[BC] AM Fidelity vs FM fidelity...

[Robert Meuser]

I think this is a systems vs theory issue. There are many transmitters that proof very well. Although a newer design, the 316F could do .5% distortion and have response to 50 kilohertz. I once fed one with an Optimod 8100 and got reasonable composite stereo through it on dummy. Properly set up for CQUAM, it could get around 40 DB seperation which certainly rivals many FM stereo generators.

The real discussion if the two transmission systems are in fact considered as a system consisting of a transmitter, transmission path and receiver is the performance in that context. An AM system is relatively spectrally efficient but has signal to noise that is directly proportional to signal strength while FM trades bandwidth for noise immunity. Both systems are constrained by both physics channel allocation issues. One either goes up or goes out wide.

There is also an issue of practicality as an FM approach on medium wave frequencies is impractical and using AM on VHF frequencies would incur an unacceptable noise penalty.
The arguement that the FM stereo subcarrier DSBSC is a red herring. The stereo subcarrier lives in somewhat protected enviroment as the noise floor is established by its' FM envelope and it does not live at the RF level but rather as part of the baseband payload where all of it has a linear relationship to the noise floor. The FM stereo system taken as a whole is interesting in that as a composite system some of the limitations of both AM and FM transmission are reaponsible for the 23 DB noise penalty.
Please do not misconstrue that last statement. The FM on FM approach would have had the same or worse.

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From: "Burt I. Weiner" <biwa at att.net>

AM can have as good a fidelity as FM and in narrower occupied bandwidth.  Consider the requirements for FM stereo and the fact that the L-R channel is AM.  Yes, it's DSSBSC, but AM non the less.  I will agree with the (unfortunate) AM noise issues in the real world when it comes to AM reception, but that doesn't alter the facts.  As far as AM transmitters - Have you ever proofed an old Raytheon RA 250 or 1000?  Amazing transmitters.  RCA BTA-5F's were also pretty good, once you remove the splatter filter, which wasn't necessary with a decent processors.

Burt

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