Message-ID: <009b01c27fac$9c288980$0700000a@parissn2>
From: "Stewart Nelson" <sn@scgroup.com>
To: rsgb_lf_group@blacksheep.org
References: <001001c27fa0$78e71420$4e60063e@main>
Date: Wed, 30 Oct 2002 01:38:03 +0100
MIME-Version: 1.0
Subject: LF: Re: sound cards...well amybe if.....
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Hi Alan and all,

Virtually all modern sound cards do sigma-delta (sometimes called
delta-sigma) A/D conversion.  See http://www.rmsinst.com/dt3.htm
for a good introduction and http://www.innovative-dsp.com/products/sd.htm
for a typical "pro" card. I don't know anything about the new Audigy,
but would be very surprised if it used a different technology.

The sigma-delta units internally oversample at 64 to 256 times
the output sample rate, and use a sophisticated digital filter to
eliminate aliases as well as reduce noise.  The analog pre-filter,
if any, only needs to attenuate in the MHz region.

It's possible that the Audigy has programmable firmware for its
digital filter, often hard-wired in silicon, but even then, it
would be very difficult to modify, because you have no chance of
getting the source code, development tools, etc.

On the other hand, it should be possible to clock an external
sample/hold circuit at 96 kHz, to recreate the desired alias response.
Of course, you now need some decent analog bandpass filtering ahead of
the S/H, to avoid *unwanted* aliasing.  For example, 152 kHz (192 - 40)
will also produce a 40 kHz output, and must be removed by the RF
filter.  The LW broadcasters on 153 kHz can be pretty strong...

73,

Stewart KK7KA


> 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