One possible issue with sound cards is that most, if not all, are 
AC-coupled; so low frequencies are rolled off.  This should be specified 
on the sound card somewhere, but 20Hz is typical.  I looked into DIY EKG 
stuff once for a project my brother was interested in, and this 
effectively prevents sound cards as input devices in that application. 
(most brainwave stuff is in the 30hz and below range).

An external A/D that isn't AC-coupled (or has the -3db point at 1Hz or 
somesuch) and sends samples digitally to a digital-in soundcard would 
solve this.  I've heard of this technique being used for various 
non-audio data acquisition projects.

Good luck, sounds fun!
J


Sergey Dryga wrote:

>  <lucid <at> magnet.ca> writes:
> 
> 
>>Can anyone guide me towards a relatively budgetary solution (ideally 
>>PC based) for logging a broad spectrum or multiple narrower spectrums 
>>of Extra Low Frequency electro-magnetic radiation in the .01-2000 Hz 
>>range? Something like a simple card for PC with external antenna of 
>>some sort would be great.
> 
> 
> I think SETI people use this tipe of equipment.  I remember seeing couple 
> amateur groups with home-brew radiotelescopes some-time ago.  They record 
> signal in this range after downconversion.  
> 
> Another option is to use PC sound card.  All you need is sample at >4Khz in 
> order to do spectrum analysis.
> 
> Sergey
> 
> 
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