UD-4

UD-4 was a discrete four-channel quadraphonic sound system for phonograph records introduced by Nippon Columbia in 1974. This system had some similarities with the more successful CD-4 process introduced by JVC and RCA in 1972.
Only about 35 to 40 LP album titles were encoded in this format, and it was marketed only in the UK, Europe and Japan. Most of these releases were marketed by the Denon label.
The UMX standard used for UD-4 contains two subsystems:

BMX, a basic 4-2-4 matrix decoder ;
QMX, a discrete 4-4-4 system.

The UD-4 process modulated both BMX and QMX into a single record. This way, a BMX decoder could be used to decode quadraphonic sound using an existing stereo record player, while by using a special phono cartridge and a specialized UD-4 demodulator discrete supplementary QMX channels could be extracted.
In theory this allowed backward compatibility with existing players and records, but the system suffered from incompatibility with regular stereo playback due to phase differences between the left and right channels.
Phono cartridge set-up for UD-4 playback was less critical than in CD-4, because the high frequency carrier signals were not as high as those found in the CD-4 system.