[Fwd: LF: Re: sound cards...well amybe if.....]

[Andre Kesteloot]

Stewart Nelson wrote:

> 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