From: "Paul Keinanen" <keinanen@sci.fi>
To: rsgb_lf_group@blacksheep.org
Cc: Alan.Melia@btinternet.com
Date: Wed, 30 Oct 2002 04:15:53 +0200
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Subject: Re: LF: sound cards...well amybe if.....
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On Tue, 29 Oct 2002 23:08:21 -0000, "Alan Melia"
<Alan.Melia@btinternet.com> wrote:

>Hi Alberto, we are all jumping on you for your over-enthusiasm........but it
>occurs to me that this is a 24bit card....than may be more significant. It
>has the potential to give the level of s/n we might need.
>
>Could not the dreaded aliasing be used to benificial effect, or am I missing
>some subtle point. It occurs to me that a 96ksps sampler will 'fold back'
>136kHz  to 40kHz....so if any anti-aliasing filter could be disabled (I
>think, where used, these are normally passive rather than active ?).....it
>might be possible to have a software 136kHz RX !!
>THERE is a CHALLENGE for you software gurus !!
>I will await my idea to be shot down in flames, before I conside buying one
>of CL new audigy units !!
>Cheers de Alan G3NYK
>alan.melia@btinternet.com

One could think about using decimation, in which the band is limited
by passive filters to say 125 kHz to 145 kHz, but as far as I
understand, in order to use decimation, you need a sample & hold
circuit ahead of the ADC with the sample time less than a half cycle
time at 135kHz (or about 3 us, preferably even less), while the hold
time should be the same as the ADC conversion time.  

In this example, even 48 kHz ADC sample rate would be sufficient, but
how do you control an external sample&hold circuit from a sound card ?

Paul OH3LWR