Their purpose was to decode the information on CD4 records by filtering the information held in those frequencies above 15 kHz and translate them into the rear channels of a quadraphonic track. The decoding was activated by the presence of a 45 kHz signal. Both demodulators were also switchable as many stereo records had some information at 45 kHz which could cause the demodulator to cycle on and off.
To be able to retrieve information at such high frequencies, special profiles of diamond stylus were required and these were provided with the Shibata profile diamond in the MMC5000 and the Pramanik profile diamond in the MMC6000, a profile unique to B&O and designed by their famous engineer, Subir Pramanik. The later MMC20CL could also provide the same service and in some ways was even better as the cantilever was even stiffer as it was constructed of a single crystal sapphire.
In addition to the decoding, the CD4 Modulators boosted all four channel signals to line level, and although CD4 records are now rarely seen, the line level output is now a most useful feature of the quad decks as it allows their use with equipment not fitted withRIAAs.
Only the Beogram 6000 was fitted with a demodulator as standard. It was an option on the 4002 and 4004 and a different model was available for the Beogram 3400. This included a red LED that replaced a small black plug on the 3400 and was lit when a CD4 record was played and the 45 kHz signal was detected.