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Re: HF SSB



>There is still a moderate amount of business in HF SSB, the main
>reason that people are going digital on HF is for anti-jam and secure voice 
>type things.

Actually, I thought the main reason for going digital on HF is to
provide greater robustness under adverse conditions and to facilitate
direct interoperability with terrestrial networks (e.g., the
Internet).  Jam resistance and the ability to encrypt are just side
benefits that come from increased robustness and going digital,
respectively.

>Anyway, one of the things I often wonder about is why since DSP is getting 
>so cheap, why most "ham" type radios still are FM only.
>It's easy to (de)modulate SSB in a cheap DSP nowadays..

That's a very good question. As you may know, some high-end HF
transceivers have been using DSP for some time now. The last hardware
downconversion is typically to an IF around 11 kHz. This IF is A/D
converted and fed to the DSP block where the final filtering and
downconversion to baseband is performed in software.

Unfortunately, the low 11 kHz sampling rate implies that only
narrowband signals can be handled. This is unfortunate. A wider
bandwidth would not only allow the DSP to implement FM, but also
various spread spectrum formats that would take advantage of frequency
diversity and produce very "thin" signals tolerant of QRM and
generating little QRM of their own.

Phil



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