From The RadioReference Wiki
What is an AM Signal?
A regular AM broadcast signal consists of a lower sideband a carrier and upper sideband. An AM signal is vulnerable to several problems. Fading can cause distortion of the signal; It can be caused by attempting to capture a signal coming from multiple angles or directions (multipath) or even a geomagnetic disturbance. Can the signal be recovered so that it's listenable? The answer is yes, it many cases it can.
Enter Synchronous AM
Using synchronous AM (sometimes referred to as AM-Sync, Sync AM, S-AM or AM-S), the original carrier is mixed with a very stable carrier generated by a circuit known as a synchronous detector. As this signal is very stable, it is not subject to distortion because of fading. Another side benefit is that the audio is generally more pleasant to hear, as any distortion is virtually wiped out by the stability of the inserted carrier.
You can accomplish something similar by using a Beat Frequency Oscillator (BFO) which is nominally used for hearing sideband (such as from hams). You tune to zero beat the carrier (you will hear a whistle as you tune; the tone will decrease as you get closer and closer until the tone is gone). Unfortunately on some radios, turning on the BFO also turns on a narrower filter which makes the sound more muffled.
Are there any radios that can do this today?
The Icom R75 and the Tecsun PL-880 has sync detection. In fact the PL-880 had it as part of its so-called hidden features. Unfortunately neither radio performs all that well in this mode. Most people that have reviewed the R75 has noted the Sync detector on the R75 as merely an afterthought. There were some outboard boxes for the R75 such as those from Sherwood Engineering, but they are very pricey and it can be somewhat involved to wire them in properly.
The king of radios with sync detection in the portable world is the long-discontinued Sony ICF-2010 (also known as the ICF-2001D in some parts of the world). The Sony ICF-7600G was also very good, as is the Tecsun PL-660 and 680. For portatops (radios that marry the characteristics of a desktop with a portable) the Eton E1 (and its cousin the E1XM) also performed well. This is to be expected, as the E1 was a joint venture between Eton/Grundig and R.L.Drake. In fact the E1 was serviced by Drake engineers at one point. The later editions of the Drake SW8 and SW8A also had this capability.
In the desktop world, the Drake R8 series, the AOR-7030 and some of the Japan Radio Company radios (the NRD-535 and 545) also had this capability. Reportedly the AOR AR5700D will have SAM.
In the world of Software Defined Radios (SDRs) the following applications are known to support Sync AM. If there are others not listed here please feel free to add them;
- Sync Detection Article from Fine Tuning Proceedings (PDF)
- Synchronous Detector (Wikipedia)
- Synchronous Detection of AM Signals (PDF) via SWLing.com
- Understanding Sync Detection in SW Radio Receivers 101
- Return to Shortwave Radiogram Gateway