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Monaural or monophonic sound reproduction (often shortened to mono) is single-channel. Typically there is only one microphone, one loudspeaker, or (in the case of headphones and multiple loudspeakers) channels are fed from a common signal path. In the case of multiple microphones the paths are mixed into a single signal path at some stage.
Monaural sound has been replaced by stereo sound in most entertainment applications. However, it remains the standard for radiotelephone communications, telephone networks, and audio induction loops for use with hearing aids. A few FM radio stations, particularly talk radio shows, choose to broadcast in monaural, as a monaural signal has a slight advantage in signal strength over a stereophonic signal of the same power.
While some experiments were made with stereophonic recording and reproduction from the early days of the phonograph in the late-19th century, until the second half of the 20th century, monaural was the rule for almost all audio recording.
Monaural sound is normal on:
- Phonograph cylinders
- Disc records made before 1958, such as records made for playing at a speed of 78 rpm and earlier 16 2⁄3, 33 1⁄3, and 45 rpm microgroove records.
- AM radio
- Some FM radio stations which broadcast only spoken word or talk radio content in order to maximize their coverage area. Examples of this would be the CBC Radio One stations on the FM dial. Sometimes listeners will not be convinced that the signal is strong since there is no ST or STEREO LED lit.
- Subcarrier signals for FM radio, which carry leased content such as background music for businesses or a radio reading service for the blind.
Incompatible standards exist for:
- Later records (monophonic records—which had almost disappeared in the United States by the end of 1967—could be played with a stereo cartridge)
- Reel-to-reel audio tape recording (depending on track alignment)
Compatible monaural and stereophonic standards exist for:
- Compact audio cassette
- FM (and in rare circumstances AM radio broadcasting)
- VCR formats
- digital audio file on many computers in many formats (WAV, MP3, etc)
No native monaural standards exist for:
In those formats, the mono-source material is presented as two identical channels, thus being technically stereo.
At various times artists have preferred to work in mono, either in recognition of the technical limitations of the equipment of the era or due to a simple preference. This can be seen as analogous to filmmakers working in black and white—such as John Mellencamp's 2010 album, No Better Than This, recorded in mono just like the mid-20th century blues and folk records it emulated were. Some early recordings such as The Beatles first four albums - Please Please Me, With The Beatles, A Hard Day's Night, and Beatles For Sale - were re-released in the CD era as monophonic in recognition that the source tapes for the earliest recordings were two track, with vocals on one track and instruments on the other (even though this was only true on the first two, while the later pair had been recorded on four-track). This was actually intended to provide flexibility in producing a final mono mix, not to provide a stereo recording, although due to demand this was done anyway and the early material was available on vinyl in either mono or stereo formats. In the 1960s and 1970s, it was common in the pop world that stereophonic versions of mono tracks were generated electronically using filtering techniques to attempt to pick out various instruments and vocals, but these were often considered unsatisfactory due to the artifacts of the conversion process.
Monaural LP records were eventually phased out and no longer manufactured after the early 1970s, with a few exceptions. For example, Decca UK had a few double issues until the end of 1970—the last one being Tom Jones' "I Who Have Nothing"; in Brazil records were released in both mono and stereo as late as 1972. During the 1960s it was common that albums were released as both monaural LPs and stereo LPs, occasionally with slight differences between the two (again detailed information of The Beatles' recordings provide a good example of the differences). This was because many people owned mono record players which were incapable of playing stereo records, as well as the prevalence of AM radio. Because of the limited quantities pressed and alternate mixes of several tracks, the monaural versions of these albums are often valued more highly than their stereo LP counterparts in record collecting circles today.
On 9 September 2009, The Beatles re-released a remastered box set of their mono output spanning the Please Please Me album to The Beatles (commonly referred to as the "The White Album"). The set, simply called The Beatles in Mono, also includes a two-disc summary of the mono singles, B-sides and EP tracks released throughout their career. Also included were five previously unreleased tracks originally mixed for a Yellow Submarine EP. Bob Dylan followed suit on October 19, 2010 with The Original Mono Recordings, a box set featuring the mono releases from Bob Dylan (1962) to John Wesley Harding (1967). In 2011, The Kinks' mono recordings were issued as The Kinks in Mono box set, featuring the releases of the band's albums from Kinks (1964) to Arthur (1969), with three additional CDs of non-album tracks that appeared on singles and EPs.
Merged stereo 
Sometimes mono sound or monaural can simply refer to a merged path of stereo channels. Over time some devices have used mono sound amplification circuitry with stereo sound compatibility since it can cut the cost of the hardware. Some consumer electronics with stereo RCA outputs have a microswitch in the red (right stereo channel) RCA output which disables merging of stereo sound into the white (left stereo channel) RCA output. Common devices with this are VCRs, DVD/Blu-ray players, information appliances, set-top boxes, etc., though video game consoles have male RCA ends of cables with a proprietary multi A/V plug on the other end, where automatic stereo merging isn't possible, unless adapers are used.
Disadvantages to merged stereo involve muffling mirror-image stereo channels that are otherwise identical to the human ear. Electrically and mechanically, it places more stress on a loudspeaker since having an array of loudspeaker can mitigate issues with electrical load for a single loudspeaker coil. In video games, merging stereo to mono sound prevents player from discerning what direction distant SFX are coming from, though reverse stereo has a similar setback too.
Mirrored mono 
Mirrored mono sound is the opposite of merged stereo, since it can be a case where media with mono sound that stereo playback devices automatically mirror it with are played on both channels of the receiver. It can also mean having a mono input mixed down to stereo amplification circuitry, or a mono system with a headphone output compatible with stereo headphones. And example of an application where both merged stereo and mirror mono apply is when a compact audio cassette respectively plays back "summed" stereo channels on a mono reading head, and when a compact cassette recorded with mono sound is played back with a stereo tape head.