gr-fosphor is choppy at audio frequency

Sylvain Munaut 246tnt at gmail.com
Fri Apr 27 17:15:00 UTC 2018


> I'm (ab)using gr-fosphor for audio spectrum visualization by using
> float to complex because it is the most colorful spectrum analyzer
> that I have. Unfortunately, for some reason gr-fosphor is choppy at 48
> kHz, while it is smooth at rtl-sdr output, about 2 MSPS or so. Is
> there any FFT configuration setting like other spectrum analyzer to
> change how many FFT bins and the framerate?

No, FFT size is fixed to 1024 and the frame rate is dictated by the
sample rate since all data is processed.

And to cope with high rate, fosphor is designed to run by "batch" of
samples and so it needs to accumulate at least 16 (IIRC) * 1024
samples before it can start computations. but of course at audio
frequencies, this make it choppy.

What you can do is artifically increase the sample rate by using
overlapping windows. So you feed in samples[0...1023] then
samples[256:1023+256] then samples [512:1023+512] and so on ...
You need a custom block to do that ...


Cheers,

   Sylvain


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