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 (J type) or 18 inches (K type).
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 (common with plate glass dichroic mirrors) 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 (red) tube sticking up, nearly at right angles to the other three. (This configuration was to be used in the Russian four-tube camera type KT-116M.Summers B. "Museum of the Broadcast Television Camera", Soviet Television Cameras)
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. (When viewed from the back of a camera the four tubes were seen as a diagonal cross). This optical arrangement defined the cross-section dimensions of the camera (which was not small – 380 × 380 mm), 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 (display tubes have, approximately, a square law characteristic with γ ≈ 2.2).
As shown below, it is beneficial, but not sufficient, to shape the luminance response to simulate that of the NTSC (PAL or SECAM) luminance characteristic (by, for example, placing an optical filter in front of the luminance tube to pass light with the required luminosity function, or by a special dichroic surface which reflects light to the luminance tube with the required luminosity function).
For a basic 3-colour system the wideband luminance signal (Y'), for NTSC, PAL and SECAM, is given by:Fink D.G., "Television Standards", Television Engineering Handbook, (Ed. Fink D.J.), Chapter 2, McGraw Hill 1957, pp. 2-1 to 2-54Mazda F.F.(ed.) "Colour Television Principles", Butterworths, 1989, Section 53.9.2, pp.53/10 to 53/12
In the case of a separate luminance tube, with appropriate spectral shaping, the output signal (Y) is given by:
which when gamma-corrected gives:
does not equal except when R = G = B, which corresponds to neutral (grey) tones.
When deriving the two narrowband colour difference signals containing RN and BN, a bandlimited version of the luminance signal (y') 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 CorrectionSproson W.N., "Colorimetric Aspects of 3- and 4-Tube Colour Television Cameras", Jour. Colour Group No. 10, July 1967, p.5 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 Vin(1). In this mode, the amplifier behaves somewhat like a 'current feedback amplifier'.Analog Devices, MT-034 Tutorial, "AD846 Current Feedback Op Amp", (1988), http://www.analog.com/media/en/training-seminars/tutorials/MT-034.pdf) The circuit maintains its bandwidth as the gain is increased (by reducing R2), unlike a conventional voltage feedback op. amp.Karki J., "Voltage Feedback Vs Current Feedback Op Amps, Application Report", Texas Instr., Dallas, Texas, Lecture Number SLVA051, Nov. 1998
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 Vin(2) 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-bandWeinberg L., "Network Analysis and Synthesis", McGraw-Hill, N.Y. 1962, pp. 499, 619 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.Guillemin E.A., "Communication Networks, Vol. 2", Wiley, N.Y., 1935, p.412
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 (f) is in degrees. At low frequencies, the propagation delays of the two filters are both the same (177 ns, approximately).
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.Lent S.J., "Pickup Tubes and Solid-state Cameras", TV & Video Engineers Reference Book, ed. Jackson K.G. and Townsend G.B., Butterworth-Heinmann Ltd., 1991, p. 10/3Vernon D., "Machine Vision", Chapter 2, Prentice Hall 1991, pp. 25-26, http://homepage.inf/ed.ac.uk/rbf/BOOKS/VERNON/Chap002.pdf"Bloom", Unity Manual, https://docs.unity3d.com/Manual/script-Bloom.html 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 1960sRedmond J., Morse D.R., Wood C.B.B, "The Plumbicon Camera Tube", BBC Report T-148, (1965/21) and all the early plumbicon cameras suffered from it,Summers B., www.tvcameramuseum.org including the EMI 2001."Tech and Trouble, the EMI Color 4 Tube Camera" http://techandtrouble.blogspot.co.uk/2013/09/the-emi-2001-color-4-tube-camera.html "EMI 2001" http://www.vintageradio.co.uk/htm/TVcollection.htm With separate mesh tubes there was some improvement, because higher beam currents could be used without loss of resolution.Lubszynski H.C. And Wardley j, "Some Problems of Resolution in Low-Velocity Camera Tubes", IEE Conf. Report, Series No. 5, Paper 4006-E, June 1963 Some cameras introduced anti comet-tail circuits to provide dynamic correction, when an overload was sensed (ACT circuits), 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.van Roosmalen J.H.T. "Suppression of Comettail Effect, US patent 3,548,250, 1970, first filed in Netherlands 6802062, Feb. 1968van Roosmalen J.H.T., "Adjustable Saturation in a Pick-up Tube with Linear Light Transfer Characteristic", Advances in Electronics and Electron Physics, Vol. 28, Part A, 1969, pp.281-288 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