Date: Tue, 29 Oct 2002 22:10:29 -0500
From: "Steve Dove" <dsp@hifidelity.com>
In-reply-to: <001001c27fa0$78e71420$4e60063e@main>
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Subject: Re: LF: sound cards...well amybe if.....
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Hi Alan,

I'm afraid that the sigma-delta convertors which almost completely
dominate the audio world nowadays don't have that aliasing 'feature',
thankfully.  Simplistically, they're a single-bit convertor running at a few 
Megs the results of which are decimated down to 'standard' PCM;  the
initial alias is several MHz away and handily disposed of with a single
RC LPF on the front.  This is cause for dancing in the street, sorry!

106dB dynamic range doesn't mean much  -  the consumer audio world
is still rife with 'creative specmanship' (remember the 500W 'music
power' stereo amplifiers of old that were lucky to puff at 80W RMS?).
The convertor set that Creative (!) are using is pretty middle-of-the-road
in terms of performance  -  but there are some very good and very fast
audio convertors around now:  Although spec'd at around 120dB-ish
dynamic range, they're measured 'A' weighted, in a sealed lead box, powered
by car batteries, in a concrete bunker deep under the Rockies at the
optimum Moon phase.  Seriously, though, in the real world and measured
honestly, around 110dB over a 20kHz bandwidth is doable through an
A/D, D/A pair of chips today.  Which is very good in anybody's terms.

It is impossible today to buy anything other than a '24-bit' convertor for
audio;  one cannot take the number of bits to mean anything much, and
especially not go all wobbly and wide-eyed at the 144dB range implied!
The difference is between how many 'real' bits and how many 'marketing' 
bits a particular device exhibits.  It can actually be quite depressing to 
terminate the front-end of an A/D and still see a LOT of low-end bits 
still wittering.

Incidentally, the faster the convertors go, the worse their noise/linearity
gets.  The good news is that they are improving at a fair clip;  they're
unrecognizable from just ten years ago.  Another unsung dirty secret is
that although systems may sample and store at 96 or 192kHz,  for the 
most part signal processing (EQ, compression etc.) is still done at 48kHz
(i.e. band-limited to 20kHz-ish), simply because the suggestion of doubling 
or quadrupling the amount of DSP hardware makes sane men twitch.

In short, the available convertors mentioned (110dB real DR,  96kHz
and so 40kHz bandwidth) are available in USB 'pods' (can't remember
the manufacturers off-hand) which gets them out of the 'orrible 'puter
environment, with drivers that emulate 'Blasters.  They are very popular
in the computer-based music-studio world.  The means already exists to 
do a *serious* convertor/dsp back-end for a software defined radio using
off-the-shelf audio stuff;  indeed, using older generation parts, this is just
what Icom did in the 'PRO, with very creditable results.

We have every reason to all get excited about all this!

        Cheers,

                Steve
 

10/29/2002 6:08:21 PM, "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
>
>
>
>