29-06-2017, 12:17 AM
The definitive answer to that question may well be lost in history.
Historically, AM broadcasting systems did not have standardized pre-emphasis and de-emphasis curves, and that was probably the reason why the UK 405-line system (which predated FM broadcasting) did not have pre-emphasis.
The triangular noise structure of FM made pre-emphasis and de-emphasis highly desirable. The original US number (evidently attributable to work by Crosby) was 100 microseconds, but BBC work showed that 50 microseconds was better. I suspect that Russian work at the same time or perhaps even a bit earlier also supported 50 microseconds. The US changed to 75 microseconds in 1945.
On the face of it, the same benefits of pre-emphasis as obtained with FM would not apply to AM with its square noise characteristic. But some work was done, and Tibbs & Johnstone, in “Frequency Modulation Engineering”, recorded that a small benefit was available by applying pre-emphasis to AM systems:
That is from the 1955 2nd edition. I don’t have the 1947 1st edition, but I imagine that it had the same commentary.
Against that background, one might infer that those who set the parameters for the French 819-line system did not regard pre-emphasis of the AM sound channel as bringing enough benefits to justify it. Perhaps too there was some inertia in that the 441-line system then in operation did not have pre-emphasis.
The Belgians evidently saw it differently, and without any potentially constraining precedent decided that whatever small benefits came with AM pre-emphasis were worth doing. That decided, then there was probably no good reason to use other than the 50-microsecond number already established for FM systems in Europe. One possible benefit in TV receivers is that any extraneous line frequency signal that gets into the sound channel ahead of the AF section will be significantly suppressed by the de-emphasis circuit. This was of greater significance for 625 lines, where the line frequency, 15 625 Hz, was just above the wanted AF range, presumed to extend to 15 kHz. It would have been less of an issue with 819 lines, as the normal AF response could be expected to be quite a bit down at 20 475 Hz. So, the Belgians, with 625 and 819 line systems, had more reason to use pre-emphasis than did the French for their 819-line system.
The French 625-line system was designed to have maximum commonality with the existing 819-line system in order to make easier the design of dual-standard receivers. That’s why it was a positive/AM system at a time when negative/FM was the norm. Accordingly, the AM sound was not pre-emphasized in order to maintain commonality with the 819-line system.
Note that the French adopted negative/FM (System K’) for their Outré-Mer territories, where backward compatibility with 819 lines was not an issue. The choice of a 1.25 MHz vestigial sideband for Systems L and K' was in line with thinking at the time. But its combination with a 6 MHz main sideband, thus “overfilling” the 8 MHz channel, was an interesting one. My guess, and it’s only that, is that it was done because it allowed enough room for symmetrical subcarrier sidebands for the SECAM colour system. The original SECAM proposal, for 819 lines, used simple AM for the subcarriers, and I think assumed simple envelope demodulation. As is well-known, simple AM demodulators produce significant distortion when presented with asymmetrical sideband signals. The SECAM system was changed to FM subcarriers quite early on, but there could have been some reluctance to have asymmetrical sideband FM, at least with conventional FM demodulators of the period.
In more recent times, standardized pre-emphasis has been applied to MF AM broadcasting. In the USA, the NRSC standard, published in 1986, was adopted in 1987. This applied 75 microseconds pre-emphasis, with a shelf at 8.7 kHz and a cutoff at 10 kHz (whereas previously 15 kHz was allowed). This was predicated on the basis of obtaining the optimum results in the conditions then prevailing. Part of the thinking seems to have been that the 75-microsecond de-emphasis roll-off could be included in the receiver IF filter curve, thus allowing better overall selectivity than would be conferred by an IF curve that was flat to plus/minus 10 kHz. Though the bandwidth restriction to 10 kHz was not welcomed by some “quality” AM broadcasters who had previously transmitted out to 15 kHz. The NRSC standard was adopted elsewhere, such as by the ABC in Australia.
Sometime before the NRSC standard was promulgated, NHK in Japan was using 100 microseconds pre-emphasis for its MF broadcasts, but I don’t know the background to this. Perhaps this number was chosen to offset somewhat the narrow IF selectivity curves of typical AM receivers.
In summary, whereas for FM systems, pre-emphasis is de rigueur, with AM systems its benefits are essentially situational, which probably accounts for its selective use.
