The four-tube television camera, intended for color television studio use, was first developed by RCA in the early 1960s.”RCA unveils new four-color tv camera”, Broadcasting Mar 19th 1962, p.81Abramson A., “The History of Television, 1942 to 2000”, McFarland 2003 In this camera, in addition to the usual complement of three tubes for the red, green and blue images, a fourth tube was included to provide luminance (black and white) detail of a scene. 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 aims of the designers of the camera were, firstly, to produce a camera that was more tolerant to mis-registration and, secondly, to achieve a lighter camera by using smaller vidicon tubes to replace some of the large heavy IO tubes. The camera had an image orthicon tube for the luminance channel and three vidicon tubes for the colour channels. In addition, the camera was fully transistorized, apart from the four pick-up tubes. The camera went into full production in 1963 and sales of several hundred of the model were achieved over the next few years.
In the mid 1960s, following RCA’s lead, other versions of the 4-tube cameras were produced (see below for details). In many cases, advantage was taken of a newly available pick-up tube (the Plumbiconde Haan E.F., Schampers P.P.M., van Vucht J.H.N., “Method of Manufacturing a Photoresponsive Device Comprising a Photoresponsive PbO Layer”, US Patent 3,372,056, March 1968, filed Netherlands 1963). This new tube allowed cameras to be smaller and lighter than before.
However, by the end of the decade, 4-tube cameras had fallen out of favour with most manufactures and customers. The picture quality and stability of pictures from 3-tube cameras had improved markedly, thanks to solid state circuitry, improvements in the Plumbicon tubes and the use of picture enhancement techniques. In addition, 3-tube cameras were smaller, lighter and cheaper than the 4-tube versions. By the early 1970s, only a very few manufacturers still made cameras using the 4-tube format.
Advantages of the four-tube configuration
Source:Hirsch C.J., “Four Tube, Separate Luminance Color Television Camera”, RCA Broadcast News, Vo. 133, April 1967, pp.30-47
In a 3-tube colour camera, the luminance (black and white) signal is derived from a combination of the three colour signals, so severe demands are placed on the registration accuracy of the three images, if sharp pictures are to be achieved.“Four Plumbicon Colour Camera”, Wireless World, Feb. 1966, pp.58-61James I.J.P., “The Separate Luminance Colour Television Tube”, Jour. Colour Group No. 10, ed. Mellerio. http://www.colour.org.uk/JCG%2010%20via%20OCR.pdf With a four tube camera, however, this registration problem is avoided because the transient edges in the luminance signal all originate from a single tube. 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. It was considered essential that these viewers should see no degradation in picture quality when viewing colour transmissions.
In addition to allowing the registration accuracy between the colour images to be relaxed, the 4-tube camera allows additional mis-registration of these three images relative to the luminance image. This is because the presence of a sharp black and white image allows significant displacement of the colour content to be tolerated by the viewer;Prichard G., “The Print Guide/registration”, http://the-print-guide.blogspot.co.uk a feature long exploited by the publishers of cheap colour comics and magazines.
RCA engineers in 1964 considered that registration tolerances could be relaxed by three times,Watson S.N., Maurice R.D.A., and Stanley A.R., “An appraisal of the RCA 4-tube colour tv camera type TK-42”, BBC Tech. Rep. T-134, (1964/54) relative to a 3-tube camera. A later study by the BBC, comparing 3 and 4-tube arrangements Taylor E.W., “The Effect of Registration Errors in Three-Tube and Four-Tube Colour Television Cameras”, BBC Res. Rep. PH-11, (1967/47) concluded that tolerances could be relaxed by at least 2:1 in the 4-tube case.
A further advantage of the 4-tube system concerned noise in the picture. As the composite signals in NTSC, PAL and SECAM systems all used band-limited colour information, so the colour channels in a 4-tube camera could be band-limited, appropriately. This ensured that there was no contamination of the luminance picture by high frequency noise from those channels.
Disadvantages of a four tube system were increased costs, a larger and heavier camera, and reduced light levels on the colour tubes (light was ‘stolen’ by the luminance tube), making such a camera less suitable for outside broadcasts in low ambient lighting.
(An alternative way of producing a sharp luminance image was the three tube luminance-red-blue camera (the 'YRB' camera).Kell R.D., “Color Television Transmitter”, US Patent 2,750,439, 1956, filed June 1950 However, this system was not favoured by most manufacturers, because it was deficient in colour hue fidelity.Bedford A.V., “Color TV Camera System”, US patent 3,196,205, July 1965, filed 1961Hunt R.W.G., "The Reproduction of Colour", Fountain Press, England, 1995 Even so, a 3-tube camera of this type was produced by Bosch/Fernseh, their type KCU40 Summers B., “Museum of the Broadcast TV Camera”, European and American TV Cameras, Bosch KCU40, http://www.tvcameramuseum.org/bosch/kcu40/p1.htmSchneider H-D, “Television Camera System”, US patent 3729580, 1973, filed Apr. 1972).
