November 1957 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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 say 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
Compatible single-sideband system may double available AM broadcast band
By 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.
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.
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
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
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. 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 April 1, 2021
(updated from original post on 4/1/2012)