Spectrum was an Australian progressive rock band which formed in Melbourne in 1969 and remained in existence until 1973. Its members also performed under the alter-ego Indelible Murtceps.

The central figure in the band was singer-songwriter-guitarist Mike Rudd, a New Zealand born singer, songwriter and guitarist from Christchurch. Mike arrived in Australia in 1966 as rhythm guitarist for the NZ group Chants R&B . That band only lasted a short time after they arrived in Australia, but Rudd remained in Melbourne, teaming up with singer/songwriter Ross Wilson and guitarist Ross Hannaford .

Wilson and Hannaford's first band The Pink Finks (which had also just broken up) worked in a similar vein to Chants, and had already had some local chart success in Melbourne. Mike was invited to be the bass player in a later lineup of their next band, The Party Machine (1967-69).

Rudd, Wilson and Hannaford then formed the more experimentally-oriented Sons of The Vegetal Mother (1969-71), which was an occasional event-based project rather than a full-time band. When that band split, Rudd formed his own band, Spectrum. This marked the beginning of his partnership with bassist Bill Putt , formerly of Melbourne bands Gallery and The Lost Souls ; they formed a lasting friendship and musical partnership, and have worked together ever since. Organist Lee Neale came from pop band Nineteen 87, and drummer Mark Kennedy had already worked with Putt in Gallery. Kennedy's considerable skill helped carry the band through a difficult first year, during which time the band honed their skills and found their sound

Initially, Spectrum drew on the work of contemporary bands like Traffic, Soft Machine and Pink Floyd and they played covers of music by these groups in the early days, but they soon developed their own style.

Twelve months of performing allowed them to develop their sound and write and refine a substantial set of original material, which became the basis for their first LP. They were regulars at the concerts events held at the various "head" venues around Melbourne like the T.F. Much Ballroom , Garrison and Sebastian’s, and they appeared with other leading progressive bands like Tully , Tamam Shud and Sons Of The Vegetal Mother.

Rudd had conceived Spectrum as a concert band, and they generally performed with a large and costly set-up, which included a large PA and a full multi-media light show. At one stage the band even supplemented their act with members of the Melbourne performance troupe Tribe. They played at all the major Australian rock festivals of the period, including Wallacia, Myponga, Mulwalla, Rosebud and Sunbury. However, their national success was limited by a lack of radio airplay in other capitals, and the fact that they rarely ventured outside Victoria, visiting other cities only intermittently.

Just prior to being signed up by EMI, Spectrum cut a demo single which they hawked around to record companies as a 7" acetate. One side was an early, folky version of one of the newer songs in their set, "I'll Be Gone "; the flip was another original, "You Just Can't Win". According to rock historian Ian McFarlane, these acetates are now "impossibly rare" and only two or three copies are known to have survived.

Once signed to EMI, the band went into the studio to make their first official recordings, under producer Howard Gable , who had established himself as one Australia's leading producers with his work for bands like The Masters Apprentices .

Unfortunately for Spectrum, the nature of the Melbourne music scene was undergoing a profound change. This was partly due to legislative changes to the age of majority (which had been recently lowered in many Australian states from 21 to 18) and to the licensing laws governing entertainment in hotels in Victoria. These legislative changes coincided with demographic changes -- the young audiences who had patronized the unlicensed dances and discos of the mid-to-late Sixties were now ageing into their late teens and early twenties, and could now enter licensed premises. Seeing the popularity of rock music and realising the financial potential, hoteliers wooed customers into the pubs by putting on popular bands, often free of charge. Consequently, while Spectrum worked to establish themselves as a concert group, the focus was beginning to shift away from the larger concert events and unlicensed city discotheques frequented by “head” audiences, whose main drug of choice was cannabis, and towards the burgeoning pub circuit, where the drug of choice -- alcohol -- was cheap, plentiful and, above all, legal. While the smaller pub venues and their rowdy, combative atmosphere suited more ‘aggressive’ bands like Billy Thorpe & The Aztecs , the erosion of the concert scene posed a major problem for Spectrum, whose complex music and upscale presentation demanded both a larger audience, and a reasonable degree of concentration from them. Consequently, as the pub circuit began to boom, the bigger engagements that Spectrum needed to survive became fewer and less regular.

Despite a loyal following and much praise from the music press (notably Go-Set) the band were virtually broke by mid-1970, when a measure of salvation came in the form of a contract with to the new EMI progressive imprint Harvest. They were signed to a recording contract and scored an surprise #1 Australian hit with their first single, "I'll Be Gone", which has become one of the most enduring Australian rock songs of that era.

They released their debut LP Spectrum Part One in late 1970, and drummer Mark Kennedy left just after it was recorded, and was replaced by Ray Arnott. Kennedy later worked with a number of important Australian acts inlcuding Ayers Rock .

Spectrum's second album, released in early 1972, was an ambitious 2LP set called Milesago, notable as the first Australian rock double album and still regarded as a landmark of Australian progressive music. Their next LP, Warts Up Your Nose (1972), was released under the pseudonym "Indelible Murtceps" ('murtceps' is 'spectrum' spelled backwards) and contained songs of a more humourous nature. Lee Neale suffered a breakdown and left the band soon after its release, quitting the music scene permanently; he was replaced by Canberra musician John Mills.

Spectrum's final studio LP was Testimonial (1973). Soon after its release Spectrum announced their breakup, a move prompted by the departure of drummer Ray Arnott; Rudd and Putt reportedly felt that it would be impossible to recreate the special feeling of the group and wso decided to end the band. They played their farewell performance at the Dallas Brooks Hall in Melbourne on 15 April 1973, and the show was recorded and subsequently released as the live LP Terminal Buzz.

Following the demise of Spectrum, Rudd, Putt and Mills joined forces with guitarist Tim Gaze and drummer Nigel Macara , ex-members of leading Sydney 'underground' band Tamam Shud , to form the group Ariel .