The four-tube television camera, intended for color television studio use, was first developed by RCA in the early 1960s. 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 Plumbicon). 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
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. 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; 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, relative to a 3-tube camera. A later study by the BBC, comparing 3 and 4-tube arrangements 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.