Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST dubbed the "Spirit of Television." Charts with various checkerboard meshes give amplifier characteristics at a glance but a cross or plus sign is usually used as a preliminary test to facilitate synchronizing receivers. The optical system continues through either a lens from an old post-card projection lantern or a reading glass, the latter being preferable as it has a somewhat shorter focus. With this lens the image is projected directly on the transmitting scanning disc upside down and with its lefts and rights reversed. Therefore, it is projected on the bottom of the disc which scans in a counter clockwise direction with the spiral progressing upward or inward to conform with the rules given for standardizing television receiver disc scanning. While the l-J inches square surface at the bottom edge of the disc acts like a projection curtain, somewhere in the picture area there is a hole moving across this projected image and letting light through to the lightsensitive photo-electric cell mounted directly behind, that is, when the hole is in a part of the picture that is bright. For every turn of the disc each part of the image is analized by the scan holes and the resistance of the photoelectric cell is varied in accordance with the intensity of light reaching it. The Scanning Disc T^HE scanning disc used for the receiver is J the same as for the transmitter except that holes are drilled with a size 40 drill in the transmitter disc and a size 44 drill for the receiver. The disc may be made from a flat brass plate 0.05" or more thick, or of somewhat thicker aluminum. From the center of the brass plate a 6-inch radius circle is drawn and sub-divided into 24 equal parts from which points lines are scratched to the center. Starting about | inch in from the edge of the onefoot diameter circle a scratch is made on each radius line iV inch nearer the center for each radius line until the 24 radii have been scratched. The points located in this manner determine the positions of the holes of the spiral. In order to increase the initial electrical signal energy above the noise level a considerable degree of overlap in the track of adjacent holes seems permissible in the transmitter disc, but the same overlap in a receiving disc would give a streaked picture, hence the different drill sizes. The holes in the disc shown in the picture are nearly a tenth of an inch in diameter, and a gain equivalent to considerable amplification is the result. WhLe silhouettes are comparatively easy to transmit because the intense light shining directly into Disc the photo cell, working with reflected rays, from the face at a distance, the light is so weak in its indirect reflected path as to produce only a very minute current in the cell. The signal in the first few stages of amplification may be easily buried under noises such as filament emission, vibration from the scanning motor, noisy B, C, or A batteries or connections, stray feedbacks, audio regeneration, a.c. induction, noisy grid leaks, plate resistors or grid blocking condensers, etc. Too much emphasis can not be put on using good parts in the first few stages at the same time obtaining a rapid gain in signal level above stray pick-up noise. In order to gain this end one of the first amplifiers built by the writer for this purpose used transformer coupling in the first two stages and between the cell and first tube input, but in spite of considerable shielding enough a.c. was induced from the motor into the windings of the transformers to spoil the pictures. Therefore, resistance coupling was resorted to and finally the advantages of the screengrid tube were used in a resistance-coupled amplifier with surprising improvement. A theoretical voltage amplification of sixty per stage is obtained, which compares well with transformer coupling, and better quality results. The disadvantages of stray induction coupling, even with only moderate shielding, are done away with and the only new difficulty introduced seems to be that only small signal potentials can be handled by this type of tube because the maximum grid swing is only about a volt and a half. However, for amplifying the very weak photo cell currents up to a moderate workable volume where one can be sure of their existance, the screen-grid tube seems admirably well fitted. Two stages are shown in the diagram although three have been used where the reflected light intensity was unusually weak. However, when three stages are used considerable isolation of battery circuits and shielding is necessary in order to realize anywhere near maximum gain. Indeed, even with the diagram shown, considerable juggling was necessary to get rid of assorted howls and squeals. In this connection it must be remembered in placing and shielding the amplifier that the resistance-coupled stages may pick up, amplify, and overload on radio frequency from the transmitter. Also, it should be mentioned that there seems to be a scarcity A close-up view of the transmitting scanner and television amplifier of amplifying transformers that will handle heavy plate current and volume (such as between a 7f-watt amplifier and a 50-watt modulator) with good response up to 9000 cycles. Transmitting Circuits IN THE matter of oscillatory circuits, any of them work but those that shift wave with different plate voltages should be avoided as they do this in modulation. Consideration should also be given the degree of cutting off of the side bands farthest from the carrier, representing the best image detail, by sharp tuning in amplifier stages as well as degree of modulation efficiency. The diagram on this page shows the complete transmitting circuit used by the writer in his experiments with amateur television. The television experimenter will at times, no doubt, loose patience with his results, or lack of results, and may even tend towards profanity. The Bible is often a great soother of sorrow and the television experimenter has not been forgotten therein and may get comfort from Habakkuk II, 3, which reads, "'For the vision is yet for an appointed time, but at the end it shall speak, and not lie; though it tarry, wait for it; because it will surely come." A complete schematic diagram of the apparatus used in the amateur television experiments • february, 1929 . . . page 249 •