Re: FFT spectrum analyzer question

[Macy]

Did a search and found pwelch() resident to my installation and recreated the 
example shown in the instructions. Wow, impressive, although I can't account 
for the 'lump' on the high side of the passband, thought it should have been 
more symmentrical.

Thanks for pointing out that function because in that function I also found 
filtering functions! did not know how to use until was forced to. I had been 
writing out the polynomial of any filter and using that and sometimes it's not 
so easy to get the polynomial. Then, I would use my own functions to filter IN 
the frequency band. One advanatage for me, I could 'comb' filter out the 
signal(s) and then monitor just the noise floor. Probably do same using 
existing functions, but didn't see it.

Interestingly, the pwelch function requires very few data points to derive the 
plot's passband. Especially as the attenuation went down to obliviou [-120dB]. 
My technique perfectly overlayed, but with only 4096 points did not go down as 
far, went down to around -60dB, and even using 2^17 points went down to only 
-75dB to -80dB, where my plots then go flat, but the two overlay perfectly.  
Scaling with these functions looked like it would be difficult to keep track 
of, but then again only been using for the first time for a very short time, 
considering complexity, not bad. To get proper scaling seems a bit daunting, 
but I'll try some other things. Like, signals with embedded noise to see what I 
get. 

There's pwelch, filter, and cheby1, must be more, will look around in the 
'signal' folder.

Regards,
Robert


--- address@hidden wrote:

From: Francesco Potortì <address@hidden>
To: Macy <address@hidden>
Cc: address@hidden
Subject: Re: FFT spectrum analyzer question
Date: Fri, 16 May 2014 16:42:12 +0200

>Thank you for the 'heads up' on pwelch function. Will look into it. 
>
>There are an amazing number of useful functions, that I never seem to
>be alerted to until AFTER I write my own.  

Generally speaking, unless you are an expert in a given field, it is
likely that a function exists somewhere that does your job, so start
looking for it before coding, or try asking on this list.

Anyway, this happened to me as well, exactly with the pwelch function,
but at that time I think pwelch was not still available, so I was
justified :)

>Worse, even when I 'know' a function exists, still difficult to find it!

Just install the "signal" package.

Have a look here, too: <http://octave.sourceforge.net/docs.html>, you'll
find pwelch there, together with much more.

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