Spreading the Stations with CSSB
November 1957 Popular Electronics
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
November 1957 Popular Electronics
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Spreading the Stations with CSSB
Compatible single-sideband system
may double available AM broadcast band space
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 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.
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
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.