# Help accessing fft bins in python script **URL:** \ **Category:** GNU Radio\ **Created:** [October 16, 2014, 7:38pm UTC](https://www.ruby-forum.com/t/help-accessing-fft-bins-in-python-script/232586 "2014-10-16T19:38:17Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [October 16, 2014, 7:38pm UTC](https://www.ruby-forum.com/t/help-accessing-fft-bins-in-python-script/232586/1 "2014-10-16T19:38:17Z")
Using qa\_fft.py for inspiration[1], I’m trying to feed a raw capture file into an fft of size 32 and then review the 32 fft bins at an instantaneous point in time[2]. I must be doing something wrong however because when I review the vector sink’s data() method, it returns a rather large array (compared to the 32-element array I am expecting). data() seems to return an array with the same number of elements as samples fed into the fft. I just want an array of the 32 fft bins so I can calculate the relative power in the given band. Any help/pointers appreciated! Thanks [1] [http://gnuradio.org/redmine/projects/gnuradio/repository/revisions/master/entry/gr-fft/python/fft/qa\_fft.py](http://gnuradio.org/redmine/projects/gnuradio/repository/revisions/master/entry/gr-fft/python/fft/qa_fft.py) [2] [https://github.com/regulatre/grfun/blob/9dbbf676d2fea013787720273af0b419699c75a4/hello-fft/decodeDump162.py](https://github.com/regulatre/grfun/blob/9dbbf676d2fea013787720273af0b419699c75a4/hello-fft/decodeDump162.py) [Brad H.] ---
**Author:** ![bhutchins](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/f1d935/32.png) [@bhutchins](https://www.ruby-forum.com/u/bhutchins)\ **Post date:** [October 16, 2014, 7:46pm UTC](https://www.ruby-forum.com/t/help-accessing-fft-bins-in-python-script/232586/2 "2014-10-16T19:46:02Z")
The vector sink is for debugging only. Don’t use it in production code, because there’s no convenient way to vacuum it out, so it grows without bound. When I’ve had this problem, I use a vector-probe block, and have a function probe running at a couple of Hz, and then leverage the dependency-tree evaluator to have my own function called with the result of the function probe. There are other ways, that’s the one I personally find most convenient. But there are message queues and sundry other methods as well. On 2014-10-16 13:36, Brad H. wrote: > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) [1] Links: ---
**Author:** ![bhutchins](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/f1d935/32.png) [@bhutchins](https://www.ruby-forum.com/u/bhutchins)\ **Post date:** [October 16, 2014, 8:03pm UTC](https://www.ruby-forum.com/t/help-accessing-fft-bins-in-python-script/232586/3 "2014-10-16T20:03:18Z")
A very simple way would just be to write your block downstream of the FFT in Python. You can then operate on the vectors in the work function. M ---
**Author:** ![bhutchins](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/f1d935/32.png) [@bhutchins](https://www.ruby-forum.com/u/bhutchins)\ **Post date:** [October 17, 2014, 7:51am UTC](https://www.ruby-forum.com/t/help-accessing-fft-bins-in-python-script/232586/4 "2014-10-17T07:51:34Z")
Sometimes when I want to grab samples and perform some periodic operation like CNR estimation, or fine-frequency estimation I sometimes use a vector\_sink, and then two function probe blocks - one to read the data and one to reset it (yes I call the reset function from the function probe). This seems to work wonderfully for certain corner cases where a vector sink doesn’t work well. And I mostly use this technique when I want to prototyping something quickly before writing a block. But am I asking for trouble with memory allocation issues??? _Also, I was thinking, it would be good to have the option, either a block parameter or an overloaded function) to reset the vector\_sink, each time .data() is called… I can code this up unless anyone thinks this functionality would lead to regressions._ But I agree the vector sink block is much more convenient for FFTs… -John ---
**Author:** ![bhutchins](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/f1d935/32.png) [@bhutchins](https://www.ruby-forum.com/u/bhutchins)\ **Post date:** [October 17, 2014, 2:39am UTC](https://www.ruby-forum.com/t/help-accessing-fft-bins-in-python-script/232586/5 "2014-10-17T02:39:03Z")
Is there any way to access the fft bins from python without writing a custom block? ----- Original Message ----- From: “Martin B.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) To: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Sent: Thursday, October 16, 2014 2:02:03 PM Subject: Re: [Discuss-gnuradio] help accessing fft bins in python script A very simple way would just be to write your block downstream of the FFT in Python. You can then operate on the vectors in the work function. M On 10/16/2014 07:44 PM, [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > and then leverage the dependency-tree evaluator to have my own > > > however because when I review the vector sink’s data() method, it [http://gnuradio.org/redmine/projects/gnuradio/repository/revisions/master/entry/gr-fft/python/fft/qa\_fft.py](http://gnuradio.org/redmine/projects/gnuradio/repository/revisions/master/entry/gr-fft/python/fft/qa_fft.py) > > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) * * * Discuss-gnuradio mailing list [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > **[Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)** ---
