I wonder if the BBC initially did some broadcasts in SQ, QS, H, & HJ without telling the listener to see if they got complaints about audio 'quality'? When I was at Uni I was playing around with my FM tuner and found a test transmission which said if you hear this please ring a phone number, I did and it was a BBC test transmission for what became RDS FM data for auto tuning the radio when driving to keep the same station.
Yes Duncan, I guess it’s possible that there were ‘stealthy’ broadcasts in various formats and indeed there was all kinds of odd stuff transmitted after normal hours (back in the day when the BBC closed down at 23.30!!) on both TV and radio when the BBC had a large research department. But if they happened, they must have been on such a small scale that no one noticed them (including me!). What is notable is that there was no mention of any such tests in the technical press that I ever saw or even more tellingly, in the BBC’s own technical reports.
The BBC Research Dept. produced two major papers on the subject of quad, namely - BBC RD 1974/29 (“The subjective performance of various quadraphonic matrix systems”)
https://www.bbc.co.uk/rd/publications/rdreport_1974_29 and BBC RD 1977/2 (“Quadraphony: Developments in Matrix H Decoding”)
https://www.bbc.co.uk/rd/publications/rdreport_1977_02 . These are very interesting reports and well worth reading.
Note that these were all tests conducted in the studio – there is no discussion of there being any off air broadcast testing or any public assessment of performance quality before Matrix H was adopted.
You will see that as early as 1974 the BBC had dismissed both SQ and QS as not suitable (without the need for test transmissions apparently) and was only recommending further research into, and the development of, the other more promising systems, notably of course, Matrix H.
Interestingly although very dismissive of both SQ and QS on the grounds of image localisation and poor mono and stereo compatibility, it was slightly more enamoured with QS decoding techniques and indeed 1977/02 explains how the first Matrix H decoder was based on a modified QS Variomatrix decoder.