Broadcasting (Oct - Dec 1945)

Engineer Finds Magnetophon Superior TO THE BROADCASTER, one of the most intriguing inventions to come out of the war was the German Magnetophon. To be accurate, this device came before the war, although its applications in Germany were divided between the Reichsrundfunk (RRG) and the military services, both ground and air. The earliest commercial reference to the Magnetophon I have found is an announcement by AEG (the German General Electric Co.) dated October 1938, but I have been informed that Dr. von Braunmuhl, the chief of the RRG central technical bureau, and AEG, were working on this device four to five years earlier. BBC's Similar Unit At this same time, the BBC and Marconi Ltd. were perfecting a similar machine, using a magnetic tape for recording. The BBC machine, now in use at Broadcasting House, utilizes a steel tape wound on drums about 24" in diameter, and fed through three heads, in the same manner as the Magnetophon. The BBC device, however, is large, tape breakage necessitates a welded repair, and a complex Thyatron motor control system is used to maintain constant tape speed. The whole device is almost the size of an upright piano: the Magnetophon is not much larger than a modern console-type radio. Principal of operation of the Magnetophon is the same as the wire recorder. The difference, and of particular interest, is that the Magnetophon gives high fidelity, noiseless, life-like reproduction of speech and music. Tape breakage is rare, and splices are inaudible. The tapes can be reused thousands of times, easily stored and classified, occupy little space; they can be easily edited and spliced for program building, and two machines can give continuous programming. (Average tape time is about 22 minutes, although as much as 28 minutes can be crowded on, but is not advisable due to "drag" on the motors.) The Magnetophon tape is an Acetylcellulose tape, 40 microns thick, 6.5 mm wide, impregnated with pulverized iron oxide. This applies to the latest, or Type L, tape. The earlier Type C tape was thicker, and the oxide was applied to one side of the tape to a thickness of 15 microns. The frequency response of the C tape was above 9 kc, and limited in most models (such as the portable), to 6 kc. The L tape extends above 10 kc, with the latest model machines giving a response from 30/50 to 10,000 cycles, 5% distortion (max.), with a quality notwithstanding the good frequency response, that makes the listener think he is standing beside the speaker or is present in the concert hall. It is known that the majority of German Recorder Combines Economy, High Fidelity, Compactness By DON V. R. DRENNER Chief engineer for AFN and Radio Luxembourg Copyright 1945, by Broadcasting Publications, Inc. BROADCASTING'S first story on the Magnetophon appeared Sept. 3, datelined Berlin. Since then, a few of the machines have been brought to this country where they are being examined by the Signal Corps and the Dept. of Commerce. The accompanying exclusive story is by DON DRENNER who worked with the Magnetophon in Europe. Mr. Drenner has an interesting background. He started at KGGF Coffeyville, Kans. Beginning in 1941 when he enlisted in the RAF as radar engineer, his accomplishments have included: running a studio for BBC, chief engineer for AFN, chief engineer on construction work with ABSIE with OWI, chief engineer for Radio Luxembourg, chief technical officer to SHAEF Mission to Netherlands. He has erected the transmitters at Cherbourg, Frankfort, and supervised construction of transmitters throughout Europe. Discharged from the service, he is back at KGGF, announcing, engineering, "writing a couple of novels, publishing some poetry, and trying to finish a symphony started overseas." RRG programs were recorded on Magnetophon tape and then played on the RRG senders, during the entire war; an obvious security measure, but one which, even to the best ear, went undetected. Functions of Magnets 1 The tape is fed over three "heads", or magnets. The first head performs the pre-magnetizing. In the earlier models, and in the current portables, this pre-magnetizing was done at D.C. potentials. (One exception to this is the small air-borne model for use in bombers, which utilizes a high-frequency oscillator, as in the latest large models.) According to current theory, this premagnetizing arranges the molecules of iron in a certain pattern. MAGNETOPHON in operation (r), with the tape about two-thirds played. It is winding on the left hand plate. Below (1), machine in foreground was used for sign-on and station breaks at Radio Frankfurt. Picture at lower right shows the two Magnetophons installed at Frankfort. The U. S. Army sergeant was formerly with George Davis, Washington engineer. The second head is for recording. In both the D.C. and the later highfrequency models, this head is of circular laminations of high-permability high-frequency steel, with dual winding cross-connected to cancel stray currents. In the D.C. model the magnetizing source is a medium voltage (about 80 v.) sup plied from a small power supply, and the recording current about 1.2 MA supplied by the recording amplifier. In the A.C., or high-frequency, model, the pre-magnetizing source is fed at 40 kc, at about 120 MA, and the recording current remains 1.2 MA, superimposed on an alternating current provided by an oscillator at 80 kc. Hysterisis losses are less, and the response is much improved with the high-frequency method. Play-Back Head The third head is the play-back head, and is similar in construction to the recording head in that it is also of circular laminated steel possessing the same high-frequency characteristics, and two windings. The electrical characteristics differ, however, and the output voltage from this head is much greater than the recording head. In earlier models the three heads were arranged in a circle, and the selection of which head was brought to bear upon the tape was controlled by the selection of the app ropriate action, "recording" "playback," etc. In later models the three heads are "in line" and the tape runs over their faces at all times. This increases the wear on all heads, two of which are obvi(Continued on Page 84) Page 36 • November 19, 1945 BROADCASTING • Broadcast Advertising