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  • In synthesizer
    A synthesizer is an electronic instrument capable of producing sounds by generating electrical signals of different frequencies. These electrical signals are played through a loudspeaker or set of headphones...

    s, capable of sounding two voices, or note
    In music, the term note has two primary meanings:#A sign used in musical notation to represent the relative duration and pitch of a sound;#A pitched sound itself....

    s, at a time. Compare: monophonic, polyphonic.

  • Duophonic is also a term used to refer to a sound process by which a monaural
    Monaural or monophonic sound reproduction is single-channel. Typically there is only one microphone, one loudspeaker, or channels are fed from a common signal path...

     recording is turned into a kind of "fake stereo
    STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...

    " by splitting the signal into two channels, delaying the left and the right channels by means of delay line
    Delay line
    Delay line may refer to:* Propagation delay, the length of time taken for something to reach its destination* Analog delay line, used to delay a signal...

    s 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
    High-pass filter
    A high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...

    , then cutting the treble frequencies in the other channel with a low-pass filter
    Low-pass filter
    A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...

    . The end result was a synthesized stereo effect, without giving the listener the true directional sound characteristics of real stereo. In some cases, the effect was enhanced with reverb and other technical tricks, sometimes adding stereo echo to mono tracks in an attempt to fool the listener.

"Duophonic" was used as a trade name for the process by Capitol Records
Capitol Records
Capitol Records is a major United States based record label, formerly located in Los Angeles, but operating in New York City as part of Capitol Music Group. Its former headquarters building, the Capitol Tower, is a major landmark near the corner of Hollywood and Vine...

 for re-releases of mono recordings from June of 1961 through the 1970s. 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 their biggest releases, including a variety of albums by The Beach Boys
The Beach Boys
The Beach Boys are an American rock band, formed in 1961 in Hawthorne, California. The group was initially composed of brothers Brian, Dennis and Carl Wilson, their cousin Mike Love, and friend Al Jardine. Managed by the Wilsons' father Murry, The Beach Boys signed to Capitol Records in 1962...

 and Frank Sinatra
Frank Sinatra
Francis Albert "Frank" Sinatra was an American singer and actor.Beginning his musical career in the swing era with Harry James and Tommy Dorsey, Sinatra became an unprecedentedly successful solo artist in the early to mid-1940s, after being signed to Columbia Records in 1943. Being the idol of the...

. Over the years however, some Duophonic tapes were confused with true stereo recordings in Capitol Records' vaults, and wound up getting accidentally reissued on CD throughout the 1980s and 1990s. In recent years, Capitol reissued some of The Beatles
The Beatles
The Beatles were an English rock band, active throughout the 1960s and one of the most commercially successful and critically acclaimed acts in the history of popular music. Formed in Liverpool, by 1962 the group consisted of John Lennon , Paul McCartney , George Harrison and Ringo Starr...

' Duophonic mixes on The Capitol Albums, Volume 1
The Capitol Albums, Volume 1
The Capitol Albums, Volume 1 is a box set compilation comprising The Beatles' 1964 American Capitol Records releases. The set, which features the official stereo versions of a number of tracks on compact disc, was released in late 2004...

 and The Capitol Albums, Volume 2
The Capitol Albums, Volume 2
The Capitol Albums, Volume 2 is a box set compilation composed of The Beatles' 1965 American Capitol Records releases. The set, which contains stereo and mono versions of all 92 tracks was announced on 22 March 2006...

, in 2004 and 2006, respectively.

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
Miles Davis
Miles Dewey Davis III was an American jazz musician, trumpeter, bandleader, and composer. Widely considered one of the most influential musicians of the 20th century, Miles Davis was, with his musical groups, at the forefront of several major developments in jazz music, including bebop, cool jazz,...

('Round About Midnight-CL 949 mono, reissued in stereo as PC 8649).

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