Radio Broadcast (May 1928-Apr 1929)

.58 RADIO BROADCAST ADVERTISER THE FINEST BROWNING -DRAKE ASSEMBLY EVER DESIGNED The new A. C. Shield Grid Browning-Drake assembly is a combination of all the most modern and advanced ideas of receiver construction. The famous Kit has been designed this summer by Professor Browning for assembly with shield grid tubes, both A. C. and D. C. For the first time this wellknown circuit has been reduced to single control, while the tickler feedback control is retained for the exceptional sensitivity for which it is noted. Complete parts list at only $59.50, the lowest list price yet reached by a kit assembly of the highest quality. Full constructional details including full scale picture wiring diagram may be obtained free on request. We have some territory open for exclusive distributors and authorized dealers handling factory-built BrowningDrake receivers. Write for our proposition on this line which brings both profit and prestige. BROWNING DRAKE CORP. Cambridge, Mass. BROWNING-DRAKE RADIO No. 235 Radio Broadcast Laboratory Information Sheet November, 1928 Television FREQUENCY BAND REQUIRED TN TELEVISION transmission a problem which must be given careful consideration is the width of the band of frequencies which must be transmitted to reproduce at the receiver end, with good quality, the scene being scanned by the television transmitter. Theoretically, a television signal contains components of all frequencies from zero to infinity. In practice the frequency band is much more restricted and depends upon various factors. The width of the band of frequencies which must be transmitted is a function of the number of elements scanned per second at the transmitter. For example, if the number of lines into which the picture is broken — which is equal to the number of holes in the scanning disc — is 50, then the number of elements into which the picture will be broken will No. of lines No. of pictures per second 10 15 20 25 3,100 4,700 6,000 50 12,000 19,000 25,000 75 28,000 42,000 56,000 100 50,000 75,000 100,000 be 50 times 50, or 2500. If we transmit 20 pictures per second, the total number of elements transmitted per second will be 50,000. The highest frequency which must be transmitted, to get good quality, can be taken as equal to half this figure, or 25,000 cycles. The table given herewith shows how the value of the highest frequency which should be transmitted varies with the number of scanning lines and the number of pictures per second. For example, a 50-line picture sent 15 times per second requires up to 19,000 cycles. A station transmitting within a broadcast band is limited to 5000-cycle modulation. Therefore, any broadcast station transmitting television programs and using a number of lines and number of pictures per second such that requires a frequency band greater than 5000 cycles must either modulate above the legal limit or suppress in the amplifiers the frequencies above 5000 cycles. No. 236 Radio Broadcast Laboratory Information Sheet November, 1928 Moving-Coil Loud Speakers THEIR OPERATION npHE important characteristic of the dynamic or, more properly named, moving-coil type loud speaker is the fact that it has a coil fastened directly to the cone, which is caused to move back and forth in an air gap in a magnetic circuit, the movements being in accordance with the frequencies flowing through the coil. The moving coil is mounted at the apex of the cone, as indicated in the diagram, and connects to the secondary of the transformer, T, the primary of which connects to the plate of the power tube. The moving coil of a well-designed unit has a fairly constant impedance over the entire range of audio frequencies and the transformer, T, is designed to "match" the coil's impedance to the output impedance of the tube. So long as the power tube works into an impedance about equal to or somewhat greater than twice the tube's plate impedance, satisfactory power transfer from the tube to the moving coil will be obtained. The instructions covering the use of one of these loud speakers should indicate what tubes or combination of tubes are recommended for use with the unit. The term "dynamic loud speaker" is not a very accurate description of a type of loud speaker whose distinguishing feature is that it has a moving coil. The word "dynamic" is defined as "mechanics treating of the motion of bodies and of the action of forces in producing or changing their motion." Since all loud speakers move they can all be called "dynamic." We have seen descriptions and adver tisements of "dynamic" loud speakers which consisted mainly of an ordinary electromagnetic unit coupled to a cone. This term used to describe such loud speakers is probably misleading to some although technically it is not incorrect. Radio Broadcast Laboratory Information Sheet November, 1928 Power Output how it depends upon impedance ratios TT HAS been proved mathematically and experimentally that a tube delivers the maximum amount of undistorted power when it works into a load resistance equal to twice the plate resistance of the tube; maximum power output, however, is obtained when the load resistance equals the tube's plate resistance. The curve on this sheet indicates relatively in TU how the power in the load varies with the ratio of the load resistance to the tube's plate resistance (sometimes called plate impedance). The X on the curve indicates where a tube is normally operated, the load resistance at the point being twice the tube resistance. We frequently see statements to the effect that the loud speaker we use must be matched to the tube to get the largest amount of undistorted power into the loud speaker. Such is the case, but the curve indicates that there can be considerable mismatching without serious loss of power. For example, even when the load resistance is about five times greater than the tube's resistance, there is only a 2 tu loss — a loss which would hardly be noticeable to the ear. It is unwise, however, to work a tube into a load resistance less than its own resistance, because under such conditions the tube's characteristic is curved (see Laboratory Sheet No. 124) and this curved characteristic introduces distortion. In cases where the element of the loud speaker has a low impedance, for example, it is necessary to use a transformer between the tube and the loud speaker to compensate the differences in impedance. A moving-coil type, i.e., dynamic loud speaker, might have an impedance of, say, 20 ohms at some frequencies, and if it is to be used with a 2000-ohm tube (171a) which requires a load impedance of 4000 ohms to get maximum undistorted power, then the coupling transformer would have an impedance ratio of 4000 divided by 20, or 200, corresponding to a turns ratio of the square root cf 200, or 14. 3 4 LOAD RESISTANCE PLATE RESISTANCE OF TUBE