Projection engineering (Sept 1929-Nov 1930)

Page 18 Projection Engineering, September, 1929 STANDARD AUDIO TRANSFORMERS AF3-AF4-AF5 PLATE a. nl/o GRID PUSHPULL IliTERMEDIATE TRANSFORMERS AF3C-AF4C-AFSC PLATEom yOGRID >GRID reiAs PUSHPULL INPUT TRANSFORMERS AF5CC PLATE (Km /O GRID .GRID BIAS PUSHPULL OUTPUT TRANSFORMERS 0P4CC 0>-r°'?r < STANDARD OUTPUT TRANSFORMERS OP 1OP 2 -OPS PLATE PICK-UP MATCHING TRANSFORMERS AF4 CORE -AF5 CORE Jlf'. •SEC. o) 1"AUDI0 TRAMS PUSHPULL OUTPUT TRANSFORMERS OP4C-OP7C-0P8C PLATE ( MICROPHONE TRANSFORMER CHOKE COILS B1-B2 -B3 SPECIAL MULTI-RATIO STANDARD AND PUSHPULL OUTPUT TRANSFORMERS ALSO FOR TUBE TO LIHC AliD LINE TO TUBE — PLATE fc> T. t 0 1-6/1 < ° 2-7/1 L PLATE PI ATE ' Fig. 3. Showing, in schematic form, a group of transformers well suited to meet the special requirements of audio-frequency transmission lines, etc. (Courtesy of Ferranti, Inc.) power work represent complex networks for use at voice frequencies and are most effective filters in some cases. Proper and enjoyable reproduction of music calls for a frequency range of from fifty to five thousand cycles but the high quality reproduction of speech requires that the range be extended in the upper register to much higher on the scale. Recent advancements in wax recording make the transmission of frequencies in excess of eight thousand cycles a necessity if full advantage is to be taken of the new standard of quality and if the aspirates and sibillants of the spoken word are to be fully evident. "As silent as the 's' in 'Talkie' " is a recent "wise crack" amongst the Broadwayites — and it's no joke from the engineer's point of view because it's all too true in the majority of cases. Proper matching of the impedances of the different elements of an installation will take care of that — but the other frequencies must appear in their proper relations one to the other and to maintain an over-all response curve that is essentially flat. Peaks in the curve may be removed through the use of resonant circuits in shunt with the transmission line and tuned broadly to the offending frequency, the degree of absorption being made variable through the use of a variable resistance in series with the trap circuit, as shown in Fig. 4. The frequencies to which such trap circuits must be tuned are determined through the use of a beat-frequency oscillator and vacuum tube voltmeters. The run is coordinated on log paper, the frequency in cycles against the level in db's so as to evidence the true audibility characteristics of the system. The schematic set-up for such a run is given in Fig. 5 and is self-explanatory. Frequency Runs Beat-frequency oscillators are rare beasts and are not to be found in the average small laboratory. In as much as the sound installations in theaters are mainly concerned with operation out of a magnetic pick-up an alternate and simpler method is shown in Fig. 6. Constant velocity records covering the entire range of frequencies are now available and with the aid of a single vacuum tube voltmeter may be used to obtain a comprehensive curve of any amplifier system. Such a curve may be plotted in db's above or below the output obtained at some arbitrary frequency Equalizer circuit to neutralize peak at 1000 cycles (400 cycles, for example). It must be remembered that these runs are effective only up to the speakers and that over-all runs including the speakers are impractical save in well equipped laboratories — and even then are of a tricky nature. I think that this covers the field fairly well. In most of the installations that the author has been called into consultation on the outstanding trouble has been the utter absence of any attempt to equalize or match the line between the amplifier and speakers. In nearly every instance of this kind the substitution of a logical transformer for those furnished with the speaker at the termination of the line has resulted in understandable speech as opposed to unintelligible mouthings. These few words are not to be construed as authoritative as the subject is too profound for the mention of more than a few practical facts. For those wishing to delve deeper into the subject the author suggests the purchase of "Transmission Circuits for Telephonic Communication," by K. S. Johnson, of the Bell Telephone Laboratories, published by Van Nostrand. CANNING SOUNDS INDUSTRY -A NEW THERE is nothing decidely new or brilliant about referring to a phonograph record plant as a canning factory. However, when it comes to canning miscellaneous sounds which have nothing to do with building up a library of recognized music, we have something decidedly new, and that is precisely the present status of a present development in the phonograph industry. "Imagine if you will," states J. E. Smith, President, National Radio Institute of Washington, D. C, "a plant devoted to canning all kinds of sounds. Yet that is precisely what is being done today by certain phonograph record companies. The rapid growth of the 'talkies' has led to a healthy demand for all kinds of sound effects. Thus the phonograph companies have prepared special sound libraries, comprising hundreds of records of sounds. There are barking dogs, crying babies, mob yells, hotel lobby backgrounds, sewing circle backgrounds, railway train, fire engine, steamship whistles, church bells, artillery fire, machine guns in action, tanks soldiers marching, etc., etc. These records are intended primarily for small theatres which cannot afford the synchronized equipment of the regular 'talkies,' but desire to employ records for incidental music, and the required sound effects." OSCILLATOR AMPLIFIER LOAD T V.T. VOLTMETER CONSTANT AMPLITUDE .' RECORD LINE HH i EQUALIZER F|Q 5 V.T. VOLTMETER .AMPLIFIER -OAD LINE Lr— ' hH HIGH QUALITY PHONO. PICKUP EQUALIZER FIG. 6 V.T. VOLTMETER Fig. 5. Set-up, with beat frequency oscillator, for making frequency runs on transmission lines. Fig. 6. An alternate method for making frequency runs, using a constant amplitude "frequency" record.