Re: [Discuss-gnuradio] Re: analysis_filterbank question

[Eric Blossom]

On Wed, Jun 10, 2009 at 05:18:37PM +0200, Markus Feldmann wrote:
> feldmaus schrieb:
>>
>> First i post a clipping from the api docu about the analysis filterbank:
>> 00043     def __init__(self, mpoints, taps=None):
>> 00044         """
>> 00045         Takes M complex streams in, produces single complex stream out
>> 00046         that runs at M times the input sample rate
>> 00047 00048         @param mpoints: number of freq bins/interpolation 
>> factor/subbands
>> 00049         @param taps:    filter taps for subband filter
>> 00050 00051         The channel spacing is equal to the input sample 
>> rate.
>> 00052         The total bandwidth and output sample rate are equal the input
>> 00053         sample rate * nchannels.
>> 00054 00055         Output stream to frequency mapping:
>> 00056         00057           channel zero is at zero frequency.
>> 00058 00059           if mpoints is odd:
>> 00060             00061             Channels with increasing positive 
>> frequencies come from
>> 00062             channels 1 through (N-1)/2.
>> 00063 00064             Channel (N+1)/2 is the maximum negative 
>> frequency, and
>> 00065             frequency increases through N-1 which is one channel lower
>> 00066             than the zero frequency.
>>
> I am now working on a Report, it would be nice if some of the developers
> explain what they mean with <channels 1 through (N-1)/2> ?
> What means <Channel (N+1)/2 is the maximum negative frequency> ?
>
> Does this formula above also valid with a bandpass_filter ?
> Or how should the algorithmus be for a bandpass ?
>
> Regards markus

Marcus,

This ordering is just the normal result of computing an FFT.
You might consider taking a look at the FFTW docs.  They do a good job
of explaining what they're actually computing and give an
interpretation of "negative frequencies".

Eric