20-03-2018, 10:52 PM
As far as I know, System C lasted until 1978. I’d guess that System F was abandoned once standards conversion equipment that was reliable enough, economic enough and had an output of sufficient quality for full-time use was readily available. Given that all of the Belgian transmitters were from the start designed to handle both Systems C and F, then at the transmission level the abandonment of System F would have been painless.
The original French SECAM work, with 819 lines and an 8.37 MHz AM subcarrier was described in Wireless World 1957 September, pp.426 to 429. I imagine that very soon after, it would have been apparent that the second French TV service, in the UHF band, was going to use 625 lines, hence development work was switched to that standard. Whether the change from an AM to a FM SECAM subcarrier occurred before or after the line-count change I do not know. Conceivably the French chose a 6 MHz vision bandwidth for the 625-line service to avoid undue truncation of the SECAM upper colour FM subcarrier sideband. Vestigial sideband FM might have been seen as being a bit awkward. (The only example of asymmetric sideband FM that I am aware of is in the CD-4 four-channel gramophone disc system, herein the subcarrier is single sideband FM for audio frequencies above 10 kHz. Any resultant harmonic distortion would have been above the audible band.)
When System L was extended to Band I, after the closure of System E, it required inverted channels and was known as System L’. Standardization decisions sometimes have a very long reach and perhaps unintended consequences. In the mid-1950s, the System E receiver IF was standardized at 28.05 MHz vision, 39.2 MHz sound. Since oscillator low was not possible at Band I, this eliminated channel F3 but allowed the use of channels F2 an F4, both inverted. So at the time it would have been a better choice than the alternative 35.7 MHz vision, 24.55 MHz sound, which would have eliminated channels F2 and F4. With System L, the chosen IFs were 32.7 MHz vision, 39.2 MHz sound, the latter being common with System E for ease of dual-standard receiver design. This was fine with the 625-line channels at UHF and Band III, where oscillator-low operation was possible. But it required inverted channels for Band I in order to fit oscillator-high operation, no problem for regular domestic receivers but awkward for multistandard receivers.
Cheers,
Steve
The original French SECAM work, with 819 lines and an 8.37 MHz AM subcarrier was described in Wireless World 1957 September, pp.426 to 429. I imagine that very soon after, it would have been apparent that the second French TV service, in the UHF band, was going to use 625 lines, hence development work was switched to that standard. Whether the change from an AM to a FM SECAM subcarrier occurred before or after the line-count change I do not know. Conceivably the French chose a 6 MHz vision bandwidth for the 625-line service to avoid undue truncation of the SECAM upper colour FM subcarrier sideband. Vestigial sideband FM might have been seen as being a bit awkward. (The only example of asymmetric sideband FM that I am aware of is in the CD-4 four-channel gramophone disc system, herein the subcarrier is single sideband FM for audio frequencies above 10 kHz. Any resultant harmonic distortion would have been above the audible band.)
When System L was extended to Band I, after the closure of System E, it required inverted channels and was known as System L’. Standardization decisions sometimes have a very long reach and perhaps unintended consequences. In the mid-1950s, the System E receiver IF was standardized at 28.05 MHz vision, 39.2 MHz sound. Since oscillator low was not possible at Band I, this eliminated channel F3 but allowed the use of channels F2 an F4, both inverted. So at the time it would have been a better choice than the alternative 35.7 MHz vision, 24.55 MHz sound, which would have eliminated channels F2 and F4. With System L, the chosen IFs were 32.7 MHz vision, 39.2 MHz sound, the latter being common with System E for ease of dual-standard receiver design. This was fine with the 625-line channels at UHF and Band III, where oscillator-low operation was possible. But it required inverted channels for Band I in order to fit oscillator-high operation, no problem for regular domestic receivers but awkward for multistandard receivers.
Cheers,
Steve
