[Synchrodyne]

It is interesting that the phasing method did not catch on for ISB communications reception. Possibly there was concern about the longer term stability of wideband phase-shift networks. And perhaps a viewpoint that where the two sidebands were radically different, e.g. voice on one and VFT on the other, the “solid” separation provided by sideband filters was more robust.

The history of “connected events” seems to be something like this.

In 1945, Crosby published details of his exalted carrier HF receiver, which was provided with synchronous demodulators (using filtered, reconditioned carrier) for both AM and PM. (IRE Proceedings, 1945 September, with a precis in Electronics 1945 March.)

Later in 1945, Belles (of Harvard) published his system for reduction of heterodyne interference. Like the Crosby receiver, it used similar synchronous demodulators to obtain the AM and PM components, and then added or subtracted the quadrature phase-shifted PM signal to the AM signal. (Electronics 1945 December.)

About a year after that, Dome of GE published his work on wideband phase shift networks, with SSB generation as one suggested application. (Electronics 1946 December, also US Patent 2566876 filed 1946 April 17, granted 1951 September 04.)

Then in 1947, Norgaard of GE put it all together in terms of SSB generation and selective sideband reception. (US Patent 2611036, filed 1947 November 12, granted 1952 September 16.) The GE YRS-1 was one outcome of Norgaard’s work.

Crosby added the Dome phase-shift network to his exalted carrier receiver, which could then receiver AM, PM, USB and LSB. This was covered in US Patent 2575047, filed 1948 July 14, granted 1951 November 13. Whether Crosby actually used this circuitry in his production receivers and adaptors I do not know. Some of these were SSB/ISB as well as exalted carrier, but as best I can determine, the SSB/ISB side was done with filters.

Thereafter it all seemed to go quiet for quite a while.

There seemed to be renewed interest in synchronous demodulation (not necessarily with the selective sideband capability) towards the end of the 1960s, perhaps because solid-state devices made it easier. When the phasing method of sideband separation reappeared in production receivers, it was for selective sideband reception of AM broadcasts rather than for ISB communications. The Sansui TU-X1 hi-fi “supertuner” of 1979 (MW only on AM) was an early example. In the early 1980s there were also some Accuphase hi-fi tuners with the same facility, again MW only.

In 1983 the Phase Track Liniplex F1 HF receiver appeared. This used a tracking PLL demodulator and the phasing method to provide selectable sideband reception of HF broadcasts. There is an interesting history there. The designer (whom I met once in the early 1990s) at one time worked for a major American manufacture of highly regarded professional communications receivers. When he was stationed overseas, he wanted to listen to the BBC World Service, but found his employer’s receivers not well suited for comfortable listening to programme content. So he looked at the problem and then designed what was a suitable receiver for the purpose. I bought the later F2, and used it for listening to the BBC WS for about 14 years or so. It was the only HF receiver I have used that passed what I called the “BBC WS Play of the Week” test. "Play of the Week" arrived on Saturday evenings when I was living in Texas, and the objective was to be able to sit back in the armchair, glass of red liquid in hand, cat on lap, and capture the play (and there were some good ones back then) without any strain. The F2 (feeding the Quad/Celestion, later Quad/KEF hi-fi system) did the job admirably. My backup receiver was a JRC NRD-525 with outboard Sherwood SE3 synchronous demodulator, but it was not a match. (The NRD-525 was more of a DX’ing receiver than one for extended listening to programme content.)

At about the same time (1983), Sony released the ICF-2001D/2010 which had PLL synchronous demodulation and sideband separation by phasing. For its price, it was fine, and it was using one of these (somehow I ended up with two of them) that convinced me that synchronous demodulation with selectable sidebands was the answer for HF broadcast listening. Sony used the same IC set that it had developed for AM stereo decoding, and which had been used for example in the SRF-A100 portable FM-AM stereo receiver. I had one of these before my first ICF2010, and when using the headphone output to feed the stereo system, it have a surprisingly good account of itself on AM, which pointed to PLL demodulation as also being a pathway to lower distortion.

An interesting comment made to me by the Liniplex designer was that where reception conditions allowed the use of both sidebands of an HF broadcast, the Costas loop actually provided slightly better audio quality than the tracking PLL. Apparently this was because, by using both sidebands to guide the loop, it was able to track the short-term phase variations of the carrier. On the other hand, the PLL was better than the Costas loop when it came to selective sideband reception. So ideally you’d want both in an HF broadcast listening receiver.

Regarding binaural listening, back in the later 1980s there was a columnist for the American magazine “Monitoring Times” who was in favour of this approach. I guess that if you used say a Sony SRF-A100 (or one of the Sansui or Sony hi-fi tuners with multi-system AM stereo decoders) set to the Kahn ISB mode, then with a mono AM broadcast you’d have more-or-less LSB on one channel and USB on the other.


Cheers,