18-03-2017, 06:51 AM
Returning to the original question, colly0410 had noted that in the colour television case, both AM (NTSC, PAL) and FM (SECAM) colour subcarriers were used.
But in the colour TV case, regardless of the nature of the subcarrier, the main carrier was AM, not FM, so that was a point of difference (obviously so) as compared with FM sound broadcasting.
Another point of difference is that in the NTSC case, the AM subcarrier signal, as well as being of the suppressed carrier type, was also of quadrature form. Not only that, but the in-phase signal (I-signal) was of asymmetric sideband form. So one might say that the subcarrier was overall of quite complex form, rather adventurous in fact for what was the first major use of a subcarrier in broadcasting. In contrast the Zenith-GE subcarrier is of quite simple form. In the PAL case, both the U and V subcarriers could be asymmetric when used in base systems of limited vision bandwidth, such as B/G/H, M and N. With SECAM, the FM subcarriers were probably close to being symmetric with systems D/K/K’ and L, but would have been asymmetric with B/G/H.
Nevertheless, aside from the FM-AM and FM-FM systems previously mentioned in this thread, other forms were used or proposed for sound broadcasting.
I know of one AM-AM example, and that is the mid-1950s French bilingual TV sound system used at the time in Algeria. This was mentioned briefly in the VHF AM thread in the former VRAT forum, see: http://www.forum.radios-tv.co.uk/viewtopic.php?f=3&t=9040&p=93909#p93909. It was basically a time division multiplex (TDM) system in which the sound carrier was switched at TV line frequency (20.475 kHz) between the two language feeds, with only the first resultant subcarrier transmitted. The subcarrier was of the DSB suppressed carrier type, which was what the TDM switching produced naturally. Audio bandwidth was restricted to 9 kHz.
Within the FM-AM realm, other forms of AM subcarrier were also proposed amongst the various systems put forward in the 1970s as candidates for FM quadraphonic broadcasting.
All candidates included a quadrature-modulated, suppressed carrier AM subcarrier at 38 kHz, with the in-phase part carrying the stereo difference signal, as with the Zenith-GE stereo system, and the quadrature part carrying the third channel signal. The fourth channel signal, optional and/or bandwidth restricted in some cases, was carried, at various frequencies and by various means, including DSB carrier suppressed AM, SSB AM and vestigial sideband AM. And it may be recalled that some of the late 1950s FM stereo proposals involved SSB AM subcarriers. These were seen as too difficult for demodulation in domestic receivers at the time. By the 1970s, IC-based PLL techniques had made them feasible, although they evidently had other drawbacks.
Use of the quadrature 38 kHz subcarrier was also a feature of the ill-fated 1980s FMX noise reduction scheme. Mutual interference between the in-phase and quadrature subcarriers due to phase shifts, etc., was a problem. Restricting the bandwidth and the modulation depth of the quadrature signal may have helped, and the IBA did this with its “2½-channel” FM surround sound proposal.
I understand that SSB AM SCA subcarriers have been used in the USA from the 1980s, after the FCC had substantially deregulated SCA activity, and no longer specified the form that they should take.
One may also imagine AM-AM and AM-FM systems in which there is quadrature AM modulation of the main channel, allowing two baseband plus subcarrier sets. The opportunities for cross-modulation would be many, though. But in a way, even this has been done, albeit with mechanical rather than electronic AM of the main channel. I am thinking of the CD-4 quadraphonic gramophone disc. This had orthogonal (quadrature) amplitude modulation of the disc grooves, in each case by a baseband signal and a 30 kHz FM subcarrier. (Apparently an AM subcarrier wouldn’t have worked in this case – too much noise and crosstalk.) The subcarrier was usually described as being a combination of FM below 800 Hz, PM from there to 6 kHz, and FM again thereafter, but that was simply the result of its having a stepped pre-emphasis curve. (One might also describe regular FM, with 50 microseconds pre-emphasis, as being FM below 3.18 kHz and PM above that frequency.) The FM subcarrier level was well below that of the baseband, so using the FM comparison its relationship to the baseband was more like that of an SCA subcarrier than the 38 kHz stereo subcarrier. Another oddity of the CD-4 subcarrier was that it had asymmetric sidebands, so in part it was SSB FM. The UD-4 disc was similar, except that the subcarrier signals were band-limited (to somewhere between 2 and 6 kHz) and had normal RIAA pre-emphasis.
