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 (universal matrix)[4] standard used for UD-4 contains two subsystems:
QMX (four channel matrix),[6] a discrete 4-4-4 system (adding band-limited localization information encoded with high frequency carrier signals[1] similar to the CD-4 system).[8][4]
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 (enhancing directional resolution).
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.[9]
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.[4]
