On 3/6/18 06:48, Joe Taylor wrote:
> Hi Phil,
> Your idea sounds reasonable.  It might be a good way to enable wideband
> receiving -- that is, reception (of FT8, say) over a passband
> significantly greater than 5 kHz, currently the practical limit.  The
> popularity of FT8 on the HF bands implies that wider sub-bands would be
> very desirable.

Great. I hadn't thought of that, but you're right. Not only would this
free up your computer's sound system for other uses, it would avoid all
of the usual computer audio system limitations.

It would be very easy to give you a "stereo" (complex I/Q) audio stream.
I did use a complex-to-real IFFT in my fast correlator for SSB mode but
I simplified the program by just using complex-to-complex for everything
and throwing the imaginary part away in SSB/CW mode. That'd be an easy
way to double bandwidth without increasing the sample rate.

If you use complex signals internally, giving you complex audio could
let you avoid the Hilbert transform (or equivalent) necessary to
regenerate the analytic signal from a purely real one (not that that's
particularly difficult).

Zero frequency isn't a problem in a purely digital format, but neither
have I had any significant analog 0 Hz problems with my AMSAT UK Funcube
SDR dongle. There's a DC offset but it's easy to remove. This leaves a
small mound of phase noise maybe 1-2 kHz wide, probably from reciprocal
mixing. I only notice it with the antenna disconnected and normally it's
completely buried. In any event, I do my own frequency translations so
DC from the front end isn't necessarily DC in my output signal.

> With MAP65 we already have significant experience with wideband
> reception of JT65 EME signals.  It works very well, and is a big
> advantage.  It can be set up to decode all JT65 signals over a 90 kHz
> range.

Yeah, why not? If your SDR front end can cover an entire band, and if
you have enough CPU cycles, you could easily extract everything in an
entire band. One program would look for JT65, a completely independent
program looks for WSPR, a third one for Winlink or Domino or PSK31 or
whatever, and so on. These independent programs can run on the same
computer or on a collection of computers, all processing a single input

> When receiving WSJT-X currently acquires audio data via the C++ class
> defined in Detector.cpp.  Your best contact person is probably Bill,
> Somerville, G4WJS.  Fair warning: Bill is in the middle of some major
> contract work, so his available time is limited.

I'll look at it, thanks.

73, Phil

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