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RE: [Discuss-gnuradio] Questions

From: Ettus, Matt
Subject: RE: [Discuss-gnuradio] Questions
Date: Thu, 10 Oct 2002 11:36:23 -0700

> -----Original Message-----
> From: David Brodrick [mailto:address@hidden
> Sent: Wednesday, October 09, 2002 5:47 PM
> To: Eric Blossom
> Cc: address@hidden
> Subject: Re: [Discuss-gnuradio] Questions

> My immediate interest is software correlators for radio 
> interferometry.

Very cool.

> I have built some simple interferometers using two direct conversion
> receiver kits slaved to a common LO (20-30MHz). I presently use the
> sound card for an ADC.

All the sound card stuff is there already
> *Real cross correlator (guts implemented in MMX). Input: 2 time domain
> signals. Output: 1 lag spectra. I see there are several correlating
> modules already, but these do not seem sufficiently general to achieve
> my goals.

Pretty simple, if I understand you correctly.  You want to cross-correlate 2
streams, I assume for finite offsets, with a stream (i.e. not blocks)

> *FFT module: input: numbers, output: FFT of input

Not hard.  The input would be a stream, the output should probably be an
array or some structure/object.

> *Maybe an Integrator, which takes lag or frequency domain input and
> averages with some time constant.

You could write a generic integrator.  If, instead of using arrays, you made
the FFT output from above into a matrix object which defined the "+"
operator (and / if you wanted to decay), then the generic integrator would
work with it.

> *Maybe a Windowing class (if there isn't already) which applies a
> tapered window to its input data. Perhaps this only needs to 
> be internal
> to the FFT module.

Window function are already there.  They just return an array.  Check out
I would make the FFT block take a window as an argument to its constructor,
and do the windowing internally.

> *AudioSource/AudioSink: I need to enable two audio input streams using
> the stereo line-in channels (if requested at runtime) and implement a
> AudioDevice base class for AudioSource and AudioSink so that the full
> duplex cap of most devices can be exploited. This would be 
> valuable for
> most people without specialised conversion hardware (eg 99% of hams).

This already works.  Don't worry about the full duplex part, it gets handled
automatically.  See gnuradio/src/gnu/examples/full_duplex_audio_demo.cc

> *I have been looking to reuse code from third party GPL QT 
> libraries as
> a versatile 2D/3D plotting class. This might end up as some kind of
> general purpose base display class. What are your plans for the GUI?

That would be great.  We briefly looked at the QWT classes, but haven't
really tried to use them.
> *If I want to utilise existing libraries (eg for neural nets) 
> how should
> I manage that? Assume the system already has the library installed?

The only difficulty is in getting the Makefile.am entry written properly,
and getting configure to check for it.

> *I have frequency control of my DDS local oscillator. Does system
> control have any place in gnuradio or should I keep such things at the
> application level or in an alternate library such as hamlib?

Check out gnuradio/src/gnu/examples/fm_demod2.cc for an example of how we
control the frequency of the cable-tv downconverter we've used.

> Sorry for being a bit long, I want to get it right.

Glad to have you using gnuradio.  I'd like to support you as much as
possible, so let me know if there are any other issues you run into.


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