With such a camera, a sharp black and white picture was always assured, as it was not necessary to combine signals from the three colour tubes to provide the luminance detail.
In the early days of colour television (from the mid 1950s to the early 1960s) studio cameras were heavy and hot-running because of the vacuum tube (thermionic valve) circuitry that they contained, in addition to three large image orthicon pick-up tubes.
With these cameras there was always a tendency for the three coloured images to drift out of registration, over time, giving a consequential loss of picture sharpness.
In 1962, in order to address these stability problems, RCA announced their prototype four-tube camera.
The camera went into full production in 1963 and sales of several hundred of the model were achieved over the next few years.
Although achieving sharp pictures had always been a desirable aim, it was of particular concern in the early days of colour television when the majority of viewers still had black and white sets.
RCA engineers in 1964 considered that registration tolerances could be relaxed by three times,[8] relative to a 3-tube camera.
However, when properly set up, these image orthicon cameras could give pleasing pictures, which had the sharp crisp look that was a characteristic of the I.O.
[2]: 96 This new camera had a conventional box- shaped housing, but it was lighter and shorter and had a 5:1 zoom lens integrated within it.
[19] In 1963 engineers from Marconi and EMI visited RCA in order to inspect this new concept camera.
BBC engineers also visited RCA with a view to using the camera in the forthcoming UK colour service.
A production version of this camera, the EMI 2000, contained a similar complement of pick-up tubes, but was never built.
[2]: 116 (The Isocon tube was more sensitive than the image orthicon so enabling the camera to operate at very low light levels, in outside broadcast use.
(This tube had first been announced ten years previously, initially for medical use,[2]: 61 [31] but much time and effort had been invested in making it suitable for broadcast TV use[32]).
Philips Research Lab., following RCA's lead, also contemplated the construction of a four-tube camera.
[2]: 124 A combination of high quality Plumbicon tubes, improved registration techniques and electronic picture enhancement methods[2]: 123 [38][39][40] made the pictures from the 3-tube cameras completely acceptable while 4-tube cameras were perceived to be big, heavy and expensive to run.
Such an outcome had been predicted by BBC engineers[41] RCA ceased production of the TK-42 in 1966 and by 1968 was offering the TK-44A (a 3-plumbicon camera, similar in concept to the Norelco PC60.
[2]: 124 [42] In 1968 Marconi brought out their new Mark VIII camera, which was a light, neat, three tube design with integral zoom and auto-registration capability.
After reception and demodulation, the composite waveform is decoded to give red, green and blue signals which are applied to the three guns of a display tube.
RCA, in their TK-42, placed an optical filter in front of the luminance tube which transmitted light according to the required luminosity function.
[4] Marconi, in the Mk VII, used a dichroic mirror to reflect light to the luminance tube which had the required luminosity function.
[5][57] EMI, in their 2001 camera, formed a low frequency luminance signal from the band-limited colour channels, according to 3-tube practice.