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[Octave-bug-tracker] [bug #43515] Specgram example is wrong


From: Doug Stewart
Subject: [Octave-bug-tracker] [bug #43515] Specgram example is wrong
Date: Sun, 02 Nov 2014 02:35:12 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/38.0.2125.111 Safari/537.36

URL:
  <http://savannah.gnu.org/bugs/?43515>

                 Summary: Specgram example is wrong
                 Project: GNU Octave
            Submitted by: dastew
            Submitted on: Sun 02 Nov 2014 02:35:11 GMT
                Category: Plotting
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: Doug Stewart
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: dev
        Operating System: Any

    _______________________________________________________

Details:

the example given in specgram has a wrong Y axis
+verbatum+
 ## Speech spectrogram
              [x, Fs] = auload(file_in_loadpath("sample.wav")); # audio file
              step = fix(5*Fs/1000);     # one spectral slice every 5 ms
              window = fix(40*Fs/1000);  # 40 ms data window
              fftn = 2^nextpow2(window); # next highest power of 2
              [S, f, t] = specgram(x, fftn, Fs, window, window-step);
              S = abs(S(2:fftn*4000/Fs,:)); # magnitude in range 0<f<=4000
Hz.
              S = S/max(S(:));           # normalize magnitude so that max is
0 dB.
              S = max(S, 10^(-40/10));   # clip below -40 dB.
              S = min(S, 10^(-3/10));    # clip above -3 dB.
              imagesc(t, f, flipud(log(S)));   # display in log scale
-verbatum-


Zero frequency is at the top left but should be at the bottom left.

Specgram works OK but the problem is with preparations and plotting with
imagesc 




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