EMI 2001

The EMI 2001 broadcast studio camera was an early, very successful British made Plumbicon studio camera that included the lens within the body of the camera. Four 30 mm tubes allowed one tube to be dedicated solely to producing a relatively high resolution monochrome signal, with the other three tubes each providing red, green and blue signals. Even though semiconductors were used in most of the camera, the highly sensitive head amplifiers still used thermionic valves in the first generation of the design.

Design

Integrating the lens within the body of the camera had both positive and negative effects. On the positive side, it meant the optical nodal point of the camera was close to the centre of gravity, which could make operation easier and more instinctive when used on movable camera mounts such as pedestals. The downside was that lens manufacturers were limited to which lenses they could adapt to fit to the camera. This made the 2001 less attractive for outside broadcasts.
The 2001 was both heavy and large. The pull-out handles at each corner needed four people to safely move the camera with the lens in place. It also required a separate remote camera control unit and the cable connecting the two was over 2 inches thick. The standard servo-controlled studio zoom lens had a 5 to 50° horizontal angle of view, with a minimum focus distance of either 36 inches or 18 inches.

Four-tube prism optics

The EMI 2001 used a four-way prism assembly to split the light into its components, using the same novel principles that had been developed by Philips for its three-way splitter. These new assemblies used the property of total internal reflection, within the prisms, to direct the light to the pick-up tubes. The techniques were described in a patent first filed in 1961. The three-way prism was also described in a description of the LDK3 camera.
The technique of using a prism assembly in this way was far superior to the earlier light-splitting arrangements, since the prism assembly was neat and compact and reproducibility in manufacture was much improved. The problems previously experienced with double imaging were also eliminated. Furthermore, because of the near-normal incidence of light onto the dichroic surfaces, sensitivity to polarised light was reduced.
Consequently, EMI chose to use a four-tube version of the prism splitter for its new colour camera, in order to retain all the advantages of the method. However, devising a single prism arrangement for four tubes was less easy than for three and several alternatives were initially considered.
In an early configuration of the prism block, shown in the thumbnail, three of the pick-up tubes were envisaged to be in a common plane, but with the fourth tube sticking up, nearly at right angles to the other three.
For the final optical arrangement in the EMI 2001, the green prism was changed to have a fully silvered mirror at about 45 degrees, to deflect the green light sideways, resulting in the final four-spoke arrangement.. This optical arrangement defined the cross-section dimensions of the camera, but it did allow the zoom lens to be located within the camera body. Also the removal of individual pick-up tubes was possible without any need to remove the scanning coils, as the tube bases were easily accessible at the outer corners.

Generating the image

The composite signals of the NTSC, PAL and SECAM systems are made up of a wideband luminance signal and two narrow-band colour difference signals containing B-Y and R-Y. If a band-limited version of the signal from the luminance tube is used to derive the colour difference signals, without modification, then colour errors would occur. This is because the luminance characteristic expected by the colour processing must be made up in a particular way, using specific proportions of red, green and blue, whereas the signal from the luminance tube has a more general monochromatic characteristic akin to that from a conventional black and white camera. In addition, the application of gamma correction to the signals further complicates the situation.
As shown below, it is beneficial, but not sufficient, to shape the luminance response to simulate that of the NTSC luminance characteristic.
For a basic 3-colour system the wideband luminance signal, for NTSC, PAL and SECAM, is given by:
In the case of a separate luminance tube, with appropriate spectral shaping, the output signal is given by:
which when gamma-corrected gives:
When deriving the two narrowband colour difference signals containing R_N and B_N, a bandlimited version of the luminance signal is required, namely:
but the band limited signal from the luminance tube is:
As before, does not equal. If the gamma corrected luminance signal is simply used instead of y' then colour errors result which can be appreciable for saturated colours.
In the EMI 2001 a process known as Delta-L Correction is used to overcome this problem.
A band-limited luminance difference correction signal, ΔL is formed, where:
This narrowband signal is used to correct the wideband luminance channel at low frequencies, so the monochrome signal transmitted becomes:
With this corrected luminance signal, the correct colour rendition is obtained, whilst still retaining the sharp luminance detail of a 4-tube camera. The narrowband R, G and B signals are gamma corrected and applied to a suitable matrixing circuit, to derive the correction. With grey scale scenes = and the signal reverts to that of the luminance tube only.