Historical background
Early three-tube cameras Up to 1962, television studios used 3-tube image orthicon cameras for programmes in colour. For example, RCA offered the TK-41,"RCA TK-40/41 Color Television Cameras", http://www.ev1.pair.com/colorTV/TVcams-in-action.html GE the PC-15 and PC-25,”TV Cameras were the Eyes of a Generation, The GE TV Cameras”, http://eyesofageneration.com/GE_Cameras.php Marconi the BD848/B3200.Summers B., “European and American Television Cameras”, http://www.tvcameramuseum.org -Marconi cameras They were all big, heavy, cameras (for example, the TK-41 with zoom lens was 5 ft long and it weighed 500 lbsReitan E., "Color Television History", http://www.earlytelevision.org/Reitan/Color_Cameras.html ). 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. pick-up tubes (see Video camera tube - Image orthicon).
The first four-tube camera In March 1962, RCA demonstrated the first 4-tube camera, the TK-42X (‘X’ for experimental), which incorporated a 4.5” image orthicon tube, as the luminance tube, together with three vidicon tubes to provide the colour information. 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. The following year, with various technical issues resolved, the camera went into production. It now had a profile similar to the TK-41, but was considerably lighter and shorter.The Ellerbee Collection - RCA TK42, http://eyesofageneration.com/bobby_TK42.php
British four-tube cameras 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.Watson S.N., Maurice R.D.A., Stanley A.R., “An appraisal of the RCA 4-tube colour camera type TK-42”, BBC Tech. Rep. T-134, 1964/54
Following their visit, EMI quickly made an experimental 4-tube camera by cobbling together parts from an EMI Type 203 image orthicon studio cameraSummers B., “European and American Television Cameras”, http://www.tvcameramuseum.org – EMI 203 data with parts from an EMI Type 204 three-vidicon industrial colour cameraJames I.J.P., “A Vidicon Camera for Industrial Colour Television”, Jour, Brit. I.R.E., March 1959, pp. 165 – 180 This camera was demonstrated to the BBC and received a lukewarm reception there. It produced sharp pictures, as expected, but there were problems with colorimetry.Wood C.B.B, “Comparisons Between Three Colour Television Cameras”, BBC Tech. Rep. T-132, 1964/53 Pictures of this camera can be found on various websites.Summers B., “European and American Television Cameras”, http://www.tvcameramuseum.org - EMI Experimental Colour 1963, data sheet A production version of this camera, the EMI 2000, contained a similar complement of pick-up tubes, but was never built.
Marconi, following their visit to RCA, also promptly set about the development of a new 4-tube camera, similar in concept to the RCA one. However, this work was soon abandoned in favour of an all Plumbicon camera (the Mark VII camera), using the newly available Plumbicon camera tubes from Philips. The camera was completed in 9 months and was the first all-Plumbicon 4-tube camera to be available worldwide.Summers B., “European and American Television Cameras”, http://www.tvcameramuseum.org, - Marconi VII data sheet It was marketed in the US from February 1966 and achieved substantial sales there.
Lagging behind by over a year, EMI completed and demonstrated their type 2001 4-tube Plumbicon camera.James I.J.P., Perkins D.G., Pyke P.J., Taylor E.W., Kent D.E., Fairbairn I.A., “The EMI Four-Tube Colour Television Camera”, Radio and Electronics Engineer, Vol.39, No. 5, May 1970, pp.249 – 270 This camera was ready only just in time for the launch of the colour television service by the BBC, but then proved to be very popular in Britain, with cameras installed in most studios, nationwide. However, its late arrival, compounded by all the early sales going into the British market, resulted in the camera missing out on sales opportunities in the USA when, in the mid 1960s, a rapid expansion of the market took place. Furthermore, it made little impact in European markets which were either not ready or already dominated by Philips' 3-tube cameras.
Other developments
Following on from the TK-42, RCA produced several more cameras with a similar format. The TK-43 was a version of the TK-42, but with an external lens, The TK-44 had an Isocon tube in the luminance channel and three Plumbicons for the colour channels. (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. However, it was a short-lived tube). There was also a version of the TK-42 in which selenicon tubes replaced the vidicons. None of these cameras sold in anything like the quantities of the TK-42. RCA also produced a four vidicon film camera, the TK-27.RCA Broadcast News, Vol. 125, Feb. 1965