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Re: [Discuss-gnuradio] Parallel Port Interface

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Parallel Port Interface
Date: Mon, 20 Jan 2003 20:22:27 -0000

> As I wrote recently, I am embarking on a project to apply SDR techniques to
ionospheric examination via VLF monitoring and still haven't settled on an
interface. Today, I'm thinking about the parallel port.

I think you're giving up on the sound card too easily.  IIRC you need 100 kHz of
BW.  There are soundcards which can provide that.

> As the sampling times are generated externally (to the PC) via hardware, is
interrupt driven data sampling appropriate? Advisable? Effect on maximum 
rate? Effect on subsystem loads?

If you are using an external sampling clock, you need the parallel port's
sampling to be synchronous with that clock.  Interrupt driven is the only way. 
Otherwise you'll sometimes have bits changing right when you sample, and some
will have completed the change while others have not.  However, I know of no
machine which can handle 400-500k interrupts per second.

The other option is to use a parallel port output bit as your clock.  This would
solve your synchronization issues, but would create others.  RTlinux has an
interrupt jitter around 10 us, IIRC, which is amazingly good, but not good
enough for this.  A 100kHz (10 us period) signal sampled with a jitter of 10 us
is essentially useless.

Going back to my soundcard suggestion, you could buy a 96ks/s stereo soundcard
for less than $100 these days.  Take your input signal, mix it (multiply) by a
48 kHz sine and cosine wave, lowpass filter those 2 outputs, and feed to the
soundcard stereo inputs.  You'll have sampled the spectrum from 0 to 96kHz at 24
bits per sample.


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