Cheers,
Steve
But in the colour TV case, regardless of the nature of the subcarrier, the main carrier was AM, not FM, so that was a point of difference (obviously so) as compared with FM sound broadcasting.
Another point of difference is that in the NTSC case, the AM subcarrier signal, as well as being of the suppressed carrier type, was also of quadrature form. Not only that, but the in-phase signal (I-signal) was of asymmetric sideband form. So one might say that the subcarrier was overall of quite complex form, rather adventurous in fact for what was the first major use of a subcarrier in broadcasting. In contrast the Zenith-GE subcarrier is of quite simple form. In the PAL case, both the U and V subcarriers could be asymmetric when used in base systems of limited vision bandwidth, such as B/G/H, M and N. With SECAM, the FM subcarriers were probably close to being symmetric with systems D/K/K’ and L, but would have been asymmetric with B/G/H.
Nevertheless, aside from the FM-AM and FM-FM systems previously mentioned in this thread, other forms were used or proposed for sound broadcasting.
I know of one AM-AM example, and that is the mid-1950s French bilingual TV sound system used at the time in Algeria. This was mentioned briefly in the VHF AM thread in the former VRAT forum, see: http://www.forum.radios-tv.co.uk/viewtopic.php?f=3&t=9040&p=93909#p93909. It was basically a time division multiplex (TDM) system in which the sound carrier was switched at TV line frequency (20.475 kHz) between the two language feeds, with only the first resultant subcarrier transmitted. The subcarrier was of the DSB suppressed carrier type, which was what the TDM switching produced naturally. Audio bandwidth was restricted to 9 kHz.
Within the FM-AM realm, other forms of AM subcarrier were also proposed amongst the various systems put forward in the 1970s as candidates for FM quadraphonic broadcasting.
All candidates included a quadrature-modulated, suppressed carrier AM subcarrier at 38 kHz, with the in-phase part carrying the stereo difference signal, as with the Zenith-GE stereo system, and the quadrature part carrying the third channel signal. The fourth channel signal, optional and/or bandwidth restricted in some cases, was carried, at various frequencies and by various means, including DSB carrier suppressed AM, SSB AM and vestigial sideband AM. And it may be recalled that some of the late 1950s FM stereo proposals involved SSB AM subcarriers. These were seen as too difficult for demodulation in domestic receivers at the time. By the 1970s, IC-based PLL techniques had made them feasible, although they evidently had other drawbacks.
Use of the quadrature 38 kHz subcarrier was also a feature of the ill-fated 1980s FMX noise reduction scheme. Mutual interference between the in-phase and quadrature subcarriers due to phase shifts, etc., was a problem. Restricting the bandwidth and the modulation depth of the quadrature signal may have helped, and the IBA did this with its “2½-channel” FM surround sound proposal.
I understand that SSB AM SCA subcarriers have been used in the USA from the 1980s, after the FCC had substantially deregulated SCA activity, and no longer specified the form that they should take.
One may also imagine AM-AM and AM-FM systems in which there is quadrature AM modulation of the main channel, allowing two baseband plus subcarrier sets. The opportunities for cross-modulation would be many, though. But in a way, even this has been done, albeit with mechanical rather than electronic AM of the main channel. I am thinking of the CD-4 quadraphonic gramophone disc. This had orthogonal (quadrature) amplitude modulation of the disc grooves, in each case by a baseband signal and a 30 kHz FM subcarrier. (Apparently an AM subcarrier wouldn’t have worked in this case – too much noise and crosstalk.) The subcarrier was usually described as being a combination of FM below 800 Hz, PM from there to 6 kHz, and FM again thereafter, but that was simply the result of its having a stepped pre-emphasis curve. (One might also describe regular FM, with 50 microseconds pre-emphasis, as being FM below 3.18 kHz and PM above that frequency.) The FM subcarrier level was well below that of the baseband, so using the FM comparison its relationship to the baseband was more like that of an SCA subcarrier than the 38 kHz stereo subcarrier. Another oddity of the CD-4 subcarrier was that it had asymmetric sidebands, so in part it was SSB FM. The UD-4 disc was similar, except that the subcarrier signals were band-limited (to somewhere between 2 and 6 kHz) and had normal RIAA pre-emphasis.
Cheers,
Steve
