[Sursound] any idea's on the background of the stereo equations in the WW1977 article?

Hello, I have a question concerning the conversion of a stereo recording to ambisonics. In the Wireless World of 1977 articles there are equations for the conversion of stereo to W,X,Y equivalent signals. The use of j*Diff term doesn't seem to make sense. As I found no background info on them,

Re: [Sursound] Anyone seen/heard about this?

Also this system is flawed. The premise is that the real personal ear response is used to create XTC filters. The source material needs to be binaural. For all other source types the space recreation is as unreal as in a normal stereo loudspeaker setup. At the recording end it is improbable