Re: SDR-4++ dual diversity SDR receiver


On 2/18/2017 1:37 PM, Chris Moulding wrote:

Listening to the combined audio output where both receivers were fed to a set of headphones was amazing. White noise appears as a wide stereo field while wanted signals appear in the centre of the stereo image. To hear 40m stations while the ionosphere changes the polarisation is fascinating, the signal level remains the same but the noise varies in each headphone as the AGC in each HDSDR session tracks the signal. You get the sense that you can feel the propagation changes rather than just hear them.

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How prominent is the shifting background noise?    Would it be significantly irritating while listening to shortwave broadcasts?

In the early 1980's, there was a push by several competing standards to be adopted as "the standard" for AM stereo broadcasting.   One of the criticisms leveled against some of the systems was that during nighttime long-range skywave reception shifting phases, multipath and selective fading across even the 10-12KHz bandpass of a single AM standard broadcast channel would make the stereo image wander drunkenly from side to side as propagation changed.  

The (US) NAB (National Association of Broadcasters) holds an annual trade show showcasing all the new radio-TV-proaudio hardware.  At one NAB show in the early '80s, one AM stereo proponent went so far as to hand out airline barf bags imprinted with a message to the effect that the rival system was going to sicken listeners with the unstable stereo image.

I once had a chance to listen to KFI AM 640KHz 50KW in Los Angeles broadcasting Motorola C-Quam AM stereo.  KFI is a monster signal routinely heard over most of the western US at night. It enjoys the rare privilege of being a "clear channel" at night; i.e. no other station on the channel in North America after sundown.   I once had the chance to listen to stereo music from KFI in San Francisco (about 400 miles/640 KM) to the north) in the early evening hours when KFI absolutely booms into northern California on a single skywave hop.    Most of the time, the stereo was very good with with an excellent image from left to right channels. But about every 5 or 10 minutes, the sound would suddenly lurch to the left or right channel only, followed by a distorted signal that sounded like SSB received on an AM receiver (i.e. when the selective fade notched out the carrier, followed by the receiver switching back to mono.  (C-QUAM mode in receivers was triggered by a continuous low-level 20 Hz pilot tone which would be lost when selective fading took out the carrier). This was followed by the return of reasonably good mono, followed by a return to full stereo.    

On the other hand, within it's stable groundwave coverage area  (about 100 miles/160KM radius day or night)  in greater Los Angeles the AM stereo performed wonderfully.  The greater Los Angeles area is very rugged terrain that is cut through with numerous mountain ranges up to 7000 feet/2100 m, riddled with hundreds of canyons. Even the City of Los Angeles is cut in half by a 1800 ft/550m mountain range.   This kind of terrain causes FM stereo to be a nightmare of multipath-induced distortion in many parts of the city, even just a few miles from transmitters located on Mt Wilson, about 5200 ft/1600m above the city.   The AM stereo signal worked perfectly anywhere in the area, even deep into canyons.     I had an AM/FM car radio at time that was equipped with one of Motorola's more advanced C-QUAM decoder chips.  This decoder could actually vary the IF bandwidth of the radio based on strength and S/N.    [NOTE: This was long before the era of DSP. This was all done with analog technology!]    On a nice strong local signal, the AM bandwidth would actually open up to 20 KHz plus, yielding almost FM-quality stereo audio on AM!

The technology worked, but it was killed by station managers obsessed with the loudness horsepower race in broadcasting.  C-QUAM couldn't tolerate asymmetrical "super"modulation that yielded positive peaks of 120-130-140% without hideous distortion.   (Stations would pay 10's of thousands of dollars to install magic mystery audio processor boxes that would produce this asymmetric audio in an effort to be louder than the next guy.)  With C-QUAM, the max modulation had to be limited to about 90-95%.  If you drove modulation % above this, the carrier power would approach zero on each negative swing, causing the receiver quadrature demodulator to loose phase lock on the carrier.  In turn this would cause a horrible "spattery" distortion that sounded similar to FM multipath.

Stephen H. Smith    wa8lmf (at)
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
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