Re: spectrogram values

[Pander <pander@users.sourceforge.net>]

On 28 January 2016 15:47:22 CET, Markus Glugla <markus.glugla@physik.tu-berlin.de> wrote:
>Am Donnerstag, den 28.01.2016, 14:00 +0100 schrieb Pander:
>> On 01/28/2016 01:51 PM, Markus Glugla wrote:
>> > Am Mittwoch, den 20.01.2016, 03:37 +0100 schrieb Thor Andreassen:
>> >> On Fri, Jan 08, 2016 at 03:31:18PM +0100, Markus Glugla wrote:
>> >>> Hi,
>> >>
>> >> Hi Markus
>> >>  
>> >>> I read the spectrogram threads and affiliate a request. I'm using
>> >>> sometimes sox spectrogram to get an spectral overview of various
>> >>> measurement datas. But i want to plot the spectrogram with other
>tools
>> >>> e.g. gnuplot.
>> >>>
>> >>> I would be very grateful if it were possible to output the
>spectrogram
>> >>> data as plotable values in a file or stream. Does anybody have
>the
>> >>> same intention and more experience than me to produce a patch?
>> >>
>> >> I have been planning to implement an amplitude and water-fall
>output,
>> >> that can be sent to Gnuplot. I will need these in the coming
>months, so
>> >> perhaps it will get done.
>> >>
>> >> A crude way of getting the raw numbers could be to use the color
>values
>> >> from the generated png file, e.g. with Imagemagick:
>> >>
>> >>     sox -n -n synth 1 synth 1k:5k spectrogram -r -o - |
>> >>     convert png:- txt:-                 |
>> >>     
>> >>     # translate into Gnuplot compatible input
>> >>     awk -v FS='[,:() ]+' 'NR > 2               { print $1, $2,
>($3+$4+$5)/3 }'  |
>> >>     awk                  'NR > 2 && $2 != prev { printf "\n" } {
>prev = $2 } 1' \
>> >>     > spectrogram.dat
>> >>
>> >> Note the use of the `-r` option to sox, which omits all
>decorations. See
>> >> `sox --help-effect spectrogram` for more.
>> >>
>> >> Now plot the data file with Gnuplot:
>> >>
>> >>     echo 'splot "spectrogram.dat" with pm3d' | gnuplot --persist
>> >>
>> >> If you want to get at the raw spectrogram, you need to patch
>> >> spectrogram.c. The attached patch works best with single channel
>files.
>> >> Use the new `-O filename` switch to save the binary data.
>> >>
>> >> The default output is a sequence of floats which are 4 bytes long.
>This
>> >> is also Gnuplots default for binary input. This means you can plot
>the
>> >> output like this:
>> >>
>> >>     echo 'plot "<sox -n -n synth 1 sine 1k:5k spectrogram -O -" \
>> >>             binary array=(513,800) flipy rotation=90d with image'
>|
>> >>     gnuplot --persist
>> >>
>> >> Note that if your output from sox is short, the spectrogram
>dimensions
>> >> will be different from 800x513 and Gnuplot will barf.
>> >>
>> > Hi Thor
>> > 
>> > Thank you for your source code and your patch. I enjoy about your
>idea
>> > which increases the productivity. 
>> > 
>> > It is also in my opinion to use gnuplot as plotting tool to create
>> > spectrogram in the same, my, style.
>> > 
>> > I also think about the grab the spectrogram datas via imagemagick.
>But
>> > in this case it is easier to use GSL.
>> > 
>> > Thank you for your help. Your code snippets are very informative.
>> > 
>> > Markus
>> 
>> Make sure to validate what the impact is of including or omitting the
>> last sample as it often is not complete and distorts the spectrum,
>see
>> my earlier analysis with graphs illustrating that.
>> 
>
>I read your hints, thank you very much. 
>
>I think, it is typical of FFT which needs a even number of samples
>(2n).
>On short sequences, the 2n FFT array is longer than the sequence you
>get
>also porblems. A FFT with a number of less than 2n will be incorrect. I
>think it's better to cut the sequence on right position to 2n before
>FFT, is'nt? What is with windowing?

More combination are OK, it is just theast sample that is dangerous. So safest is to skip it. That means either calculating the number of samples or parseing it twice or with a delay. 

>
>Thank you for your hint,
>Markus
>
>
>> > 
>> > 
>> >
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