EMI 2001

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).

First produced in 1966, by the early 1970s almost all of BBC Television's studios and many outside broadcast (OB) units were equipped with the 2001.[1] Several ITV companies purchased or leased the camera including Thames Television, Yorkshire Television, Associated Television/Central Independent Television, Granada, HTV, Anglia and London Weekend Television. Independent outfits such as the early cable television stations Rediffusion Cablevision, Sheffield Cablevision and the educational television arm of the Inner London Education Authority also purchased the camera.

Although there was no predicted lifespan for the camera, the heavy hot-running four-tube design was considered somewhat outdated even when it was new, which contributed to the camera's near-total failure to sell to broadcasters outside the UK. Furthermore, when EMI closed-down the Broadcast Equipment Division in the late 1970s, studios were deprived of technical and spares support for their cameras. Consequently, several ITV companies began replacing them in the late-1970s with the last commercial operators (Yorkshire & Central) both phasing them out in 1986 (in the main, Central had disposed of them in 1984, however they were used for continuity and presentation from their Birmingham operation until 1986). However the BBC kept a number of such cameras in operation at BBC Television Centre, their various regional outposts and their BBC Elstree Centre for some years afterwards, the last being at Elstree until July 1991; they were kept running by "cannibalising" identical cameras left behind by Central when the BBC purchased Elstree from them in 1984 as well as BBC EMI 2001s disposed of in previous years.

In 1963, prior to the development of the 2001, an experimental four tube camera[2] was constructed by EMI engineers. This experimental camera had been inspired by RCA's new four tube camera, the TK-42,[3] and used the same tube arrangement, i.e. a 4½ inch image Orthicon tube in the luminance channel and three 1” Vidicon tubes in the colour channels. In addition, the experimental camera had an integrally mounted Varotal III zoom lens. It was demonstrated to the BBC in 1964[4] where it received a mixed reception. Pictures from the camera had disappointing colorimetry, but sharp luminance detail.

A production version of this camera was planned, the EMI 2000, but this camera was never built. Instead, EMI opted to use the newly available Plumbicon tubes[5] supplied by Philips, in its new camera, the EMI 2001.[6] After successful trials, production quantities of this camera became available only just in time for the launch of the BBC's colour television service, in 1967. (The BBC also purchased cameras from other suppliers, namely the Marconi Mk VII four-tube cameras,[7][8] which were used in the weather, news and presentation studios in Television Centre, and Philips' LDK3 three-tube cameras[9] mainly used for outside broadcasts).

When sold abroad, the EMI 2001 was carried under the Thomson SA brand – hence "Thomson TH.T 2001". How this came about is unknown as EMI and Thomson SA did not have business links.

The Thomson 2001's, like the EMI's, also used Plumbicons; however, due to a brochure which was printed in French, it was presumed that they used Vidicon tubes.[10] But, apart from the silver viewfinder squares (instead of white) and the brand name change on the front and sides, the cameras were the same.

In the United States, the cameras were marketed by International Video Corporation as the IVC/EMI 2001-B (four tubes), with another version, the IVC/EMI 2001-C, consisting of three tubes.[11][12][13] Only one U.S. station is known to have purchased the 2001: WSNS-TV in Chicago, in the early years of its operation.[14]

Early 'concept prism assembly', where the individual prisms were machined from Perspex

Plan view of the system in the plane of the Red-Blue channels

Plan view of the system in the plane of the Luminance-Green channels

The EMI 2001 used a 4-way prism assembly to split the light into its components, using the same novel principles that had been developed by Philips for their 3-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.[24] The 3-way prism was also described in a description of the LDE3 camera.[21]

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.[25]

Consequently, EMI chose to use a 4-tube version of the prism splitter for their new colour camera,[6] 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 4-tube camera type KT-116M.[26])

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 4-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 x 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

Rank Taylor Hobson declined to offer a zoom lens for EMI's new camera, claiming they were fully committed elsewhere, but the Angenieux Company (see Pierre Angenieux) expressed their interest in supplying zoom lenses for the project. The French company offered two zoom lenses for the camera; the first was a 10:1 zoom for studio use and the second a larger unit for outside broadcasts. Both could be accommodated within the body of the camera, although the O.B. lens did protrude a little.

