 The early SQ decoders could not produce more than 3 dB of separation from front to back.Early "Front-rear logic" circuits were introduced to enhance separation to 12 dB and later "Full logic" circuits 20 dB, but both provided poor performance, very noticeable gain-pumping and an unstable 'swaying' sound field.The SQ system also faced resistance from broadcasters since, while essentially a two-channel system and stereo-compatible, it could have substantial mono compatibility problems, which posed serious problems with all televisions and monophonic radios of the era.