Re: [Discuss-gnuradio] Re: gnu radio with a soundcard

[Martin Braun]

On Thu, Sep 18, 2008 at 07:39:40AM +0000, Mathew George wrote:
> This seems to be fully functional in 150 kHz - 30 MHz band, and needs a
> supported sound card, for its operation. Pre-assembled boards costs about 110
> euros (tested units from Elektor), where as one can consider building the unit
> on his own tor educe the costs. Pre-fabricated PCBs for this project is
> available for 28 euros or so per piece.
> 
> Can someone comment, whether it is possible to make the unit working with GNU
> radio? If the answer is yes, how to proceed? What would be the first steps?

Hi Mathew,

I haven't tried it myself, but this board (and similar ones) has been used a lot
together with the Dream DRM receiver (http://drm.sourceforge.net/).
There are two types of these receivers (you'll have to check the details
yourself): the standard type works just like an HF AM receiver and
downconverts any signal up to 30MHz into the audible band (center
frequency around 6-12 kHz). You simply plug the output signal into the
line-in connector of your sound card and (in GNU Radio) use the
audio.source-block to start the DSP chain. Other receivers actually
downconvert to complex baseband and put the I and Q paths on the L and R
inputs, which just means you take the same source and connect it to a
real-to-complex GR block, but I haven't seen this work myself.

Sound cards have some quirks when (ab-)using them for this kind
of stuff (highpasses for DC-filtering, for example). Chances are very high
you will need a new soundcard anyway.
The main consideration is the limited bandwidth: even high-end
soundcards have ADCs running at no more than 96kHz, and due to some
audio-specific processing you won't be able to use the full bandwidth
(it's best to put the lower frequency not too close to 0Hz), so you'll
be limited to HF bands (AM broadcasting, DRM, HAM Radio etc.).

The HF-receiver-to-soundcard is not uncommon among HAM folk and used for
packet radio, PSK31 and likewise. You will find tons of
documentation for this on the nets, and the Dream website is a good
place to start (receiving OFDM is a lot more demanding concerning the
hardware than PSK31, so if you can listen to DRM your receiver is OK).

Your receiver setup will be limited to small bandwidths and low
frequencies, though.

Regards,
mb

-- 
Dipl.-Ing. Martin Braun           Phone: +49-(0)721-608 3790
Institut fuer Nachrichtentechnik  Fax:   +49-(0)721-608 6071
Universitaet Karlsruhe (TH)       http://www.int.uni-karlsruhe.de/

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