Spreading the Stations with CSSB
November 1957 Popular Electronics
It's hard to believe that
even by the end of 1957, single-sideband broadcasting was still in its infancy. The claim that, "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," is still a matter of dispute amongst radio aficionados. Just as many audiophiles swear that
even the most sophisticated solid-state driver is not as good as a vintage vacuum tube circuit, there are those
who saw that single-sideband reception is clearly distinguishable from standard double-sideband. Doubt me? Here I
quote from page 2-7 of the "ARRL General Class License Manual for Ham Radio, "SSB transmitters tend to optimize
the signal characteristics for strength at the expanse of some fidelity. AM transmitters, on the other hand, tend
to give a 'warmer' sound to the speaker's vioce." QED
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Spreading the Stations with CSSB
Compatible single-sideband system may double available AM broadcast
By Philip James
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
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.
Developed by Leonard Kahn, a young research engineer, the system essentially filters out one of the sidebands of
the signal on transmission, theoretically doubling the bandspace available. Actually, because of technical
complications, it would work out to only an 80% increase, which still would prove to be quite a help in relieving
the overcrowded broadcast spectrum.
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.
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.