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
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.
Last updated: October 13, 2001