EMI 2001 - Comet Tails

Comet Tails

A very common artifact of early vacuum-tube cameras was a "comet tail". This is where a bright light or anything white builds up so much charge on the imaging surface of the camera tube that the charge cannot be removed in one "field" (1/25 of a second, the rate at which television pictures are made on PAL and SECAM broadcast TV systems). So when the camera is moved, the object causing the overload is moved, or something moves between the camera and the bright light source it causes a bright streak of where the source of the image flaw was. This is because that particular picture element on the camera tube's imaging surface is supposed to be imaging a dark surface, but it still has electric charge on that picture element and is read by the camera circuitry as light instead.

Most models of colour television camera had ACT (Anti-Comet tail) circuits for each individual camera tube and if they are not matched precisely, the comet tails appear to be coloured rather than white. The EMI 2001s didn't have ACT circuits. This means it is often very easy to tell if a programme used EMI 2001s (or any other first-generation PAL colour camera) to capture the images as the comet tails would often be coloured "blobs" or "splodges" (usually caused by a light source or light reflecting off a highly reflective or polished surface) simply because the camera did not have ACT circuits, but some were later modified to include ACT circuits. The ACT circuits were adjusted so that the Comet Tail doesn't appear to be a "blob".

Another artefact about the comet tails produced by the 2001s was that sometimes the comet tails would either be a mix of two separate colours, one colour inside the other (e.g. a comet tail that is red with a smaller comet tail inside that one that may be green). This is where both of the (in this example) red and green ACT circuits are not to the same adjustment of the other two tubes, but are adjusted at different levels themselves. In other cases, comet tails have been produced by 2001s that are not primary colours of light, e.g. pink. This is where the adjustments of both of the (in this example) red and white tubes are not adjusted to the same adjustment of the blue and green tubes, but are both themselves placed at the same adjustment.

ACT and the EMI 2001/1. As supplied by EMI the 2001 and the later 2001/1 did not have any form of ACT (anti-comet tail) or HOP (highlight overload protection). 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.

While some broadcasters may have modified their cameras to have ACT, retro fitting ACT/HOP was not an easy modification as 4 new HOP camera tubes would be needed, the tube bases, wiring harness, 4 head amplifiers and 4 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.

There was another method, ABO or DBO (Dynamic Beam Optimisation) that worked by measuring the tubes output and if it is more than a pre-set level above peak white, increasing the beam current in proportion. This DBO method could be used with standard tubes. If this is not implemented well it can lead to positive feedback, with loss of picture and damage to the tubes.