CD-4 Quadradisc

CD-4, or Compatible Discrete 4-Channel, recordings contain a sum signal and a difference signal on each wall of the LP record groove.  To decode these signals, you need a CD-4 disc demodulator.  The demodulator uses the sum and difference signals on each groove wall to reconstruct the left front and back and right front and back quadraphonic speaker signals.  The technique is the same as used in FM stereo.

The difference signals ride on a very high frequency carrier.  As a result, to play CD-4 records you must use a phono cartridge that meets many requirements not satisfied by the typical stereo cartridge.

• First, the cartridge must have a upper frequency response of 45kHz or higher, and response, even if not flat, must be relatively smooth throughout this range, especially in the supersonic band above 20kHz.  Nevertheless, at least one CD-4 aficionado has reported good success with the Audio-Technica AT-331LP, a cartridge that claims a frequency response from 10Hz to only 30kHz.  If this cartridge works as reported, then it (or at least most samples) have a much higher response than advertised.



• Second, the stylus must have a shape that provides greater contact with the record grooves than many stereo styli.  A stylus with a line contact shape gives the best results.  This includes Audio-Technica's Shibata, Stanton's Quadrahedral, Bang & Olufsen's Pramanik, and Shure's Micro-Ridge.  Elliptical and hyperelliptical styli will work but cause more record wear.  Conical and spherical styli do not work with CD-4.


model freq. resp. stylus shape cartridge type output AT-331LP 10Hz-30kHz linear contact moving magnet 3.0 (mV at 1 kHz, 5 cm/sec) AT-OC9ML 15Hz-50kHz Microline Dual Moving MicroCoil 0.4 (mV at 1 kHz, 5 cm/sec) AT-15E 5Hz-45kHz Shibata AT440ML 5Hz-32kHz Microline Dual Moving Magnet Cartridge 5.0 (mV at 1 kHz, 5 cm/sec)
• Third, the cartridge must maintain channel separation in its supersonic range.  Lack of separation results in interference between the two carriers, which causes the demodulator to momentarily unlock and mistrack and produce annoying buzzing and spitting noises.  A poor quality demodulator also causes this common problem, so don't necessarily blame the cartridge if you encounter it.



• Fourth, the design of most CD-4 cartridges calls for a load resistance (or load impedance) of 100,000 ohms.  Most stereo cartridges, however, specify 47,000 ohms.  CD-4 demodulators are fitted with resistors for the 100,000 ohm load, but cartridges made for a 47,000 ohm load seem to work satisfactorily.  Use a 100,000 ohm load cartridge if you can, but don't lose sleep if you can't.

Fourth, total per channel load capacitance of the tonearm wiring and of the phono cables from the turntable to the CD-4 demodulator should not exceed 100 picofarads, and the lower the better.  In addition to using low capacitance phono cable, make it as short as possible.  And plug the cable directly into the demodulator, not through an external pickup switch box, which adds to the total capacitance.

The sum signal on each record groove is what makes CD-4 records compatible with two-channel systems.  If you do not have a CD-4 demodulator or high frequency phono cartridge, you'll still hear all the music the recording artist and engineer intended.