[Lf] [Fwd: LF: Sound Cards]

[Andre Kesteloot]

James Moritz wrote:

> Dear Sandy, LF group,
>
> As I recall, the average sound card ADC claims about 70 or 80dB dynamic
> range - while I have no doubt some have much better performance, the RX
> IF/detector/AF sections are almost never going to be good enough to see any
> benefit from an improved ADC.
>
> This is understandable - as far as the designers are concerned, in a normal
> HF/LF communications receiver design, all the channel filtering is
> performed by the IF filter, and whatever signal is present at the filter
> output is the "wanted" signal. So all the subsequent stages of the receiver
> are expected to do is maintain an adequate signal to noise/distortion ratio
> - for communications purposes, 40dB would be more than enough.
>
> With LF narrow-band modes the situation is different - the selectivity and
> unwanted signal rejection is defined entirely by the software, so all
> circuits up to and including the sound card ADC are effectively the
> front-end of the receiver, and have to handle large amounts of unwanted
> signals and noise without distorting a weak wanted signal - not something
> they were ever designed to do. This is the same as the argument against
> add-on CW audio filters. However, in practice it usually seems to be
> possible to set the gain so that neither receiver nor sound card are
> overloaded, and the receiver noise level is only a tiny fraction of the
> band noise, so it probably does not matter much most of the time.
>
> Where it is a problem is when there are large unwanted signals in the IF
> passband - this sometimes happens here when you are trying to copy a weak
> QRSS signal, and there is a local station operating at the same time.
> During the transatlantic tests I found that with the right adjustment of
> receiver and sound card gain controls, along with the display parameters,
> it was possible to copy a weak signal 50 - 60 dB below a strong local
> signal, when 30s dots were being used.
>
> It would certainly be possible to design an RX with high dynamic range, low
> noise IF/detector/Audio stages with optimised gain distribution to
> complement a high performance sound card. A more integrated approach is a
> receiver with IF DSP, with the ADC digitising the IF signal instead of the
> audio. But with current LF usage, especially in the US, it would probably
> be a benefit only 1% of the time.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
> PS - even for the top-notch sound cards on the web site you mention, it
> still says nothing about the sample rate accuracy!
>
> At 23:11 22/07/2002 -0400, you wrote:
> >   I have a question? How important is the sound card to the operation of
> > DSP software (Argo etc). I think we have proved that almost any card will
> > work but would we gain any thing by buying a card with a better signal to
> > noise ratio or better dynamic range?
> >  This web page has test results showing many DB difference between cards
> > (click on the card name for details on each card).
> >
> >  http://www.pcavtech.com/soundcards/summary/index.htm
> >
> >                                         Sandy
> >                                         WB5MMB
> >
> >