Transistor circuits – the ring-of-three

The circuitry in the 2001 was all solid state apart from the pick-up tubes and, in the early cameras, the first stage of the head amplifiers. The circuitry made extensive use of the 'ring-of-three' amplifier configuration, shown simplified in the figure. This circuit was easily adapted for various uses.
In the normal, non-inverting mode, the bottom of resistor R2 is grounded and the input is via V_in. In this mode, the amplifier behaves somewhat like a 'current feedback amplifier'. The circuit maintains its bandwidth as the gain is increased, unlike a conventional voltage feedback op. amp.
The circuit has a 'virtual earth' point at 'A' so that inverting or summing amplifiers are possible. In this mode, the base of TR1 is grounded and the input is via V_in and the series resistor R2.

Band defining linear phase filters

The EMI 2001 used band defining filters in all four channels. For the colour channels and narrow-band luminance, the low-pass filters had a Gaussian shaped pass-band and, although such filters were not 'sharp-cut', they were linear phase and gave negligible overshoots on transients. The wide-band luminance channel had its bandwidth defined by a linear phase low pass filter with a 3 dB cut-off at 6.8 MHz. Its design follows the lattice filter methods of Bode.
The amplitude responses of the two filters are shown below. Also shown are the phase deviations of the two filters from the linear phase/frequency characteristic, given by:
where is the frequency in Hz and is in degrees. At low frequencies, the propagation delays of the two filters are both the same.

Comet tails and blooming

"Blooming" refers to the situation where bright areas in a picture 'bleed' into adjacent dark areas, with a consequential loss of picture sharpness and detail. The condition leads to "comet-tails" which streak across a picture, following moving highlights.
The pictures from early Plumbicon cameras were susceptible to comet tails and blooming and, although these effects had not been a major concern in the previous generation of cameras, which employed vidicon or image orthicon tubes, they were an annoying feature of Plumbicon tubes.
The problems arose when the beam current of a pick-up tube was insufficient to fully discharge the target in very bright areas of an image. Reducing the target voltage and increasing the beam current of the pick-up tube helped to mitigate the problem but, with early Plumbicons, this resulted in loss of resolution and increased lag.
This problem with Plumbicon tubes was already of concern in the early 1960s and all the early plumbicon cameras suffered from it, including the EMI 2001. With separate mesh tubes there was some improvement, because higher beam currents could be used without loss of resolution. Some cameras introduced anti comet-tail circuits to provide dynamic correction, when an overload was sensed, but these were not used in the EMI 2001.
The problem was not satisfactorily resolved until the late 1960s when an extra 'anti-comet-tail' gun was introduced into Plumbicon tubes. New camera designs, produced in the 1970s, were able to include the new improved tubes, and usually did so. Some 2001 cameras were modified to take the new tubes, but it was a difficult retrofit procedure, because of the complexity of the additional circuitry.

ACT and the EMI 2001/1

As supplied by EMI, the 2001 and the later 2001/1, did not have any form of ACT or HOP. This is why its performance was poor in this respect, when compared with the next generation of cameras supplied in the 1970s. None of the first generation of true broadcast cameras in the middle to late 1960s had ACT, so the EMI 2001 was not unusual.
When observing old recordings, such as those from the 2001, it is very easy to tell if a programme used EMI 2001s to capture the images as the comet tails would often be coloured "blobs" or "splodges" simply because the camera did not have ACT circuits.
Some broadcasters modified their cameras to have ACT, but retrofitting ACT/HOP was not an easy modification as four new HOP camera tubes would be needed, the tube bases, wiring harness, four head amplifiers and four video amplifiers and the tube beam current boards would all have needed work done to them. ACT and HOP works by using an extra electrode in the tube to 'flood discharge' the target during the flyback period. Great care was needed in setting up the HOP voltages as damage to the tube's emission could occur. Once fitted, the ACT circuits were adjusted so that the comet tail didn't appear as a "blob".
Even when ACT circuits had been retro-fitted, comet tails would sometimes occur, consisting of either a mix of two separate colours, one colour inside the other, or the comet tail may be a non-primary colour, such as pink. The problems occurred when the settings of the ACT circuits were not well matched.