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Duophonic sound was a trade name for a type of audio signal processing used by Capitol Records on re-releases of mono recordings from June 1961 through the 1970s. In this process monaural recordings were reprocessed into a type of artificial stereo. Generically, the sound is commonly known as fake stereo or mock stereo.
This was done by splitting the mono signal into two channels, then delaying the left and the right signals by means of delay lines and other circuits, desynchronizing the two channels by fractions of a second, and cutting the bass frequencies in one channel with a high-pass filter, then cutting the treble frequencies in the other channel with a low-pass filter. The result was an artificial stereo effect, without giving the listener the true directional sound characteristics of real stereo. In some cases, the effect was enhanced with reverberation and other technical tricks, sometimes adding stereo echo to mono tracks in an attempt to fool the listener.
Capitol employed this technique in order to increase their inventory of stereo LPs, to satisfy retailer demand for more stereo content (and help promote the sale of stereo receivers and turntables). For nearly ten years, Capitol used the banner "DUOPHONIC – For Stereo Phonographs Only" to differentiate their true stereo LPs from the Duophonic LPs.
The process was used for some of the biggest Capitol releases, including albums by The Beach Boys and Frank Sinatra. Over the years however, some Duophonic tapes were confused with true stereo recordings in Capitol Records' vaults, and were reissued on CD throughout the 1980s and 1990s. Capitol intentionally reissued some of The Beatles' Duophonic mixes on The Capitol Albums, Volume 1 and The Capitol Albums, Volume 2, in 2004 and 2006, respectively.
On rare occasions some artists deliberately used fake stereo for artistic effect. Fake stereo can be used because elements of a mono mix cannot be reproduced for a stereo remix. The Beatles used this effect in the stereo version of the song "I Am the Walrus"; the first portion of the song is true stereo but switches to fake stereo at about the two-minute mark for the remainder of the song because the live radio feeds from a BBC broadcast of King Lear were mixed directly into the mono mix of the song and could not be replicated for the stereo mix. Later remixes of the song, such as that included in the Love soundtrack album, are in true stereo for the complete song. Similarly, the mono mix of the song "Only a Northern Song" featured sound effects which were made during the mixing process and were difficult to be remade for a stereo remix, so the song was released in fake stereo in the 1969 album "Yellow Submarine". However, the 1999 album Yellow Submarine Songtrack features a full stereo remix of the song and the 2009 remaster of the original 1969 album restores the song to its original mono mix because enhanced stereo has fallen out of favor.
Other record companies used similar processing of monophonic material to create a stereo effect, but referred to the process by other names. For example, Columbia Records used the logo, "Electronically Re-channeled for Stereo" on records issued with their particular process. As with Capitol, Columbia's fake stereo issues included albums by major artists, such as Miles Davis ('Round About Midnight, CL 949 mono, reissued in stereo as PC 8649).
- Brennan, Joseph. "The Usenet Guide to Beatles Recording Variations".
- Yellow Submarine 2009 Remaster Booklet