[linrad] Simple front-end; Max-BW band scope; load/delay; Re: [linrad] Re: Linrad focus: .............

['Zaba' OH1ZAA <zaba@xxxxxx>]

Thanks Leif!

That already answers a couple of my questions! In fact I would
NOT be satisfied to look at a plain 3 kHz output of a regular
radio, but indeed the full I/Q processing of the full bandwidth
of a regular inexpensive 16-bit sound card. I guess the famous
'Linrad' noise-canceling would show its best features there.

After all we would only need a preamplifier, an oscillator,
a pair of mixers and a couple of op-amps to achieve a fair
dynamic range for effective monitoring (for 99.9% of time)
on the relatively quiet bands (e.g. 50/70 MHz in Winter).

Using 48000/8000 sampling rates this would allow 40 kHz input
bandwidth and about 3.5 kHz output bandwidth (filter margin).
It would provide a nice band scope too. The FFT-algorithm must
be pretty optimized these days to allow quick calculations,
but it is a fact that it takes time to fill the buffers with
enough samples to get sufficient data for the FFT-transforms
to commence. So with these additional remarks it would be nice
to get some (delay/load) comments still on my previous mail.

                         73, "Zaba"  OH1ZAA/2


At 19:54 24.4.2005 +0200, SM5BSZ wrote:

> Hi Zaba and all,
>
> There was one thing in your posting:
>
> > I do not think that one saves very much by implementing
> > the old 486-series, as cheap boards and capable CPU's
> > are available (new) in the range of 40-60 euros.
> It is quite clear that 486 machines are a bit too small
> but the early Pentium machines are adequate.
>
> > Another issue is the signal delay that will grow with slow CPU's.
> Not really. The signal delay is determined essentially by the filter.
> Even on a 100MHz machine the processing time is negligible.
> Of course it is possible to get some CPU-induced delay by selecting
> inappropriate processing parameters, but when using Linrad to
> process the output of an ordinary radio, the input bandwidth
> is only about 3kHz and the mode is CW. By setting a reasonably large
> first mixer conversion ratio, the sampling rate in the baseband
> becomes so low that even a 386 would be mighty fast:-)
> The limited maximum bandwidth one can set the filter to should
> be of no concern in this case.
>
> > If you buy a used PC, make sure that it works.
> Oooh! Do not buy any. People have them and you can pick them up
> for free - but they are 486-machines now. Very soon people will be
> happy when you carry away old Pentiums for them so they do not
> have to transport them to the recirculation junk-yard themselves:-)
>
>
> 73
>
> Leif / SM5BSZ


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