Cheers,
Steve
Historically, AM broadcasting systems did not have standardized pre-emphasis and de-emphasis curves, and that was probably the reason why the UK 405-line system (which predated FM broadcasting) did not have pre-emphasis.
The triangular noise structure of FM made pre-emphasis and de-emphasis highly desirable. The original US number (evidently attributable to work by Crosby) was 100 microseconds, but BBC work showed that 50 microseconds was better. I suspect that Russian work at the same time or perhaps even a bit earlier also supported 50 microseconds. The US changed to 75 microseconds in 1945.
On the face of it, the same benefits of pre-emphasis as obtained with FM would not apply to AM with its square noise characteristic. But some work was done, and Tibbs & Johnstone, in “Frequency Modulation Engineering”, recorded that a small benefit was available by applying pre-emphasis to AM systems:
That is from the 1955 2nd edition. I don’t have the 1947 1st edition, but I imagine that it had the same commentary.
Against that background, one might infer that those who set the parameters for the French 819-line system did not regard pre-emphasis of the AM sound channel as bringing enough benefits to justify it. Perhaps too there was some inertia in that the 441-line system then in operation did not have pre-emphasis.
The Belgians evidently saw it differently, and without any potentially constraining precedent decided that whatever small benefits came with AM pre-emphasis were worth doing. That decided, then there was probably no good reason to use other than the 50-microsecond number already established for FM systems in Europe. One possible benefit in TV receivers is that any extraneous line frequency signal that gets into the sound channel ahead of the AF section will be significantly suppressed by the de-emphasis circuit. This was of greater significance for 625 lines, where the line frequency, 15 625 Hz, was just above the wanted AF range, presumed to extend to 15 kHz. It would have been less of an issue with 819 lines, as the normal AF response could be expected to be quite a bit down at 20 475 Hz. So, the Belgians, with 625 and 819 line systems, had more reason to use pre-emphasis than did the French for their 819-line system.
The French 625-line system was designed to have maximum commonality with the existing 819-line system in order to make easier the design of dual-standard receivers. That’s why it was a positive/AM system at a time when negative/FM was the norm. Accordingly, the AM sound was not pre-emphasized in order to maintain commonality with the 819-line system.
Note that the French adopted negative/FM (System K’) for their Outré-Mer territories, where backward compatibility with 819 lines was not an issue. The choice of a 1.25 MHz vestigial sideband for Systems L and K' was in line with thinking at the time. But its combination with a 6 MHz main sideband, thus “overfilling” the 8 MHz channel, was an interesting one. My guess, and it’s only that, is that it was done because it allowed enough room for symmetrical subcarrier sidebands for the SECAM colour system. The original SECAM proposal, for 819 lines, used simple AM for the subcarriers, and I think assumed simple envelope demodulation. As is well-known, simple AM demodulators produce significant distortion when presented with asymmetrical sideband signals. The SECAM system was changed to FM subcarriers quite early on, but there could have been some reluctance to have asymmetrical sideband FM, at least with conventional FM demodulators of the period.
In more recent times, standardized pre-emphasis has been applied to MF AM broadcasting. In the USA, the NRSC standard, published in 1986, was adopted in 1987. This applied 75 microseconds pre-emphasis, with a shelf at 8.7 kHz and a cutoff at 10 kHz (whereas previously 15 kHz was allowed). This was predicated on the basis of obtaining the optimum results in the conditions then prevailing. Part of the thinking seems to have been that the 75-microsecond de-emphasis roll-off could be included in the receiver IF filter curve, thus allowing better overall selectivity than would be conferred by an IF curve that was flat to plus/minus 10 kHz. Though the bandwidth restriction to 10 kHz was not welcomed by some “quality” AM broadcasters who had previously transmitted out to 15 kHz. The NRSC standard was adopted elsewhere, such as by the ABC in Australia.
Sometime before the NRSC standard was promulgated, NHK in Japan was using 100 microseconds pre-emphasis for its MF broadcasts, but I don’t know the background to this. Perhaps this number was chosen to offset somewhat the narrow IF selectivity curves of typical AM receivers.
In summary, whereas for FM systems, pre-emphasis is de rigueur, with AM systems its benefits are essentially situational, which probably accounts for its selective use.
Cheers,
Steve
