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iv accompanying illustrations shows the elements of a Radiophoto ma- chine — the rotating cylinder, the lead screw, and the carriage mounted on the lead screw which carries either a scanning or a re- cording head, depending on whether the equipment is for sending or receiving. Once the copy to be ti-ansmitted is broken down into varying elec- trical current* the problem is to convey this intelligence over the radio circuit. In early systems the varying current from the scanner was used to control the length of a series of dots sent out by the trans- mitter. When the subject scanned was white, the dots were shortened to pin-points, and when the sub- ject was black the dots were elon- gated until they almost overlapped. This Constant Frequency Variable Dot (CFVD) system, developed by RCA, produced a finished copy sim- ilar in appearance to the ordinary halftone used in newspaper re- production and which, if viewed through a magnifying glass, would appear as a number of small dots of varying weight. New System Evolved While this system gave fairly' good results, there was much to be desired and with the improve- ment in radiophone circuits, a new system known as Sub-Carrier Fre- quency Modulation (SCFM) was developed. The operation of this system can be understood if we think of the transmitter as a tele- I'RINCIPAL ELEMENTS OF THE MODERN RADIOPHOTO SYSTEM ARE THE ROTATING CYLINDER, THE LEAD SCREW (CENTER HOD OF THE THREE) AND THE SCANNING i)R RECORDING HEAD IN THE BLACK BOX. phone into which we might whistle tones of various pitch or frequency. When the scanner is on a white portion of the copy being trans- mitted, a relatively low pitch is be- ing sent out on the radio circuit and when the scanner sees a black por- tion, the pitch rises. Shades be- tween pure black and white are represented by an intermediate pitch. This varying pitch tone when received at the receiving station is used to control the brightness of the recording lamp focused on the film or paper rotating on the cyinder. The choice of the upper and lower pitch tones representing black and white and referred to as "limits" has been farily well standardized at 1500 cycles per second for white and 2300 for black. The 800-cycle difference is about the same as that used for frequency shift telegraph keying so that ultimately the same equipment can be adapted for both telegraph and Radiophoto transmis- sions. One might think that in such a Radiophoto system the speed of handling might be increased by merely increasing the speed of the rotating cylinder. This is true, how- ever, only to a certain extent for there is another factor known as keying speed which is definitely dependent upon the radio circuit. In general the greater the detail or intelligence, the higher the keying speed and the better the quality of the radio circuit required for its transmission. For analysis we can consider that the picture being transmitted is composed of very small areas or squares, each square equal in width and height to the width of a scan- ning line or about 1/100 of an inch. (Continued on page 31) PRESENT RADIOPHOTO SYSTEMS CAN TRANSMIT AND REPRODUCE ILLUSTRATIONS SUCH AS THIS, RAPIDLY, CLEARLY AND WITHOUT DISTORTION. [RADIO AGE 15]