To accommodate a 4-way prism splitter, extra distance was needed, from the back of the lens to the image focal plane, when compared to a 3-tube splitter. This set severe demands for the lens designer, but Angenieux were able to achieve EMI's requirements, provided that field flattening lenses were fitted in front of each pick-up tube. Early cameras used this arrangement, but with later zoom designs these lenses became unnecessary.

The servo motors and the servo amplifiers were supplied by Evershed Power Optics.[27] The driver amplifiers for the servo motors were mounted in the camera body alongside the lens. The servo-driven zoom lens and the associated amplifier circuitry added considerably to the weight of the camera. In addition, incorporating the servo drivers within the camera body precluded the use of other makes of zoom lens.

Ring-of-three amplifier circuit

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(1). In this mode, the amplifier behaves somewhat like a 'current feedback amplifier'.[37] The circuit maintains its bandwidth as the gain is increased (by reducing R2), unlike a conventional voltage feedback op. amp.[38]

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(2) and the series resistor R2.

Band defining filter circuits

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[39] 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.[40]

Band defining filter responses

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:

${\displaystyle \phi (f)=-6.36\times 10^{-5}\times f}$

where ‘f’ is the frequency in Hz and φ(f) is in degrees. At low frequencies, the propagation delays of the two filters are both the same (177 ns, approximately).

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,[15] or by a special dichroic surface which reflects light to the luminance tube with the required luminosity function[7]).

For a basic 3-colour system the wideband luminance signal (Y'), for NTSC, PAL and SECAM, is given by:[41][42]

${\displaystyle Y'=0.3R^{\frac {1}{\gamma }}+0.59G^{\frac {1}{\gamma }}+0.11B^{\frac {1}{\gamma }}}$

In the case of a separate luminance tube, with appropriate spectral shaping, the output signal (Y) is given by:

${\displaystyle Y=0.3R+0.59G+0.11B}$

which when gamma-corrected gives:

${\displaystyle Y^{\frac {1}{\gamma }}=(0.3R+0.59G+0.11B)^{\frac {1}{\gamma }}}$

${\displaystyle Y^{\frac {1}{\gamma }}}$ does not equal ${\displaystyle Y'}$ except when R = G = B, which corresponds to neutral (grey) tones.

When deriving the two narrowband colour difference signals containing R_N and B_N , a bandlimited version of the luminance signal (y') is required, namely:

${\displaystyle y'=0.3R_{N}^{\frac {1}{\gamma }}+0.59G_{N}^{\frac {1}{\gamma }}+0.11B_{N}^{\frac {1}{\gamma }}}$

but the band limited signal from the luminance tube is:

${\displaystyle y^{\frac {1}{\gamma }}=(0.3R_{N}+0.59G_{N}+0.11B_{N})^{\frac {1}{\gamma }}}$

As before, ${\displaystyle y^{\frac {1}{\gamma }}}$ does not equal ${\displaystyle y'}$. If the gamma corrected luminance signal ${\displaystyle y^{\frac {1}{\gamma }}}$ 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[6][43] is used to overcome this problem.

A band-limited luminance difference correction signal, ΔL is formed, where:

${\displaystyle \Delta L=y^{\frac {1}{\gamma }}-y'}$

This narrowband signal is used to correct the wideband luminance channel at low frequencies, so the monchrome signal transmitted becomes:

${\displaystyle Y^{\frac {1}{\gamma }}-\Delta L=Y^{\frac {1}{\gamma }}-(y^{\frac {1}{\gamma }}-y')}$

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 ${\displaystyle y^{\frac {1}{\gamma }}}$ = ${\displaystyle y'}$ and the signal reverts to that of the luminance tube only.

Four-tube television camera

• Demonstration of EMI 2001 'lining up process by ex BBC OB operators
• Ex BBC OB camera operators discuss their experience working with the EMI 2001
• BBC Tech Ops History Site
• Museum of the Broadcast Television Camera
• The website of Martin Kempton on London television studios contains much on the industries use of various cameras over the decades, including the EMI 2001.