A variant of the pilot-tone system has been investigated in the Netherlands using a single-sideband modulation of the sub-carrier, having a vector amplitude twice that of the sidebands generated in the case of normal sub-carrier modulation, making it compatible for stereophonic receivers having a synchronous type of decoder.
ITU-R REPORT BS.300-7-1990 history
1990ITU-R REPORT BS.300-7-1990 Stereophonic or multi-dimensional sound in frequency-modulation sound broadcasting