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SGRAM - Spectrogram


sgram(f,op1,op2, ... );
sgram(f,fs,op1,op2, ... );
C=sgram(f, ... );


sgram(f) plots a spectrogram of f using a Discrete Gabor Transform (DGT).

sgram(f,fs) does the same for a signal with sampling rate fs (sampled with fs samples per second);

sgram(f,fs,dynrange) additionally limits the dynamic range of the plot. See the description of the 'dynrange' parameter below.

C=sgram(f, ... ) returns the image to be displayed as a matrix. Use this in conjunction with imwrite etc. These coefficients are only intended to be used by post-processing image tools. Numerical Gabor signal analysis and synthesis should always be done using the dgt, idgt, dgtreal and idgtreal functions.

Additional arguments can be supplied like this:


The arguments must be character strings possibly followed by an argument:

'dynrange',r Limit the dynamical range to r by using a colormap in the interval \([chigh-r,chigh]\), where chigh is the highest value in the plot. The default value of [] means to not limit the dynamical range.
'db' Apply 20*log10 to the coefficients. This makes it possible to see very weak phenomena, but it might show too much noise. A logarithmic scale is more adapted to perception of sound. This is the default.
'lin' Show the energy of the coefficients on a linear scale.
'tfr',v Set the ratio of frequency resolution to time resolution. A value \(v=1\) is the default. Setting \(v>1\) will give better frequency resolution at the expense of a worse time resolution. A value of \(0<v<1\) will do the opposite.
'wlen',s Window length. Specifies the length of the window measured in samples. See help of pgauss on the exact details of the window length.
'posfreq' Display only the positive frequencies. This is the default for real-valued signals.
'nf' Display negative frequencies, with the zero-frequency centered in the middle. For real signals, this will just mirror the upper half plane. This is standard for complex signals.
'tc' Time centering. Move the beginning of the signal to the middle of the plot. This is useful for visualizing the window functions of the toolbox.
'image' Use imagesc to display the spectrogram. This is the default.
'clim',clim Use a colormap ranging from \(clim(1)\) to \(clim(2)\). These values are passed to imagesc. See the help on imagesc.
'thr',r Keep only the largest fraction r of the coefficients, and set the rest to zero.
'fmax',y Display y as the highest frequency. Default value of [] means to use the Nyquist frequency.
'xres',xres Approximate number of pixels along x-axis / time. The default value is 800
'yres',yres Approximate number of pixels along y-axis / frequency The Default value is 600
'contour' Do a contour plot to display the spectrogram.
'surf' Do a surf plot to display the spectrogram.
'colorbar' Display the colorbar. This is the default.
'nocolorbar' Do not display the colorbar.

In addition to these parameters, sgram accepts any of the flags from setnorm. The window used to calculate the spectrogram will be normalized as specified.


The greasy signal is sampled using a sampling rate of 16 kHz. To display a spectrogram of greasy with a dynamic range of 90 dB, use:


To create a spectrogram with a window length of 20ms (which is typically used in speech analysis) use