Radio broadcast .. (1922-30)

can be stated that frequencies between 35 and 7000 cycles are successfully transmitted in this system. The audio-frequency characteristic of a typical network circuit is shown by the two curves (Fig. 2) representing the red and blue network circuits from New York to Chicago. Considering the length of this circuit, and the fact that it is made up of varying types of transmission facilities and involves a number of telephone re- peaters, the frequency characteristic from 100 to 4500 is flat within plus or minus 2 DB. This condition, of course, varies somewhat with temperature and weather conditions but in general it is typical of the circuit. Low-Frequency Cut-Off Generally speaking, both of these cir- cuits cut off quite sharply at 90 cycles, and, as stated before, this cut-off point varies from day to day within a few cycles as shown by the difference between the red and blue network in the particular curves. Two or three years ago this cut-off at 100 cycles was not noticeable to the average listener but with the advent of electrodynamic loud speakers and ampli- fiers which will handle the bass in the later radio receiving set models, this loss of frequencies below 100 cycles is now more serious. The cut-off at the upper end in the vicinity of 5000 cycles is extremely sharp but this end of the spectrum is not so serious at this time as most radio receivers cut off quite sharply at 3000 cycles. Much work is being done by both the Telephone company and the National Broadcasting Company to extend the range of the present telephone facilities to make the general transmission of net- work circuits comparable with that shown in the curve for the Bellmore circuit. Fig. 4 shows a typical frequency char- acteristic of the transcontinental circuit from San Francisco to New York. It will be noted that the added line facilities and the additional repeaters make the cut- off at 100 and 4500 cycles much more definite. Considering that this represents 3000 miles of cross-continent circuits the frequency characteristic over the range transmitted is reasonably stable. Another Problem Another factor enters into this problem, and that is the volume variation which the average radio receiver can handle. The more modern sets are now constructed so that they can handle considerably more volume than was possible two years ago, but the problem of background noise brought in by the radio receiver and in- troduced by the vacuum tubes and the power supply is a limiting factor. As conditions stand to-day, the average radio set operating at some distance from a radio transmitter has a higher noise level to contend with than that which is found on telephone wires, so that even if greater volume ranges could be transmitted over telephone circuits, the ether medium through which the program must be trans- mitted eventually, introduces a problem which is not as readily overcome as our first difficulty. The cure for static and interference by electrical machinery operating in the neighborhood of receiving sets is a further increase in power of the radio transmitter, thus increasing the ratio of signal strength to noise at the listener's antenna. The first condition we have some con- trol over—that is, the energy levels trans- mitted over telephone wires to the radio transmitters; but the broadcaster has no control over the manner in which the broadcast listener operates his radio set. All the care taken by broadcasting com- panies and the telephone companies in maintaining energy levels and prevent- ing overloading in any of the equip- ment under their control, is completely offset by the listener when he overloads his amplifier. Conclusions I still feel that, perhaps, the real limiting factor which makes volume control neces- sary is the ether medium and the radio receiver where the margin is even less than exists in telephone circuits. The amplifiers and the radio transmitter in use by broad- casters have been designed so that the volume variation that can be handled by this equipment is as great as that of the symphony orchestra itself. I use the symphony orchestra in this explanation because it probably has the greatest volume range that we have to consider. A jazz orchestra, for example, which plays almost at the same tempo and volume at all times, for dancing purposes, presents no problem. In most cases volume control is not necessary during the playing, as the orchestra maintains its dynamic range within a volume variation of twenty-seven decibels. The diagram on page 261 (Fig. 3) shows the limitations on a deci- bel scale as compared with the volume vari- ations of a symphony orchestra. CHARACTERISTICS OF PENTODES (Continued from page 255) be representative of the final tube in several respects, there is a step-up of 8 to 1 between the high-voltage grid and plate. Hence any a.c. hum reaching to the high- voltage grid will be amplified by a factor of 8 by the time it gets to the plate. At the same time the superior sensitivity of the tube indicates that hum appearing in the plate circuit of the preceding stage will reach considerable proportions in the loud speaker circuit. For this reason it may be that greater filtering than is now necessary will be required. Similar experimental tubes are being built by other tube manufacturers. Those made by Champion have been tested in the Laboratory and are not appreciably different from the Arcturus tubes. Screen-Grid Tube Developments If another grid will improve a power tube and get rid of secondary emission, tube manufacturers reason that an extra ele- ment may improve a screen-grid tube. CeCo has spent considerable time in ex- perimenting with such tubes and some character! sties of such a screen-grid pen- tode tube are given here (Fig. 5). Table III compares it and present-day screen-grid tubes. The greater possible amplification from such multi -element tubes is obvious. Whether or not receiver manufacturers pre- fer to get a lot of r.f. gain per stage or to get the total amplification in several stages is a matter that time only will indicate. At present it seems more economical to use several stages and to get the total amplification by cascading. The problem of shielding a stage in which there is a voltage gain of 100 is different than that of preventing stray fields from a stage in which the gain is only 30 times. On the other hand, the economy to the consumer of operating fewer tubes and the sales value of a physically smaller set may indicate use for this new high-gain screen- grid tube. As an a.f. amplifier tube, it should prove to be quite valuable. It rep- resents a gain of 6 DB over present screen- grid tubes. Tube manufacturers working on power- output and screen-grid pentodes will wel- come suggestions from manufacturers and designers of receiving sets. At the same time the Editors of RADIO BROAD- CAST will welcome communications on this interesting subject. U. S. Pat. 1676869 SATISFACTION Sell ELECTRADS SINCE the birth of radio, the name ELECTRAD on a resistance has stood for highest quality at right prices — the unbeatable combination that spells in- creased profits and good-will for your parts department. TRUVOLT Safest for Eliminators Distinctly different air- cooled winding gives greater accuracy and longer life. Variable model (illustrated) with knob control and panel mounting. 22 sizes. List $2.50. Fixed types have exclusive sliding clip for adjust- ment of resistance val- ues. Made in all usual sizes. TONATROL The Expert's Choice The TONATROL line of volume controls is characterized by su- perior workmanship and longer life. Made in a variety of sizes and resistance values, with or without filament switch at- tached. List $1.50 to $3.00. Super-TONATROL For Power Receivers New type resistance element per- manently fused to an enameled metal plate with pure silver float- ing contact insures smoother ac- tion, longer wear and rapid heat to 5 • RADIO BROADCAST FOR MARCH 293