Re: [Discuss-gnuradio] sensed spectrum in dB???

[kaleem ahmad]

Thanks Brian,

But I am still confused what this code is doing:

        power = 0 
        for tap in mywindow: 
            power += tap*tap 
            
        # FIXME the log10 primitive is dog slow 
        log = gr.nlog10_ff(10, self.fft_size, 
                          
-20*math.log10(self.fft_size)-10*math.log10(power/self.fft_size)) 


Especially why this factor
"20*math.log10(self.fft_size)-10*math.log10(power/self.fft_size)" is used???

moreover nlog10_ff block actually computes -> n*log10(input) + k, can
someone explain what this k is doing and how can we compare this formulae to
traditional dB formulae i.e. 10log(input), or 20log(input).

Best Regards


Brian Padalino wrote:
> 
> On Mon, Sep 22, 2008 at 9:19 AM, kaleem ahmad <address@hidden>
> wrote:
>>
>> Hi All,
>>
>> I am using USRP+RFX2400+SUSI 10.3.
>>
>> Many people have already discussed "usrp_spectrum_sense.py" and I read
>> all
>> those threads but unfortunately it didnt solve my problem and didnt help
>> me
>> to understand the file completely, I have two questions in this file and
>> will be thankful if some one can help me.
>>
>> First, what is in m.data???
>>
>> First, what the following piece of code is doing??? Is it converting
>> m.data
>> to power in dB?? If not please help me to find how this code can be
>> changed
>> so that I get power dB at any given center frequency.
>> ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////
>> power = 0
>>        for tap in mywindow:
>>            power += tap*tap
>>
>>        c2mag = gr.complex_to_mag_squared(self.fft_size)
>>
>>        # FIXME the log10 primitive is dog slow
>>        log = gr.nlog10_ff(10, self.fft_size,
>>
>> -20*math.log10(self.fft_size)-10*math.log10(power/self.fft_size))
>> ////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
> 
> It sure looks like it's calculating power in dB with all those
> log10's.  A quick Googling led me here for a discussion on FIR filters
> and normalization:
> 
>     http://www.hydrogenaudio.org/forums/index.php?showtopic=43195
> 
> Good luck!
> 
> Brian
> 
> 
> _______________________________________________
> Discuss-gnuradio mailing list
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> 
> 

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