This article is from the Ambisonic Surround Sound FAQ, by Martin Leese with numerous contributions by others.
Matrix H and HJ were surround sound encodings used by the British
Broadcasting Corporation in the late 1970s for experimental FM radio
broadcasts. Matrix H was based on the QS quadraphonic system and was
modified to HJ which was based on Ambisonic principles. The system was
not adopted by the BBC because some mono transmissions sounded
"phasey". The other rumoured problem was that someone high-up in the
BBC thought sound engineers might have to be paid more if they worked
with twice as many speakers.
The 'H' has no meaning. The BBC called the first matrix they assessed
Matrix A, and then worked up the alphabet.