01-11-2016, 08:22 PM
I have never seen any explanation as to why South Africa chose System I, but I imagine that there would have been a combination of technical and political factors involved. Being a late entrant as it were, I very much doubt that the choice would have been from amongst other than the 8 MHz channel variants of the 625-line system, that is Systems D/K, I, K’ and L. And of those, probably only I and K’ were real contenders.
System I came out of the UK TAC deliberations as reported in 1960. The TAC had initially determined that System B was not good enough, and that the System D parameters were much better. Then after further study it concluded that better use of the available 8 MHz channel would be obtained by extending the vestigial sideband to 1.25 MHz and consequently using a 5.5 rather than a 6.0 MHz main sideband.
The French developed System L in about the same time period as the TAC was deliberating, although it was formally adopted earlier than either the UK or Ireland adopted System I. The reasons for choosing the 1.25 MHz vestigial sideband were no doubt the same as those of the TAC, but the French stayed with the 6 MHz main sideband. This combination really occupied an 8.5 MHz channel, but that did not seem to be an obstacle. Previously, System E, originally designed for a 14 MHz channel, had been fitted to the tête-bêche channelling system that had 13.15 MHz spacing, so there was a precedent for “overfilling” the channel. Just possibly one reason for the retention of the 6 MHz main sideband was that it would have allowed enough room to avoid any single-sidebanding of the SECAM colour subcarriers. System L was chosen as positive/AM to align with System E and so minimize the complexity of dual-standard receivers. System K’, for the French Outré Mer territories, was effectively the negative/FM version of System L. At the time, the prevailing viewpoint seemed to be in favour of negative/FM (at least for monochrome, contrary opinions being expressed in respect of NTSC (and derivative) colour systems.), and whether the TAC just accepted this or whether it deliberated on the issue is unknown.
So if one accepted the TAC viewpoint on bandwidth allocation, System I was better than System D/K. Logically System K’ was a bit better still, but perhaps its overlapping of the channel lower edge might have been unacceptable to some of the in-country frequency assignment authorities.
The irony is that about the time that the South African TV service started, changing receiver technology, in the form of integrated circuit quasi-synchronous demodulators and SAW IF filters, meant that the size of the vestigial sideband and the consequent steepness of the Nyquist slope was becoming much less of an issue in terms of minimizing quadrature distortion in the vision channel. And quasi-split sound, which essentially eliminated the .faults inherent in standard intercarrier sound (caused by quadrature distortion from vision sideband asymmetry and the Nyquist slope), was not too far in the future.
My best guess on the non-use of the Band I channels in South Africa is that they were already otherwise occupied, and that there were anyway sufficient channels available in Bands III and IV/V. Given its size and the distances between major population centres, South Africa strikes me as a place where Band I could have been used advantageously, as it was in the USA, where “low band” channels were prized allocations.
South Africa did though choose the CCIR standard 38.9 MHz receiver IF, rather than the UK 39.5 MHz number.
Geordie, thanks for the flattering words in post #3. I am inclined to characterize myself simply as a casual lay observer, though. Way back in the 1960s, I noticed that there were multiple 625/50 transmission variants, and I wondered why. None of the books or magazine articles that I read over the years really answered the “why” question; they tended to focus on the “what”, or parts of it anyway, and what was in fact the TAC summary justification for the System I parameters was about as much as could be found on the 'why" side. Even the TAC origins of System I tended to be obscure. I had gleaned the impression from my reading over the years that it was developed by the BBC, and it was only the complete absence of any development history amongst the wealth of BBC technical publications available on-line that prompted a closer look. Anyway, around 2004 my interest was rekindled when I discovered Alan Pemberton’s web site and I entered into some correspondence with him. Thereafter now-and-again internet searches, coupled with much additional information supplied by forum contributors, have unearthed enough additional information to construct a plausible outline history of the 625/50 transmission systems, including answers to some of the “why” questions. The problem is, my findings are somewhat scattered over several threads in two, now three forums. Consolidating these into a single summary is indicated, but that is a possibility that I am approaching rather slowly......
