Good questions.
The different modems A, B, etc. were remnants of my modem improvement experiments.
They were all based on the principle of having a bandpass filter for each of the two tones and seeing which one was stronger.
There were cases where it was beneficial to run more than one in parallel.
This was especially true for the faster phase shift keying (PSK).
Later, I added multiple slicers to compensate for imbalance in the strengths of the two tones. (caused by pre-emphasis & de-emphasis).
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1. A few years ago, I had a revelation that I was doing it all wrong.? The AFSK demodulator was rewritten to be simpler, faster, better.?? Letters B, C, D, and E were mapped into A for backward compatibility.
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2. At one time, D was optimized for 300 baud, rather than 1200.? It might map into B now.? Not sure without looking.
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3. Not for A, C, E.? Maybe with A & B.
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New demodulator B is? based on a different principle.? It operates like an FM-discriminator, producing a signal proportional to the audio frequency.? In this case, the multiple slicers can compensate for mistuning of an SSB signal.
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73,
John WB2OSZ
