A different format, CD-4 (not to be confused with compact disc), by RCA, encoded the front-rear difference information on an ultrasonic carrier, which required a special wideband cartridge to capture it on carefully calibrated pickup arm/turntable combinations. CD-4 was even less successful than the two matrixed formats. (A further problem was that no cutting heads were available that could handle the HF information. That was remedied by cutting at half the speed. Later, the special half-speed cutting heads and equalization techniques were employed to get a wider frequency response in stereo with reduced distortion and greater headroom.)

What was a major issue with CD-4 technology?