Re: Historic AM audio quality
Somewhere – I can’t find it right now - I have an SAE Paper on the use of pre-emphasis in MF AM broadcasting as it existed in the USA in the 1980s. It was all ad hoc, and some fairly severe curves – beyond first order – were in use. That chaotic situation had led to the development of the NRSC pre-emphasis curve, basically 75 microseconds with a shelf at 8.7 kHz (originally it might have been 9.5 kHz). Cutoff at 10 kHz was also part of the definitive NRSC specification.
Much of what has been discussed in this thread is neatly summarized by this 1992 article from Audio magazine:
Cheers,
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