I avoided most of the discussion in the last week or so but finally decided to see what the ARRL Handbook had to say. At first I thought it was totally unhelpful but after it sank in a bit found it some help.
What I gleaned is that many digital modes use spectrum spreading techniques. The handbook seemed quite clear on this point. I am still trying to understand what spectrum spreading means. There is an implication in there of using more spectrum than. something. For analog, i.e. voice, this is somewhat clear. If you are sending voice up to 2.5kz then the spectrum 'something' is around 2.5 kHz SSB, or double that for AM. Spectrum spreading would utilize some additional spectrum. Consider a hypothetical mode where you took the voice signal, spread the audio by 4 times to generate a 10 kHz signal, and used that audio to modulate the RF. That would be a spectrum spreading technique. I simply cannot get a handle on what spreading means for a digital signal. Is the base 'something' CW and PSK31? >From the Handbook, and I gather from the discussion here, there is another aspect which concerns the way in which the signal is encoded. In my hypothetical analog mode you might somehow invert or fold the frequency spectrum. The reverse technique would be required to decode the signal. It is my sense that some types of encoding are not allowed, while others would be acceptable. Not trying to start the entire debate but hoping to get a better understanding of the meaning of all this. - 73 - Rud Merriam K5RUD ARES AEC Montgomery County, TX http://mysticlakesoftware.com/
