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[TenTec] Enhanced SSB

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Subject: [TenTec] Enhanced SSB
From: geraldj@isunet.net (Dr. Gerald N. Johnson, electrical engineer)
Date: Fri Apr 18 13:16:09 2003
SSB achieves its superior communications performance compared to double
sideband AM by saving the carrier power and concentrating the
transmitter  power into just one sideband AND by allowing half the
receiver bandwidth so for threshold level signals, the received noise
power is half thus improving the received signal to noise ratio.
Reduction of equipment weight and power supply capacity are incidental
side effects. By the reduction in transmission and receiver bandwidth,
SSB allows at least a doubling of the number of QSOs per MHz.

Generating SSB by phasing network was MORE bandwidth limited by
circuitry than SSB by filtering. True there's a LF roll off from the
slope of the filter getting LF opposite side band (and some enhanced
carrier) suppression, but the audio phase shift networks of the past
were always compromises. Adding a 100 Hz to the low end required taking
1 KHz from the top end in those RC networks. I did an analysis of all
the known phase shift networks back about 1971 or '72 while exercising
main frame network analysis programs. That is once I found such a
network analysis program that was portable enough to run on the ISU main
frame. I found that most had an average phase difference closer to 80
degrees than 90 degrees. The commercial B&W was the best of the pack.
This effectively prevented switching sidebands by the simple inversion
of the audio, one simply couldn't achieve good sideband suppression on
more than one position of the sideband selection switch without
shuffling the RF phase shift to match the error in the audio phase
shift. That phase shift error problem is why we use LSB on 80 and USB on
20. The first ham SSB was with phasing and was on 20 meters, 9 MHz IF
and 5 MHz VFO (command set). 9 - 5 gave 75 meters but the opposite
sideband. Three times the VFO - 9 gave LSB for 40 meters.

The modern phase shift network is ten times more complex than that used
in the 50s and 60s and is still bandwidth limited. Modern analog phasing
equipment still very carefully limits the audio bandwidth and the
harmonic distortion of the audio amplifiers (transmitter AND receiver)
to minimize bandwidth. Distortion products from the transmitter audio
amplifier will show up on BOTH sidebands coming out of the phasing
exciter, whether DSP or analog.

Spectrum studies for audio communications dating back nearly to
Alexander Graham Bell have shown that the LF components of male voices
contribute little to communications but a great deal to power
dissipation in the communications circuit, so its considered practical
to roll off rapidly below 300 Hz. Hurts naturalness a bit but not
articulation. The same thing is true for high frequency components much
above 2400 Hz. Silabent energy does extend further, but adds virtually
nothing to understanding of the voice communications. Hence the
classical filter rigs, the Collins KWS1/75A4 and S-line use 2.1 KHz
filter bandwidth and set the carrier 300 Hz from the passband corner. As
do Ma Bell's frequency multiplexed wire and microwave voice circuits
since the 30s when frequency multiplex was developed. Collins supplied
LF mechanical filters by the TON for such frequency multiplexed wire
line and microwave service. The emphasis for elimination of low
frequency energy was to save circuit power without loss of
understanding. Remember that in the early days of long distance, there
weren't any rural electric cooperatives hand to supply DC power to the
necessary amplifiers, they had to be supplied through the phone wire and
efficiency was very necessary to work at all.

Use of SSB has a conflict with hearing quality, that comes from tuning
accuracy. It takes more care to tune SSB for quality hearing than AM or
FM. New hams seem to have great difficulty tuning for quality, often
tuning for bare intelligibility and accepting a 200 Hz or more error. In
commercial circuits that has been attacked by leaving in a little
carrier or for VHF by transmitting a pilot tone out about 3 KHz (with a
strong LP filter to take out the pilot tone). Operator skill can help
the quality immensely by tuning so that the transmitted harmonics of the
lower frequency components of the voice come out of the speaker at the
harmonic frequency.

Back in the December 1956 SSB issue of the Proceedings of the IRE (the
classic reference work at engineering level about SSB and
communications, recommended reading), John B. Costas pushed DSB
suppressed carrier because it gave most of the transmitted energy
savings, and could be received with the SSB receiver (though those
running SSB that noticed I had both sidebands on 40 meters back about
1958 did tend to stop talking to me), and better could be received with
a receiver whose LO was phase locked to the two double sideband
components to be exactly in phase with the original suppressed carrier.
Hence tuning was very easy, the receiver was more complex, but the
transmitter was simplified and the received quality was always high (at
least in the Costas receiver).

I find that I often want a narrower SSB filter than the 2.4 of my
Corsair II, fortunately with the passband tuning, that is easily
achieved with a tweak of a knob. Then to clean up harmonic artifacts
created in the radio I have a switchable cutoff frequency passive low
pass filter in the speaker circuit. Typically that LP filter is set for
about 2.1 KHz.

As for spectral efficiency, I have found by decades of experiment, that
opposite sidebands can overlap further than same sidebands for the same
effective interference to communications. E.g. I can run USB on 75
meters with my carrier about 3 to 3.5 KHz from a LSB signal and still
communicate while running that close both on LSB the interference is
much more severe. I think that has to do with the energy density of the
male voice above 1.2 KHz being small. And in my receiver, interference
in that range is less annoying than if its from the low audio portion of
the male voice spectrum. E.g. overlap of SSB spectra is less annoying if
its from the opposite sideband than from the same side band for the same
amount of overlap.

There is a rule in part 97 about using the minimum spectrum required for
the communications, right next to the one about using the minimum

So as I see it, "HIFI" SSB is contrary to good communications principles
and to the letter of the FCC rules covering amateur radio. Other than
catering to an ego that demands greater bandwidth and "fidelity" is
accomplishing nothing but to waste spectrum. Perhaps some future digital
mode (I have one in the works) will allow fitting 10 or 20
communications within the same spectrum as normal bandwidth SSB and
perhaps 40 independent communications within the spectrum wasted in a
single "HIFI" SSB signal.

73, Jerry, K0CQ
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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