Spectrum are an Australian progressive rock band which formed in April 1969 and broke up in April 1973. The original line-up was Mark Kennedy on drums, Lee Neale on organ (ex-Nineteen87), Bill Putt on bass guitar (ex-Lost Souls), and Mike Rudd on guitar and lead vocals (ex-Chants R&B, The Party Machine). In August 1970 Kennedy was replaced by Ray Arnott on drums. These members also performed under the alter ego, Indelible Murtceps, from 1971 to 1973. Spectrum had a number-one hit, "I'll Be Gone" (January 1971), on the Go-Set National Top 60 singles chart. After Spectrum and Indelible Murtceps disbanded, Putt and Rudd formed Ariel. In 1999 the pair formed Spectrum Plays the Blues, which later trimmed their name back to Spectrum. On 7 August 2013 Bill Putt died, after a heart attack.

Spectrum were formed in April 1969 in Melbourne, as a progressive rock group, by Mark Kennedy on drums (ex-Gallery), Lee Neale on organ (ex-Nineteen87), Bill Putt on bass guitar (ex-Gallery, The Lost Souls), and Mike Rudd on guitar, harmonica and lead vocals (ex-Chants R&B, The Party Machine, Sons of the Vegetal Mother). Initially the group drew on the work of contemporary bands such as Traffic, Soft Machine and Pink Floyd; they played cover versions of their material. Spectrum developed their own style as Rudd began writing original material. Alongside Kennedy's drum solos, Putt's bass playing and Neale's keyboard work, a feature of Spectrum's sound was Rudd's guitar playing — he eschewed the near-universal use of guitar picks – using a finger-picking style on a vintage Fender Stratocaster to develop a characteristic sound.

During twelve months of regularly performing on the local dance and discothèque circuit, Spectrum refined their original material. They appeared at various "head" (see cannabis slang) venues around Melbourne: T.F. Much Ballroom, Garrison and Sebastian's, alongside other progressive rockers, Tully, Tamam Shud and Sons of the Vegetal Mother. Spectrum used an elaborate set-up which included a large PA and a full multi-media light show; often supplemented by a performance troupe, Tribe. Early in 1970 they cut a demo single which they hawked to record companies as a 7" acetate. One side was an early, folk version of "I'll Be Gone". The B-side was another original, "You Just Can't Win". According to rock music historian, Ian McFarlane, these acetates are now "impossibly rare" and only two or three copies are known to have survived.

Despite a loyal following and praise from the music press – including from Australia's pop newspaper Go-Set – the band were almost broke by mid-1970. After they signed to EMI under their progressive rock imprint, Harvest Records, the band went into the studio in August 1970 to make their first official recordings, using producer, Howard Gable (The Masters Apprentices). Rudd described the recording sessions, "[we had] gone into the studio in order to record 'Launching Place Parts I & II', to help promote the Launching Place Festival. When we had finished recording those, Howard asked us did we have any others and I said, 'Yeah, I've got this one called "I'll Be Gone"', which we recorded as an afterthought. But then it became a hit single." Rudd had decided to add a harmonica part to the song.

The festival at Launching Place occurred in December 1970. "I'll Be Gone" was released in January 1971, which became a surprise No. 1 Australian hit on the Go-Set National Top 60 singles chart in May that year. It became one of the most enduring Australian rock songs of that era. McFarlane described it as a "shuffling, hypnotic" track, for which "suddenly the band was in big demand."

Spectrum released their debut studio album, Spectrum Part One, in March 1971, although it did not include the hit single. Rudd had refused to add it as "It didn't fit with the innovative roaming style of the rest of the music." The album reached No. 10 in April. Kennedy had left in August of the previous year just after it was recorded, he had "lost patience" as the group "struggled for gigs (promoters found them 'too progressive')." He was replaced on drums and vocals by Ray Arnott (ex-Chelsea Set, Cam-Pact, Company Caine). Rudd praised Kennedy's musicianship: "Mark really carried us through the first year because people would say 'Wow, look at that drummer, they must be a good group'... He used to play things like drum solos!, but he was very good. By the same token, I was almost relieved to get away from that 'cause the emphasis swung back to the material and the band in general rather than one player." Kennedy later worked with Leo de Castro, Ayers Rock and then Marcia Hines.

Spectrum's follow up singles, "Trust Me" (June 1971) and "But That's Alright" (November), did not reach the top 60. During October that year Spectrum formed a side project, Indelible Murtceps, using the same line-up of Arnott on drums, Neale on electric piano, Putt on bass guitar and Rudd on vocals and guitar. They performed at pubs, and local dances, playing a more dance/pop-oriented repertoire and using a simpler set-up than when performing as Spectrum.

Their second album, Milesago, was released in December 1971, as the first Australian rock music double album. It is also the first Australian rock album to be recorded using a 16-track recorder – newly installed – at Armstrong Studios during the previous September. McFarlane stated that it "remains one of the landmark releases of the Australian progressive rock era." It peaked at No. 9 on the Go-Set Top 20 Albums chart. By September 1972 Neale had left both bands; he was replaced by John Mills on keyboards. Neale quit the music scene permanently after leaving Spectrum and Indelible Murtceps.