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

[Andre Kesteloot]

Steve Dove wrote:

> 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
> >
> >
> >
> >