[Laser] Sound card stability and DSP

[Tim Toast]

This may or may not help since I don't know a lot
about dsp and fft really. I was thinking since you
have 12 hertz of total bandwidth (20 - 32 Hz) and i'm
assuming 40 characters (A-Z and 0-9 plus 4 extra maybe
to make an even 40) 
12 / 40 = 0.3
so each character is centered in a space 0.3 Hz wide
(300 millihertz) so the widest the bins could possibly
be is 300 mhz (150 mhz above and below the center of
each channel)
 
If you're using bins 122 mhz wide then it seems like
there will be a lot of dead space between them where
if the frequency is off then it will be ignored. 

Why not make the bins as wide as possible with no dead
space between them? And if noise is an issue with
wider bins then what about narrowing the total
frequency spread a little to compensate. So that you
might use a band 5 or 6 Hz wide with the widest bins
possible. Or say an 8 Hz wide band; 8 / 40 = 0.2 (200
mhz) with 200 mhz bins. 

Since you ran into trouble so soon with 122 mhz bins,
perhaps only 50% of sound cards are accurate enough at
that bin size (wild guess!) and changing the bin size
to 200 or 300 mhz would lead to compatibility with
most cards out there. My math is very sketchy on this
of course :) 

Also since your writing code, maybe there is a way to
have manual fine tuning "offsets" of some kind in the
software to compensate for individual card variations
in both transmit and receive.

It may be that this kind of thing will always have the
best results with a "better grade" of soundcard
anyway, especially for demanding things like decoding
narrowband stuff. Not to say that ultra-hi-fi cards
would automatically be better in the fine frequency
domain than a cheaper card. 

>From reading some of the fine print and disclaimers
manufacturers always bundle with their hardware, they
don't really even guarantee it wont permanently damage
your system ("use at your own risk"), much less
conform to some frequency stability accuracy standards
(if there are any). I think the standards are pretty
flexible right now on some things.

on google i found these places that may or may not
help but maybe some have links to specific issues with
sound cards etc..:
http://www.marshlands.org/HamRadio/sound_card_tests.htm

http://www.google.com/search?num=20&hl=en&lr=&ie=UTF-8&safe=off&q=%22sound+card%22+%22frequency+stability%22

http://www.google.com/search?num=20&hl=en&lr=&ie=UTF-8&safe=off&q=%22sound+card%22+%22frequency+standards%22


>Date: Thu, 24 Jun 2004 21:31:39 -0400
>From: "Andrew T. Flowers, K0SM"
><aflowers at frontiernet.net>
>Subject: [Laser] Sound card stability and DSP
>To: laser at mailman.qth.net
>Message-ID: <40DB807B.6090203 at frontiernet.net>
>Content-Type: text/plain; charset=us-ascii;
>format=flowed

>Hi folks,

>I'm writing a program  for troposcatter
>communication.  Does anyone know, or can anyone find
>out, what kind of frequency stability and frequency
>accuracy you get with a typical soundcard?  Right now
>the program uses the frequencies between 20 and 32 Hz
>and bins with a width of 122 mHz (that is millihertz,
>not MHz).  This sounds perfectly reasonable, but I
>imagine there is quite a bit of variance between 
>cards.  
...




=====
Tim
toasty256 at yahoo.com
http://www.aladal.net/toast/


		
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