{"id":50355,"date":"2021-09-26T08:56:43","date_gmt":"2021-09-26T12:56:43","guid":{"rendered":"https:\/\/swling.com\/blog\/?p=50355"},"modified":"2023-04-13T20:21:50","modified_gmt":"2023-04-14T00:21:50","slug":"guest-post-a-synchronous-detector-crash-course","status":"publish","type":"post","link":"https:\/\/swling.com\/blog\/2021\/09\/guest-post-a-synchronous-detector-crash-course\/","title":{"rendered":"Guest Post: A synchronous detector crash course!"},"content":{"rendered":"

Many thanks to\u00a0SWLing Post<\/em>\u00a0contributor,\u00a013dka<\/a>, who shares the following guest post:<\/strong><\/p>\n

\n

\"\"<\/a>Revisiting the Belka’s “pseudo-sync detector”: A sync detector crash course!<\/h1>\n

by 13dka<\/strong><\/p>\n

“It\u2019s usually hard to assess whether or not a sync detector helped with a particular dip in the signal or not, unless you have 2 samples of the same radio to record their output simultaneously and compare.”<\/em>*<\/p>\n

That’s what I wrote about the “pseudo sync detector” in my review of the Belka DSP last year<\/a>.<\/p>\n

Since I was recently upgrading to the Belka DX in order to pass on the Belka DSP to a friend, I had briefly two examples of almost<\/em> the same radio on the table at the dike. I tuned them to the same stations and recorded some audio clips with one radio on sync detector, the other in regular AM mode, to answer the question whether or not sync has “helped with a particular dip in the signal”. Then I thought that demonstration would be an opportunity to try an explanation on what exactly (I think) sync detectors are all about anyway, hoping to find a middle ground between “technical” and “dumbed down beyond recognition”.<\/p>\n

\"\"<\/a><\/p>\n

The trouble with sync detectors<\/h1>\n

Perhaps no component of a shortwave receiver is surrounded by so much misconception and confusion as sync detectors. Full disclosure: Until quite recently, I had an, at best, vague concept on what they do myself. It seems it’s not so much that people don’t know how they work, what they actually do when they work is where the ideas often diverge. Contributing factors might be that explanations on what problem sync detectors are supposed to solve are usually high-level theoretical and just as hard to digest as the explanations on how they solve that problem, and a few old magazine reviews and now internet buzz probably did their own to misguide or inflate expectations instead of explaining when to turn them on.<\/p>\n

The outcome is that sync detectors tend to disappoint: I have often read things like “I turned it on but I couldn’t hear a difference” because people turn them on when there’s nothing for the circuit to do and then they think they do nothing, or they believe their purpose is “magically recovering weak signals” and get disappointed by the mixed results. Contrary to what some YT videos state, sync detectors are not “great when a signal fades a lot” (that’s what the automatic gain control is meant to keep in check) and no, they were not “invented by Sony”.<\/p>\n

A very brief history of sync detectors<\/h1>\n

From the very beginning of radio broadcasting, people noticed that their favorite non-local stations were usually sounding just fine during the day (if they came in at all) and in the early evening, but not so great anymore when the family gathered around the radio on long, dark winter evenings or at night (on the soon to become popular shortwave even during the day). It didn’t take long until the sheer wonder of wireless entertainment wore off enough that listeners and engineers started asking why the tone of these stations was changing continuously and even worse, why some of them exhibited not only fading but some recurring and sometimes very nasty distortion. That’s why the first concepts on addressing these issues by somehow “exalting the carrier” started to emerge as early as 1922<\/a>, and it took 30-50 more years until these early ideas evolved into the sync detectors we know today.<\/p>\n

\"\"<\/a><\/p>\n

Not only in this historical context it seems obvious that dealing with the well-known old problem outlined above is (or was) basically all they were supposed to do. But by the time the theories could materialize in practical and affordable circuits, FM and TV were the all the rage and the downmarket didn’t care that much for AM broadcasting anymore. Apart from a few professional receivers and ham radio projects, sync detectors didn’t catch on in the consumer radio world. Until the 1970s, when they started to be marketed to demanding shortwave listeners – in usually quite technical or vague and hence confusing ways, and somehow the focus started shifting:<\/p>\n

Because they occasionally do recover DX stations buried in the mud to some very varying extent and under some pretty elusive circumstances, this (dare I say) byproduct of the concepts understandably became a sales pitch and often also the one benchmark used to rate sync detectors in reviews, also understandably because rating the elusive thing they were meant to do is difficult.<\/p>\n

A “selective fading” crash course<\/h1>\n

Most of us have learned that “multipath propagation” is the source of the trouble, because it causes…<\/p>\n

Selective fading<\/strong><\/h3>\n

The difference between fading and “selective fading” is that the latter affects only portions<\/em> of the signal within the part of the spectrum a station occupies, hence the name. The somewhat unlucky term may have contributed to the confusion too, because (frequency-) selective fading and fading are two completely independent phenomena that can happen both separately and simultaneously<\/strong>! Still, a lot of explanations on the net and even very technical articles often lump them together… Anyway, still very abstract, right?<\/p>\n

Why not just have a radio showing us<\/em> what selective fading is? Here is a “waterfall” plot<\/a> of a signal, showing the history of a station’s (HF-) spectrum over a span of several seconds, the station is subject to selective fading (and in the middle of the clip also regular fading):<\/p>\n