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Spreading the Stations with CSSB |
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See all articles from Popular Electronics. Spreading the Stations with CSSB Compatible single-sideband system may double available AM broadcast band spaceBy Philip James  The
harried Federal Communications Commission is keeping its fingers
crossed. One of these days it may get a totally unexpected bonus
in the way of doubled broadcasting space. Tests are under way with
a completely new method of broadcasting that promises to allow "space"
for many new stations. The FCC's big trouble up to now has been those tricky little devils called "side-bands." Stations allotted an AM broadcast (or carrier) frequency must also be given added room on each side of the assigned frequency. This is because the two side-bands take up that room. What Sidebands Are. These side frequencies normally are inescapable. They are created when the carrier is modulated with voice or music. It's because of the sidebands that you can pick up a station even if your receiver isn't tuned on the nose. However, if you were able to eliminate one of these sidebands, you would automatically cut down on the bandwidth needed for the station. That's what the new compatible single-sideband system (CSSB) does.  Personnel at the Voice of America installation in Munich, Germany adjust their receiving equipment. VOA is now using the new compatible single-sideband technique. This is one of the few pictures ever released of an overseas Voice installation. Below is the adapter equipment developed by Leonard Kahn to transmit CSSB; it is located in the Kahn Research Labs test center, Freeport, N. Y. CSSB's most remarkable feature is that although it uses but one sideband, a broadcast will still sound the same to even the simplest home radio. It will be recalled that in standard single-sideband reception, receivers with a local oscillator to reinsert the carrier frequency must be used to receive an intelligible signal. Otherwise, you would get "Donald Duck" chatter. Tested by VOA. One huge Voice of America station has been using CSSB in order to crack though Russian jamming. They found that it reduces interference and manages almost to double signal strength without increasing the size of the transmitter. The American Broadcasting Company (ABC Radio Network) has been so impressed with the new system that it is planning a thorough tryout. Should the network consider the system a complete success, it would probably kick off the biggest race for new station permits since the inception of broadcasting. Areas now considered "full up" would no longer be so. HOW CSSB WORKS In the compatible single-sideband system, conventional SSB is first generated without a carrier in the usual manner. From this point, the system follows a new path. The carrier is reinserted and then, with an adapter unit, the distortion which this process has introduced is cut out. It's done by changing the shape of the wave which has amplitude and phase modulation (and a high degree of distortion) into a non-distorted AM wave. The final result is a single-sideband wave with carrier almost completely lacking in distortion. Thus, it can be picked up by the ordinary AM receiver.
By using a spectrum analyzer, we see the ordinary c.w. or code signal as a single carrier in A (above, right). In B, we see the ordinary double-sideband AM wave with carrier as the taller center line and the two shorter lines on each side as the sidebands. C is the compatible single-sideband analysis. Actually, you get a choice of cutting off either the right or left sideband. In the conventional single-sideband analysis (not shown), the center (center line) would also be cut off; this, of course, requires a local oscillator to reintroduce the signal. D shows a frequency-modulated wave analysis. Because of the constant change in frequency, however, these sidebands also change constantly, so that the picture is different at any given moment. What is spectrum analysis? If a signal is examined using a relatively rare instrument known as a spectrum analyzer, we get a very different picture of what takes place during modulation, as seen in the scope traces above. Instead of the usual time-based trace, we get "spectra," in which the horizontal base line is a measure of increasing frequency rather than increasing time. A detailed description of the individual traces (A, B, C and D) is given in "How CSSB Works" at left. Although the FCC could not permit too many new stations to go on the air, since that would defeat CSSB's purpose of wiping out interference, it would probably allow quite a few new ones. Finer Tuning. For the listener, CSSB would mean that much finer tuning of a receiver is necessary. Right now you can pick up most stations 2 or 3 kc. away from the station's authorized frequency. That is, if a station is broadcasting at 1500 kc., you can get a pretty fair signal anywhere from 1498 to 1502 kc. - although the optimum signal is still at 1500 kc. A station broadcasting with CSSB would use either the lower or upper of the two sidebands available to it. It could use either the area from 1490 to 1500 kc. or from 1500 to 1510 kc.-depending on which sideband it picks. If it picks the upper - 1500 to 1510 kc. - the listener would get optimum signal strength at about 1502 or 1503 kc., thus detuning slightly from the station's authorized frequency. However, if you should stay at 1500 kc., you would still get as good a signal as if the station were transmitting a standard double-sideband signal. Posted 4/1/2012 |
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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ... All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged. My Hobby Website: AirplanesAndRockets.com
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