Radio broadcast .. (1922-30)

NOVEMBER, 1927 HOW THE COOLEY "RAYFOTO" SYSTEM WORKS 25 vented by the use of low-resistance batteries, a very large condenser across batteries of moder- ately high resistance, or by the use of inde- pendent batteries for the audio amplifier. The first pictures transmitted will contain sufficient contrast so that imperfect amplifying charac- teristics will not appear very noticeable. Never- theless, the progressive experimenter should try to keep one step ahead of the game. The plate current drain on the B batteries due to the Cooley "Rayfoto" printer will be about 10 or 15 milliamperes, so that the total current drain of the printer and an ordinary five-tube receiver will be in the neighborhood of 45 mil- liamperes. However, this additional drain of 15 milliamperes will only be present when the printer is being used and, since it will not be operated for long periods at a time, an ordi- nary set of B batteries should be good for many months of service. A total voltage of about 200 volts is required. Naturally an amplifier that can be operated without oscillating is much more efficient than one that tends to oscillate and which therefore requires the introduction of some loss to prevent oscillations. In many cases, however, it is more convenient to use an amplifier we already have and which can be "doctored" up a little to make it serviceable for " Rayfoto" work. A resistance across the secondary of one or more of the trans- formers will prevent the amplifier from oscillat- ing. The required resistance may vary between 100,000 ohms and 2 megohms. Many broadcast receivers have sufficient am- plification in their own system so that additional audio amplification is not necessary. You may test out your receiver in the following manner to determine whether any additional amplifica- tion is required to operate the recorder: Place a milliammeter in the plate circuit of the last amplifier stage; cut the current down to 0.2 milliamperes by increasing the C battery poten- tial; short-circuit the loud speaker terminals; then tune-in a local broadcasting station. If the milliammeter jumps up over 15 milliamperes, no additional amplifier stage is needed. Even if it only goes to ten mils, it will not be necessary to use the added stage but this amount of current will allow only a very small margin of safety. If an added stage of amplification is required, a special transformer should be used, one that is capable of operating without saturation and which will not produce oscillations in the audio system. Special transformers for this work will soon be available. The Cooley "Rayfoto" printer is the device for producing the corona discharge that affects the photographic recording paper. It converts the received audio-signal into a fluctuating source of light corresponding to the transmitted signal. This unit consists of a modulated oscillator feed- ing a corona coil. The corona discharges are secured from the high-voltage side of the corona coil secondary winding. Readers may wish to have some explanation of the nature of the corona we refer to here. Visually, the corona discharge at the needle point riding on the paper is a small spray of blue sparks similar in appearance to those produced by a violet-ray machine. This discharge occurs when a difference of potential of 13,000 to 26,000 volts per centimeter (which, incidentally, won't hurt you) exist around the needle point. This potential is produced by the radio-frequency amplifying transformer, known as the corona coil. The pri- mary of this coil is part of a vacuum-tube oscilla- tor operating at a frequency of 333 kc. (about 900 meters). The plate circuit is supplied by the sig- nals from the radio receiver. After being amplified to supply enough power to the modulation trans- former, these signals are strong enough to produce a strong corona discharge when strong signals are received. For the sake of efficiency and shading, about one hundred volts of direct current is sup- plied, in series with the modulation transformer to the plate of the oscillator. This boosting voltage must not be sufficient to produce a corona that will print when weak signals are coming through. The oscillator of the "Rayfoto" printer radi- ates for some distance if the frequency is high, and to prevent such interference we have chosen the reasonably low frequency of about 333 kc. We do not recommend an oscillator frequency corresponding to more than this unless careful shielding is used. The "Rayfoto" recorder is the mechanical unit of the system which consists of the re- ceiving drum driven by a motor and controlled with a "stop-start" system of synchronizing. A screw feed arrangement feeds the corona needle along the drum as it revolves so that the needle moves along at approximately the same speed that the converter drum as the transmitter moves along its shaft. The "stop-start" mechanism consists of a slip clutch between the motor drive and the drum, and a trip magnet arrangement that stops the drum at the end of each revolution until the synchroi.izing impulse is received. This impulse trips the armature of the magnet which operates through a relay. By this system, the transmitter and receiver are synchronized about twice a second, thereby eliminating much delicate and expensive synchronizing apparatus. The recorder drum is the same size as the one at the transmitter but since the recorder has a slight "lead," that is, runs slightly faster than the converter drum, the received picture will be stretched out a small amount, depending upon the amount of lead. To compensate this, the gears between the drum and screw feed shaft will be of such a ratio that the needle will feed along a little faster than the transmitter drum so that the proper proportions are restored. As a result the received picture will be slightly larger than the one transmitted. It is desired to keep the lead as small as possi- ble so as to prevent excessive stretching of the picture. Also, if the lead is too much, the trip magnet may be tripped from a subject signal instead of the synchronizing signal. If the lead is too small, the synchronizing signal may be re- ceived before the recorder drum has finished its revolution and has switched the relay in the circuit. Consequently the relay will be operated by the next strong subject signal. Complete constructional data for a Cooley Spot Of lighten picture " Ra y foto " receiver which may easily be made at home is scheduled for next month's RADIO BROADCAST. Lens to focus _- • lighten picture Glass prism to change direction of light _ Photo-electric cell- Output signals from cell consist of audio- freqency currents of varying amplitude. Frequency is determined by speed of disk," and amplitude by shading of picture To amplifier and radio transmitter Light is broken up by passing through holes in revolving disk. Frequency determined by speed of disk Picture being transmitted "Converter" drum moves^downward along threaded shaft y 8 o every revolution -- Holes in disk interrupt light from lamp L Motor drives disk and drum A PICTURE DIAGRAM OF THE COOLEY TRANSMITTER This drawing shows graphically how the Cooley picture transmitter produces electrical sig- nals varying in strength in accordance with the shading of the picture being transmitted