Radio Broadcast (May 1928-Apr 1929)

Complete Equipment For Television' Reception Fig. 1. Zeh Bouck's Televisor Building Receivers For Television By ZEH BOUCK t; 'ELEVISION signals are transmitted to-day to a rapidly increasing number of experimenters throughout the world. The interest and pleasure associated with their reception is found in the novelty and fascination of the achievement with home-made apparatus rather than in esthetic considerations associated with the reproduction. Television signals when properly transmitted and received portray the instantaneous characteristics of the object being scanned at the transmitter. This is accomplished by interpreting at the transmitter the visual aspects of the televised object as electrical variations which, at the receiver, are reconverted into properly distributed shades and high-lights. The quality of the received picture depends upon how good a signal is transmitted in the first place, and upon how well it is reproduced at the receiving end. Here the amplifier used following the detector plays the important part. Theoretically the signal to be received should contain important frequencies from as low as the number of pictures per second to those lying far above the highest audio frequency used in music. However, television programs in the broadcast band cannot contain frequencies above 5000 cycles since this is the highest frequency at which a broadcasting station is permitted to modulate, and even when the transmissions are on short waves, the improvement in reception obtained through the use of a special amplifier going up to 15 or 20 kilocycles isn't worth the expense of constructing it — at least it doesn't appear to be worth doing until the quality of the transmissions are much improved over what they are to-day. A start can be made with any good amplifier and, after the best possible results have been obtained from it, there will be time to construct an amplifier designed especially for television reception. It is the purpose of these notes to describe the amplifier and scanning combination used by the writer. This apparatus will reproduce television images when attached to any receiver capable of delivering a signal of requisite strength from the desired transmitting station. {Continued on page 36) I JN THIS article are given descriptions of the work of two experimenters well known to readers of Radio Broadcast — Zeh Bouck and James Millen — both of whom have set up apparatus and successfully received television programs. The differences in the apparatus — especially the amplifier equipment— will give the reader a good idea of the circuits which may be adapted to present day television reception. As the art develops more stringent requirements will be set up as to the characteristic of the audio amplifier but it is evident from the data given here that for the present at least almost any good amplifier may he used. — The Editor. "NSTALLING and operating an experimental television receiver is not difficult for the average radio set builder or amateur experimenter. In this connection a description of the equipment used by the writer should be of considerable interest, especially as the same apparatus may be employed by experimenters in other parts of the country for receiving any television station by merely selecting a scanning disc of the proper type for the particular signal being received. For instance, when using a 24-hole disc the wgy 21meter television signal may be received all along the West coast. Then, there are the kdka experimental signals which require a 60-hole receiving disc. The majority of stations, however, are transmitting a 48-line picture. Any good receiver capable of being tuned to the wavelength of the desired television transmitting station may be used. The short-wave receiver employed by the writer is the standard kit sold by the National Company, all of the parts of which are available in the open market. It was described in detail on page 286 in the August, 1928 issue of Radio Broadcast. The 222-type r.f. tube is followed by a regenerative detector. This system prevents radiation— a problem which would soon become quite serious if all the short-wave receivers were of the radiating variety. In building any type receiver, especially for short-wave reception, and particularly one for television work where a motor and scanning disc are located in the same room, considerable attention must be given to rigidity of construction. This applies to the coils and their mountings as well as the wiring and other parts of the set. We find from experience that the ordinary audio amplifier, such as you use at present for speech and music, is good enough to provide picture reproduction sufficiently clear to distinguish forms, such as the outline of a hand or head, and to follow motion. This is perfectly O. K. for a starter, and your present audio amplifier can be used, provided it has at least the gain of a good two-stage transformercoupled unit. For receiving 3XK in Boston it was found ad(Continued on page 37) 35