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
Table of Contents
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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.
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
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.
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.
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
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