Casio MT-40

The MT-40 has a built-in bassline (internally referred to as the "rock" preset) that, along with the keyboard's suggested 1/16 note fill for that preset, formed the basis of a seminal reggae track, 1985's "Under Me Sleng Teng". The track's riddim went on to spawn nearly 500 cover versions. This song's success is widely credited with single-handedly transitioning reggae from analog to computerized production. This transition to a music production that depended on digital instruments and sequencers is also seen as catalyzing the computerization of hip-hop music, which was, like reggae, an analog musical tradition until the mid-1980s. Given the lasting consequences of the Sleng Teng riddim, the MT-40's "rock" preset has been the subject of considerable speculation. The famous preset was composed for Casio in 1980 by a then newly-hired music engineer, Hiroko Okuda. She was assigned to create several of the keyboard's presets, among them a "rock" rhythm. Okuda had been hired fresh out of music school, where she had produced one of Japan's first graduate theses on reggae. In 2022 she described herself as having been immersed in reggae for several years before her 1980 hire by Casio (the previous year, during Bob Marley's only visit to Japan, Okuda had attended more than three performances from the tour). Okuda only learned of her part in "Under Me Sleng Teng"'s global success in August 1986, when she read an article titled “The Sleng Teng Flood” in Japan's Music Magazine. The article described the wave of dozens and dozens of reggae songs being produced in Jamaica, all based on a Casiotone keyboard preset.

After the worldwide success of Sleng Teng many speculated as to the ultimate source of the "rock" preset. At first Eddie Cochran's 1958 song "Somethin' Else" or the Sex Pistols' 1976 track "Anarchy in the UK" were thought to be prime candidates, with "Somethin' Else" being widely accepted for decades by connoisseurs. In 2015 Okuda was quoted as saying the source was a track on an unnamed 1970's British rock album. This was later speculated to be David Bowie's "Hang On to Yourself", the 8th track on his 1972 album The Rise and Fall of Ziggy Stardust and the Spiders from Mars. However, referring to the question in 2022, Okuda said, “I did use to listen to a lot of British rock, so I’m sure there must have been songs that influenced me. But really, the bassline was something I came up with myself. It wasn’t based on any other tune.” According to Okuda, there was some talk at the Casio corporation of attempting to defend their intellectual property from its free use in "Under Me Sleng Teng" with lawsuits. However, other voices at Casio (among them the head of the Musical Instrument division, co-founder of the company, and second-eldest Kashio brother, Toshio Kashio) prevailed. Toshio Kashio in particular felt strongly that the company's mission ought to be “bringing the pleasure of playing a musical instrument to everyone.” Despite the minute size and financial importance of the Musical Instrument division compared to the company's calculator division, then its main breadwinner, Toshio Kashio's defence of free use set a decisive corporate precedent. To the present day Casio's response to clearance requests for the "rock" preset has been an acknowledgement that the song “uses a sound file taken from a Casio MT-40”, and no fee. The preset is accessed by pressing the "synchro" button and then the "D" bass button (second from left) while the MT-40 rhythm slider is in the "rock" position.