[RTTY] (no subject)

[Kok Chen]

On Aug 23, 2011, at 8:29 PM, Bill, W6WRT wrote:

> Somewhere along the line, the term AFSK came to be used to describe audio tones
> being used to modulate an SSB transmitter, which does in fact produce a
> transmitted signal which is identical to a frequency-shifted carrier.

Irv Hoff (K8DKC back then, before he became W6FFC) wrote the article "Audio Frequency-Shift Keying for RTTY" in the June 1965 issue of QST.

In the introduction, he wrote:

"Audio FSK originally applied in the VHF bands where tone-modulated keying is permissible, has come into use on lower frequencies because it appears to be an "easy" way to get FSK with an SSB transmitter.  There are inherent dangers, in terms of meeting the purity-of-emissions requirements of the FCC regulations."

> Way back, the original meaning of AFSK was a carrier, either AM or FM, which was modulated with two audio tones. If you recall your modulation theory, this is very different from shifting the carrier frequency back and forth by 170 Hz, as we do it on HF today. 

What we do today (and what Hoff did) is precisely the same as modulating an AM transmitter and then removing the carrier and one of the AM sidebands.  The result is the two audio tones are shifted into the RF spectrum by the modulation process (i.e., convolution of the AFSK waveform with an RF carrier).

73
Chen, W7AY