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Re: [Discuss-gnuradio] Parallel Port Interface

From: Joseph DiVerdi
Subject: Re: [Discuss-gnuradio] Parallel Port Interface
Date: Mon, 20 Jan 2003 14:40:58 -0700

>> 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 
>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 
>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 
>bits per sample.

Dear Matt,

Thanks for your comments. For the record I haven't yet given up on the sound 
card; I'm pursuing the evaluation in parallel. (No pun intended)

You've understood the requirement exactly regarding the interrupt-driven aspect 
of this work and I share your concern about the effect of the high interrupt 
rate. We'll see what happens with my measurements...

Regarding the suggestion of mixing and the use of a dual channel sound card: 

Do 96ksample/s cards have 48kHz of actual bandwidth or will some analog filters 
need to be disabled?

How wide of a hole surrounding 48kHz will this scheme have?

Can the sampling clock for a sound card be derived externally?

Your scheme gives me another idea to use the sound card: use a pair of 
track/hold devices to sample the raw analog voltages, one at a time, at 192kHz 
and offer these alternating time points to the two channels of the sound card 
at 96kHz, generating the timing externally as I already have as a requirement.

Best regards,
Joseph A. DiVerdi, Ph.D., M.B.A.          
http://diverditech.com/           970.980.5868 (voice) 
http://xtrsystems.com/            970.224.3723 (fax)
PGP Key ID: 0xD50A9E33

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