**Author:** ![bhutchins](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/f1d935/32.png) [@bhutchins](https://www.ruby-forum.com/u/bhutchins)\ **Post date:** [October 17, 2014, 6:30pm UTC](https://www.ruby-forum.com/t/help-accessing-fft-bins-in-python-script/232586/6 "2014-10-17T18:30:49Z")
See attached screen shot. You’ll need to change some things around for 3.7. Use a probe vector instead and change the call in the function probe “data” accordingly. Probe buffer is from a proprietary out-of-tree module used with 3.6… -John ---
**Author:** ![bhutchins](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/f1d935/32.png) [@bhutchins](https://www.ruby-forum.com/u/bhutchins)\ **Post date:** [October 17, 2014, 6:42pm UTC](https://www.ruby-forum.com/t/help-accessing-fft-bins-in-python-script/232586/7 "2014-10-17T18:42:44Z")
Again, if one uses the vector-probe block, one can call the appropriate function from a function-probe, at that point, you have a variable with the contents of the FFT output vector in it, which can be used in a call to your own Python code. On 2014-10-17 12:01, Brad H. wrote: > SUBJECT: Re: [Discuss-gnuradio] help accessing fft bins in python script > > > M > > > > > function probe running at a couple of Hz, > > > > > > > file into an fft of size 32 and then review the 32 fft bins at an > > > > > > > > Discuss-gnuradio mailing list > > > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) [3] Links: ---
**Author:** ![bhutchins](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/f1d935/32.png) [@bhutchins](https://www.ruby-forum.com/u/bhutchins)\ **Post date:** [October 17, 2014, 7:22pm UTC](https://www.ruby-forum.com/t/help-accessing-fft-bins-in-python-script/232586/8 "2014-10-17T19:22:37Z")
k1gto wrote > Is there any way to access the fft bins from python without writing a > custom block? See this: [http://ha5kfu.sch.bme.hu/fft\_test](http://ha5kfu.sch.bme.hu/fft_test) Lou KD4HSO – View this message in context: [http://gnuradio.4.n7.nabble.com/help-accessing-fft-bins-in-python-script-tp50854p50891.html](http://gnuradio.4.n7.nabble.com/help-accessing-fft-bins-in-python-script-tp50854p50891.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![bhutchins](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/f1d935/32.png) [@bhutchins](https://www.ruby-forum.com/u/bhutchins)\ **Post date:** [October 17, 2014, 6:03pm UTC](https://www.ruby-forum.com/t/help-accessing-fft-bins-in-python-script/232586/9 "2014-10-17T18:03:22Z")
This sounds interesting - do you have any sample code? Can anybody explain the concept of “size” of an fft (Arg 1 of fft . fft\_vcc ) comes into play if the output of that FFT generates as many samples as it receives? ----- Original Message ----- From: “John M.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) To: “Brad H.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Cc: “Martin B.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid), [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Sent: Friday, October 17, 2014 1:50:31 AM Subject: Re: [Discuss-gnuradio] help accessing fft bins in python script Sometimes when I want to grab samples and perform some periodic operation like CNR estimation, or fine-frequency estimation I sometimes use a vector\_sink, and then two function probe blocks - one to read the data and one to reset it (yes I call the reset function from the function probe). This seems to work wonderfully for certain corner cases where a vector sink doesn’t work well. And I mostly use this technique when I want to prototyping something quickly before writing a block. But am I asking for trouble with memory allocation issues??? Also, I was thinking, it would be good to have the option, either a block parameter or an overloaded function) to reset the vector\_sink, each time .data() is called… I can code this up unless anyone thinks this functionality would lead to regressions. But I agree the vector sink block is much more convenient for FFTs… -John On Thu, Oct 16, 2014 at 5:37 PM, Brad H. \< [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) \> wrote: Is there any way to access the fft bins from python without writing a custom block? From: “Martin B.” \< [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) \> To: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Sent: Thursday, October 16, 2014 2:02:03 PM Subject: Re: [Discuss-gnuradio] help accessing fft bins in python script A very simple way would just be to write your block downstream of the FFT in Python. You can then operate on the vectors in the work function. M On 10/16/2014 07:44 PM, [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > and then leverage the dependency-tree evaluator to have my own > > > however because when I review the vector sink’s data() method, it [http://gnuradio.org/redmine/projects/gnuradio/repository/revisions/master/entry/gr-fft/python/fft/qa\_fft.py](http://gnuradio.org/redmine/projects/gnuradio/repository/revisions/master/entry/gr-fft/python/fft/qa_fft.py) > > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) * * * Discuss-gnuradio mailing list [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > **[Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)** * * * Discuss-gnuradio mailing list [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > **[Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)**