16-02-2017, 07:35 PM
The short answer is yes. The NSRC (National Stereo Radio Committee) and the FCC did consider both FM-FM and FM-AM proposals (including FM-AM with an SSB subcarrier). In fact some of the FM-FM proposals, notably that from Crosby, predated the FM-AM proposals. But there were cogent reasons for choosing FM-AM, i.e. the Zenith-GE system. Primary was the fact that within the constraint of ±75 kHz maximum deviation, the fact that an AM difference signal subcarrier was interleaved ensured that there was minimal effect on modulation depth, and hence signal-to-noise ratio and service area, as seen by mono receivers. With an FM subcarrier, whatever maximum modulation depth was assigned to the subcarrier was necessarily subtracted from that available for the mono baseband signal, thus effectively reducing transmitter range. Additionally, the use of an AM subcarrier still allowed room for the SCA service, which provided essential income for many US FM stations. That was not always the case with FM subcarriers, particularly if the subcarrier deviation was made large enough to provide a good signal-to-noise ratio for the difference channel. The Crosby proposal (which was runner-up in the FCC “contest”) used a 50 kHz FM subcarrier with ±25 kHz deviation, and occupying 25 to 75 kHz. That meant that there was no room for an SCA channel within the 75 kHz maximum modulation frequency then allowed by the FCC. (Later- early 1980s I think - that upper limit was relaxed to 99 kHz.
Note that FM subcarriers are used for the American SCA service provided by FM radio. Also, the Japanese (EIAJ) TV stereo sound system used an FM subcarrier alternatively for the stereo difference channel and the second audio programme (SAP) channel. In part that was because for SAP, where the main channel-to/from-subcarrier cross-talk requirements were more stringent than for stereo, FM provided greater isolation.
The American TV stereo sound system used an AM subcarrier for the stereo difference signal and an FM subcarrier for SAP. The BTSC (Broadcast Television System Committee) and the FCC considered proposals with both AM and FM stereo subcarriers, in the latter case a system derived from Japanese practice. In this case, interleaving was much less of an issue because additional deviation was to be made available, and the subcarrier channels would be companded. AM was chosen in part because it had noticeably lower distortion than what was a relatively narrow-band FM channel.
For some additional background on the FM stereo case, perhaps look here: http://www.vintage-radio.net/forum/showt...hp?t=56858.
Cheers,
Steve
Note that FM subcarriers are used for the American SCA service provided by FM radio. Also, the Japanese (EIAJ) TV stereo sound system used an FM subcarrier alternatively for the stereo difference channel and the second audio programme (SAP) channel. In part that was because for SAP, where the main channel-to/from-subcarrier cross-talk requirements were more stringent than for stereo, FM provided greater isolation.
The American TV stereo sound system used an AM subcarrier for the stereo difference signal and an FM subcarrier for SAP. The BTSC (Broadcast Television System Committee) and the FCC considered proposals with both AM and FM stereo subcarriers, in the latter case a system derived from Japanese practice. In this case, interleaving was much less of an issue because additional deviation was to be made available, and the subcarrier channels would be companded. AM was chosen in part because it had noticeably lower distortion than what was a relatively narrow-band FM channel.
For some additional background on the FM stereo case, perhaps look here: http://www.vintage-radio.net/forum/showt...hp?t=56858.
Cheers,
Steve
