The greater thickness and width of 35mm magnetic film prevented tape layer print-through and pre-echo and gained extended frequency range and transient response. The Mercury Living Presence recordings were remastered to CD in the 1990s by the original producer, using the same method of 3-to-2 mix directly to the master recorder.
The development of quadraphonic music records was announced in 1971. These recorded four separate sound signals. This was achieved on the two stereo channels by electronic matrixing, where the additional channels were combined into the main signal. When the records were played, phase-detection circuits in the amplifiers were able to decode the signals into four separate channels. There were two main systems of matrixed quadraphonic records produced, confusingly named SQ (by CBS) and QS (by Sansui). They proved commercially unsuccessful, but were an important precursor to later "surround sound" systems, as seen in SACD and home cinema today. A different format, CD-4 (not to be confused with compact disc), by RCA, encoded rear channel information on an ultrasonic carrier, which required a special wideband cartridge to capture it on carefully-calibrated pickup arm/turntable combinations. Typically the high frequency information inscribed onto these LPs wore off after only a few playings, and CD-4 was even less successful than the two matrixed formats.
In the late 1970s and 1980s, a method to improve the dynamic range of mass produced records involved highly advanced disc cutting equipment. These techniques, marketed as the CBS DisComputer and Teldec Direct Metal Mastering, were used to reduce inner-groove distortion.
Also in the late 1970s, "direct-to-disc" records were produced, aimed at an audiophile niche market. These completely bypassed the use of magnetic tape in favor of a "purist" transcription directly to the master lacquer disc. Also during this period, "half-speed mastered" and "original master" records were released, using expensive state-of-the-art technology. A further late 1970s development was the Disco Eye-Cued(TM) system used mainly on Motown 12" singles released between 1978 and 1980. The introduction, drum-breaks or choruses of a track were indicated by widely separated grooves, giving a visual clue to DJs mixing the records. The appearance of these music records is similar to an LP, but they only contain one track each side.
The early 1980s saw the introduction of "dbx-encoded" music records, again for the audiophile niche market. These were completely incompatible with standard record playback preamplifiers, relying on the dbx compandor encoding/decoding scheme to greatly increase dynamic range (dbx encoded disks were recorded with the dynamic range compressed by a factor of two in dB: