial larynx. Chapter two is devoted to the recording and characteristics of speech waves ; to the principles, development, and operation of such recording instruments as Koenig's "phonoautograph," Miller's "phonodeik," the phonograph, and the oscillograph ; to the differentiation of speech sound ; the general characteristics of speech ; and to tabular data showing these characteristics. The third chapter deals with speech power. The various kinds of speech power and the units expressing difference in speech powers are defined. The power required in producing speech sounds is discussed, and something of the manner in which this power is determined is explained. The chapter closes with a section on the distribution of speech power into frequency bands. Chapter four develops the frequency with which speech sounds occur under the headings of words, syllable combinations, and fundamental sounds. Tables are included that show relative frequencies of occurrence. The opening chapter of Part Two takes up the characteristics of typical music sound waves, the operation of the electrical harmonic analyzer, the determination of acoustic spectra of musical instruments, and musical range and intensity. The second chapter affords a treatment of noise ; its physical properties, methods for its measurement, and the results obtained from noise surveys. The third section begins with an explanation of the mechanics of hearing ; giving a full description of the ear, its structure, and the functions of its several parts. The Helmholtz theory is developed with relation to recent work, and the reaction of the nerves in transmitting auditory stimuli is explained. The "threshold of audibility" and the "threshold of feeling," together with other factors governing the limits of hearing, such as feeling intensity and pitch are treated in chapter two. The applications of the Weber-Fechner law and the determination of minimum perceptable differences in sound are studied in chapter three. In this connection the work of such experimenters as Knudsen, Riesz, Wegel, and Lane has been reviewed. Chapter four treats of the masking of sounds — one by another, taking up in this respect pure tones, subjective tones, complex sounds, and the calculation of the form of vibration of the basilar membrane. The masking of sounds is a phenomenon long known to physicists, but which has been accurately measured for the first time in Bell Telephone Laboratories. Binaural phenomena, including complex sounds, binaural beats, and sound images are taken up in chapter five. In chapter six are explained the various tests used in determining the acuity of hearing. This chapter also offers a description of the audiometer, an instrument for the measurement of hearing, developed under the author's direction. A discussion of loudness, which covers loudness of sound, pure tones, complex sounds, the calculation of loudness losses, and the comparison of observed and calculated values introduces the fourth section. Chapter two explains how the pitch of musical tones are recognized. Speech sounds and their recognition, intelligibility tests, and probability applied to articulation are covered in chapter three. In chapter four the effects of change in intensity on the recognition of speech sounds are shown, together with a description of the method and apparatus used. Chapter five shows how frequency distortion affects the recognition of speech sounds, and describes the performance of tests that indicate the importance of frequencies in recognizing sounds. In the sixth chapter the influence of other types of distortion on the recognition of speech sounds are pointed out. Those considered are : the "overload" on the vacuum tubes in amateur radio receiving equipment, reverberating qualities of rooms, the variance in speed between a phonograph turntable and the turntable used for producing a given record, and the difference in the frequency of sending and receiving carriers in carrier telephone systems. Chapter seven is given to noise and deafness and their effects upon the recognition of speech sounds. Experiments and discussions are included that are explanatory and descriptive of these relations. The book is brought to a close by five appendices : the first of which relates to the calibration of a condenser transmitter through the use of a thermophone ; the second to the derivation of equation relating to the properties of a plane wave ; the third applies Hook's law to the transmission of sound in the inner ear : the fourth establishes the mathematical relation between hearing loss and the maximum distance for hearing and understanding speech ; the fifth gives a method for computing the loudness loss with a known characteristic impedance, and the sixth relates to the mathematical determination of the velocity in singlyresonant systems.