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[TenTec] Slightly OT: SSB vs AM

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Subject: [TenTec] Slightly OT: SSB vs AM
From: geraldj@isunet.net (Dr. Gerald N. Johnson, electrical engineer)
Date: Tue Apr 22 13:42:06 2003
The carrier and other sideband of AM make for simple detection. E.g. the
crystal set, just a diode. The carrier does nothing but bias the diode
detector, the other sideband is just the result of the amplitude
modulation and has redundant information. When received with a suitably
locked local BFO and synchronous detection, the carrier shows precisely
where the local oscillator should be in phase and the simple synchronous
detector will add information from both sidebands to the output, so long
as propagation has not changed the phase of the carrier and the
sidebands. Propagation can destroy the phase consistency necessary for a
synchronous detector's great reception and much more so for the simple
diode detector's reception. Then SSB reception works better. Back in the
50's John B. Costas proposed that DSB sans carrier gave enough
information for the phase locked receiver to recreate the local
oscillator phase with perfect accuracy, at the cost of twice the
spectral bandwidth. Many times in the past, communications quality has
been sacrificed to enable narrower spectrum and thus more users.

The data rate in voice communication is very low. How about close to the
syllabic rate. For sure much lower than the maximum transmitted
modulation frequency.

The voice spectrum has two major groups, one at a few hundred Hz and
another out maybe three times that frequency with a gap in between.
There was a proposed scheme in the ARRL handbook a few years back to
separate those with separate bandpass filters, then heterodyne the upper
one into the gap at the transmitter, then shift it back to its normal
range in the receiver. Saved at least a third of the spectrum, maybe
more, but it went over like a lead balloon.

For really great bandwidth reduction one might detect phonemes, signal
their sequence and timing with PSk-31 style signals, then use them to
run a speech synthesizer in the receiver. Then the RF part would be
running at Nyquist rate. The major cost would be naturalness. The
simpler voice synthesizers are being used on NWS weather radio. Igor
takes a bit of listening practice for good understanding. Then one might
compress TV a great deal by sending a bit of digital data to establish
character positions and attitudes, then let the user's TV set do
animation based on those voice phonemes. Perhaps "live" TV in 100 Hz
bandwidth... Looses a bit of quality, but talk TV wastes much spectrum
just showing talking heads. The same information could be transmitted as
plain audio in 1/1000 th the bandwidth using AM, less using SSB.

73, Jerry, K0CQ

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