Re: [linux-audio-dev] Radio receiver.

From: fons adriaensen <fons.adriaensen@email-addr-hidden>
Date: Fri Oct 28 2005 - 00:36:02 EEST

On Thu, Oct 27, 2005 at 10:23:33PM +0100, James Courtier-Dutton wrote:

> I want to plug a ham radio receiver into the sound card of my PC, and
> use the CPU of the PC to tidy up the signal, to hopefully make it more
> readable.
> One form of communications is called Morse Code, where a single tone is
> switched on and then off to pass a signal over the radio.
>
> I know it is easy to do band pass filters in Linux so that only the tone
> gets through, but the other really useful thing would be a noise filter.

There really is no such thing as a 'noise filter' - noise usually
occupies the whole band, so a noise filter would remove all of the
signal ! The only thing you can do is remove the noise in those parts
of the spectrum you don't want to hear.

For Morse, a bandpass will let through the tone you want and attenuate
all the rest, so only the noise very close (in frequency) to the tone
remains. This will improve the S/N ratio if there is wideband noise.

There will be a tradeoff for the bandwidth of the filter you need.
If you make it too narrow, the clear on/off edges will be smeared out
and less clear, and it will also be difficult to track the signal if
its frequency is not very stable. If you make it wider, more noise
will get through.

Also, human hearing is quite good at detecting a single frequency in a
noisy background, so filtering will help only if there are other forms
of interference (and that's of course quite likely on the SW bands).
That's one of the reasons why Morse works quite well even with very
primitive equipment.

There must be a number of LADSPA plugins that do exactly what you
need. Have a look at http://www.ladspa.org. As a host, you can
use jackrack, AMS, ecasound, and many others.

-- 
FA
Received on Fri Oct 28 04:15:07 2005

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