Cheers,
System I came out of the UK TAC deliberations as reported in 1960. The TAC had initially determined that System B was not good enough, and that the System D parameters were much better. Then after further study it concluded that better use of the available 8 MHz channel would be obtained by extending the vestigial sideband to 1.25 MHz and consequently using a 5.5 rather than a 6.0 MHz main sideband.
The French developed System L in about the same time period as the TAC was deliberating, although it was formally adopted earlier than either the UK or Ireland adopted System I. The reasons for choosing the 1.25 MHz vestigial sideband were no doubt the same as those of the TAC, but the French stayed with the 6 MHz main sideband. This combination really occupied an 8.5 MHz channel, but that did not seem to be an obstacle. Previously, System E, originally designed for a 14 MHz channel, had been fitted to the tête-bêche channelling system that had 13.15 MHz spacing, so there was a precedent for “overfilling” the channel. Just possibly one reason for the retention of the 6 MHz main sideband was that it would have allowed enough room to avoid any single-sidebanding of the SECAM colour subcarriers. System L was chosen as positive/AM to align with System E and so minimize the complexity of dual-standard receivers. System K’, for the French Outré Mer territories, was effectively the negative/FM version of System L. At the time, the prevailing viewpoint seemed to be in favour of negative/FM (at least for monochrome, contrary opinions being expressed in respect of NTSC (and derivative) colour systems.), and whether the TAC just accepted this or whether it deliberated on the issue is unknown.
So if one accepted the TAC viewpoint on bandwidth allocation, System I was better than System D/K. Logically System K’ was a bit better still, but perhaps its overlapping of the channel lower edge might have been unacceptable to some of the in-country frequency assignment authorities.
The irony is that about the time that the South African TV service started, changing receiver technology, in the form of integrated circuit quasi-synchronous demodulators and SAW IF filters, meant that the size of the vestigial sideband and the consequent steepness of the Nyquist slope was becoming much less of an issue in terms of minimizing quadrature distortion in the vision channel. And quasi-split sound, which essentially eliminated the .faults inherent in standard intercarrier sound (caused by quadrature distortion from vision sideband asymmetry and the Nyquist slope), was not too far in the future.
My best guess on the non-use of the Band I channels in South Africa is that they were already otherwise occupied, and that there were anyway sufficient channels available in Bands III and IV/V. Given its size and the distances between major population centres, South Africa strikes me as a place where Band I could have been used advantageously, as it was in the USA, where “low band” channels were prized allocations.
South Africa did though choose the CCIR standard 38.9 MHz receiver IF, rather than the UK 39.5 MHz number.
Geordie, thanks for the flattering words in post #3. I am inclined to characterize myself simply as a casual lay observer, though. Way back in the 1960s, I noticed that there were multiple 625/50 transmission variants, and I wondered why. None of the books or magazine articles that I read over the years really answered the “why” question; they tended to focus on the “what”, or parts of it anyway, and what was in fact the TAC summary justification for the System I parameters was about as much as could be found on the 'why" side. Even the TAC origins of System I tended to be obscure. I had gleaned the impression from my reading over the years that it was developed by the BBC, and it was only the complete absence of any development history amongst the wealth of BBC technical publications available on-line that prompted a closer look. Anyway, around 2004 my interest was rekindled when I discovered Alan Pemberton’s web site and I entered into some correspondence with him. Thereafter now-and-again internet searches, coupled with much additional information supplied by forum contributors, have unearthed enough additional information to construct a plausible outline history of the 625/50 transmission systems, including answers to some of the “why” questions. The problem is, my findings are somewhat scattered over several threads in two, now three forums. Consolidating these into a single summary is indicated, but that is a possibility that I am approaching rather slowly......
Cheers,
