by chris_vk3aml » Thu Apr 24, 2008 9:09 am
Actually the 240 line Baird system WAS transmitted for the consumption of the general public. In fact, it was the system used for the very first official public program from the Alexandra Palace on 2 November 1936. The usage of the Baird 240 line system went ahead on a back-to-back public comparison against the Marconi-EMI system until 30 January 1937.
Doug Pitt told me years ago that he saw one or two of the transmissions, the general consensus being that the Baird system was much the superior for telecine, but that Marconi-EMI had the edge on practically every other account - camera mobility, multi-camera switching, flexibility of production, and lack of breakdowns. Rather more obviously, the lack of interlaced scanning on the Baird 240 line system gave it a flicker level that was obtrusive on the few sets capable of producing really good overall picture brightness at that time. (Perceived flicker reduces when the screen illumination is dim, which it then was on many sets having no screen aluminisation and EHT voltages of 6KV or lower).
The scanning discs used for telecine did not have 240 holes, as one might expect. The disc, with a relatively limited number of holes, spun at very high speed in an evacuated chamber. Its holes were not in the spiral pattern, but were concentric, and the light from a high intensity arc was focussed through them onto a constantly-moving film in a gate having no intermittent advance mechanism. A gearing-down from the rotational speed of the scanning disc to the steady movement of the film set the 240 line per frame scan. Effectively, the disc only provided the horizontal scan while the steady film advance set the frame (low speed) scan. Baird could have produced interlaced scanning by setting the holes in the disc in a spaced spiral pattern, but his rejection of interlace may have had something to do with EMI patent rights - Baird, however, had specified interlace in patents of the 1920s but not in this instance.
The optical inefficiency of Nipkow discs at this very high scanning rate meant that Baird was always battling for more light - see my notes on Nipkow disc efficiency in another recent posting here. The 240 line disc system could never be used for 'live' pickup without recourse to the flying spot pickup system, so all link announcements were made in total darkness by either Elizabeth Cowell or Jasmine Bligh who had to memorise their lines - no teleprompt or reading from scripts was possible in total darkness!
For "live" shows the Baird 240 line system used intermediate film - and this system permitted of only using one fixed camera position in the studio. Lens changes were made during transmission and were clearly visible to the viewing audience in necessary but obtrusive brief blackouts.
This was mechanical scanning pushed to its uttermost limit - some would even say "beyond".
The problem with the Marconi-EMI system on telecine involved the Emitron camera tubes producing appalling shading and smear artifacts which were, of course, absent in Baird's. And the Emitron gave a rather soft image which went nowhere near exploiting the 405 line definition of which the scanning system was eventually capable. The difference in perceived definition between Baird's 240 and EMI's 405 was actually almost impossible to choose at the time, according to contemporary reports and the memories of today's few survivors who saw the transmissions.
The Baird pioneers were brave, but Marconi-EMI - some would say unfortunately - had access to a patent pool involving Blumlein, Brown, RCA and Zworykin which became a formidable joint force.
Hope this sets the record straight,
All the best,
Chris Long VK3AML