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Date: Mon, 04 Feb 2008 23:58:41 +0100
From: Alberto di Bene <dibene@usa.net>
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Subject: LF: Re:  Mini Whip and local noise
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Roelof Bakker wrote:
> Hello Alberto,
>  
> Your program Spectran is in wide use amongst NDB chasers.
> Someone had the following question:
> --- snip ---

  Hi Roelof,

    that question was posed in the SDR-IQ Yahoo group, and I have answered there. For the benefit of the readers of this 
group that are not subscribed to that other group, I'm doing a copy-and-paste of my answer here below.

73  Alberto  I2PHD
-------------------------------------------------------------------------------



 > When using a sound card with programs such as WinRad or Spectran, how
 > important is the sound card quality for signal processing? Does the
 > sound card do any processing in decodind or displaying the signals or
 > is this all done in the Spectran software?
 >
 > Sound cards are available for $20, others cost over $250. Will the
 > higher cost sound card imrove processing or decoding?

Al, the short answer is yes. The sound card does not do any processing
of the signal, apart from converting it from analog form to the digital
realm. So what really matters in a sound card is the quality of its ADC
and of the analog stages that are in front of it. All that is what
determines the dynamic range, which is the ratio between the strongest
signal that the card is capable to digitize without overloading and the
weakest signal that can be reliably extracted from the background noise
(explained in simplified form).

All the processing is then done by the program that runs on the PC. So
when you buy a sound card, read carefully the specs about the dynamic
range and the noise. The problem is that those specs are generally
related to the intended use of a sound card, i.e. acquiring and playing
back audio... for SDR the requirements are a bit more stringent. So
that's why the M-Audio Audiophile 192 card, excellent on paper, is a
mediocre performer for SDR applications... it has been measured as
having an excellent noise performance up to about 30 kHz, but then the
noise pedestal grows significantly when you go up in frequency... so its
use at 96 or 192 kHz sampling rate for applications where you have
significant signal components above 30 kHz , like an SDR application, is
not recommended.

If you are in search of a good sound card, the measures performed not by
me, but by some other SDR enthusiasts, all point to the EMU-1212M, which
seems to have a very good performance/price ratio, especially if you
buy it second hand on eBay. If you want to spend a little less, but
still obtaining decent performance, albeit inferior to the 1212M, a good
candidate could be the M-Audio Delta 44, that is widespread about the
SDR community.

All of the above of course is valid if you use the sound card as input,
i.e. if you have a Tayloe-type mixer like the various Softrock
incarnations. Instead if you use a direct RF sampling receiver, like
the SDR-IQ, the subject of this group, disregard all of the above, as in
this case the sound card is used only for the playback of the
demodulated audio, and the cheap AC'97 chipset found on every
motherboard is more than enough for this.

73 Alberto I2PHD