Also known as: single sideband, SSB
Single sideband (SSB) is a refined form of amplitude modulation that removes the carrier and one of the two redundant sidebands, transmitting only one sideband — upper (USB) or lower (LSB).1 Because ordinary AM spends half its bandwidth and most of its power on redundant information, SSB is the efficiency-maximising member of the AM family and the backbone of long-distance HF voice.
How it works
Standard AM produces two sidebands that are mirror images of each other and a carrier that conveys nothing but a frequency reference. All the message information is present in either sideband alone, so SSB throws away the carrier and one sideband entirely. The result uses about half the bandwidth of AM — roughly 2.7 kHz for a voice channel instead of 6 kHz — and, with no power wasted on a carrier or a redundant sideband, puts essentially all transmitter power into the information. That efficiency lets modest transmitters reach across continents on HF via ionospheric propagation.
Two classic methods generate SSB. The filter method creates a double-sideband suppressed-carrier signal in a balanced mixer, then uses a very sharp crystal or mechanical filter to keep just one sideband. The phasing method uses a Hilbert transform — a 90° broadband phase shift — so that the unwanted sideband cancels while the wanted one adds; this is the form an SDR implements naturally, since generating a complex (analytic) signal and shifting it is just arithmetic on IQ samples.
The cost of suppressing the carrier is that the receiver must supply its own. A beat frequency oscillator (BFO) reinserts a local carrier at exactly the right frequency to demodulate the sideband. If that reinjected carrier is off by even 50–100 Hz the voice sounds unnaturally high- or low-pitched (“Donald Duck”), which is why SSB tuning must be precise and why USB versus LSB matters — pick the wrong sideband and the audio is gibberish. By convention, amateur HF below 10 MHz uses LSB and above uses USB.
Variants
SSB sits on a spectrum of carrier- and sideband-trimming schemes. Double-sideband suppressed carrier (DSB-SC) removes the carrier but keeps both sidebands. Vestigial sideband (VSB) keeps one full sideband plus a filtered remnant of the other, used where the message extends down to DC (analog TV video). Variants of SSB itself include AME (amplitude-modulation equivalent, with a reduced pilot carrier for easier tuning) and independent-sideband (ISB), which sends different information on each sideband to carry two channels in one AM-width slot.
Relevance to SDR
SSB is where SDRs shine: a software receiver can generate the analytic signal, shift it, and low-pass filter to isolate a single sideband with no analog filter at all, and can switch USB/LSB or fine-tune the reinserted carrier with a slider. Receiving SSB needs an HF-capable SDR — an upconverter ahead of an RTL dongle, a direct-sampling receiver, or a dedicated HF SDR like the Airspy HF+ — plus accurate tuning to reinsert the missing carrier. GopherTrunk targets digital trunking rather than HF voice, so SSB is out of its decode scope, but the same phasing/analytic-signal math underpins how it forms complex baseband.
Sources
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Single-sideband modulation — Wikipedia, for the suppressed-carrier definition, USB/LSB, and bandwidth/power efficiency. ↩