pspectrum-spectrogram analysis

Hi I am trying to plot the spectrogram of a time frequency analysis. Please see the attachment. The trend shown by the plot is absolutely correct. The frequency reduces as the time increases. But the plot is not quite sharp as I want. I have tried doing all the combinations like, changing the time resolution, frequency resolution, changing overlap percent. I also tried with reassign parameter being set to true. But the quality is not that sharp. What can be the reason for this? Shall I filter data more and then analyse, so that the sharpness of the plot may increase?

Please advice.

[tkd]

Where is the attachement?