Path: utzoo!utgpu!ugw.utcs.utoronto.ca!CUVMA!SWL-L Date: Tue, 16 Jan 90 13:17:00 EST Reply-To: Ed Schwalenberg Sender: Short Wave Listener's List From: Ed Schwalenberg Subject: What does one do with an SSB switch? X-To: albert%endor.uucp@BBN.COM, To: UofToronto LAN redistribution References: The message of 16 Jan 90 10:51 EST from David Albert Message-ID: <90Jan16.132700est.57931@ugw.utcs.utoronto.ca> Newsgroups: bitnet.swl-l Distribution: ut Approved: devnull@gpu.utcs.toronto.edu Date: Tue, 16 Jan 90 15:51:32 GMT From: David Albert My Sony 7600DS has an SSB switch, but I have no idea what I might want to use it for. I'm also not sure what it does (i.e. physically); is it the equivalent of a narrow-band switch? SSB stands for Single SideBand. Ordinary AM modulation consists of a carrier (what you hear when nobody's talking) and two mirror-image sidebands, which carry the information. It's cheap to build transmitters and receivers for AM, but AM is wasteful of energy and bandwidth, since the carrier and one of the sidebands can be discarded without losing information. SSB does exactly that: an SSB transmission is an AM transmission with the carrier and one sideband eliminated. If you try to listen to one in AM mode, it sounds like Donald Duck. The SSB switch activates some circuitry which basically tries to duplicate the missing carrier, so the signal can be detected like an AM one. (The details vary with the kind of receiver, so I'm glossing over things here.) The bandwidth of an SSB signal is exactly half that of an equivalent AM signal; a narrow bandwidth filter of between 2.5 and 5KHz will give you all of the desired signal and filter out adjacent interference. (A corresponding AM filter, being twice as wide, allows twice as much interference through.) Selecting SSB mode does not have any direct effect on bandwidth in most receivers; the choices are orthogonal in theory and usually in practice as well. Virtually all SW broadcast stations use AM, since that's what common receivers use. You can hear SSB signals almost anywhere in the shortwave spectrum that is NOT part of a SW broadcast band. Most non-broadcast stations use SSB because you can get twice as many stations in a given frequency band and because SSB provides more readable signals than the equivalent AM (all the transmitter power goes into one information sideband so you get 2 or 3 times the effective power, and the receiver can use narrower filters to avoid more interference). Tuning an SSB signal is harder than tuning an AM signal: if you tune 1KHz off dead center, the tones in the modulation will be shifted by 1KHz too, so you'll get squeaks or growls instead of voices. There are two possible sidebands: lower and upper, or LSB and USB. If you tune the wrong one, you'll get inverted speech, with bass and treble inverted. This also means that successful SSB radios must be very stable; if your radio drifts by 1KHz on a 30 MHz signal, it's only off by 1 part in 30,000 but you won't be able to understand anything. A variant on the SSB idea is used by the VOA for getting programming from the US to foreign transmitters (so-called "feeders"): ISB or Independent SideBand sends two different programs via one transmitter on one frequency, using LSB for one and USB for the other. It sounds like Huey, Dewey and Louie all talking at once on AM. Another variant is the ECSS feature in the Sony 2010 receiver discussed here so often: ECSS amounts to pretending that an AM signal is an SSB one. You can do that with any SSB receiver by tuning an AM signal in SSB mode until the zero-beat goes away, but even the slightest drift will cause a 1 or 2 Hz "motorboating" sound which is extremely annoying. ECSS locks the receiver to the transmitter frequency, preventing the drift. You can then choose the sideband with the least interference, and use a narrow SSB filter to eliminate even more interference.