How to convert a spectrogram to 3d plot. Python

Question

I am trying to achieve waterfall graph of wav file. In my attempts I noticed that this is basically a spectogram in 3d (or as close to what I need). I am trying to do this in Python with numpy and matplotlib.

My main problem is I don't know how to change the plot of specgram from matplotlib into a 3d plot.

Sample of my "code":

sample ,data = wavfile.read('file.wav')
F = Figure()
a = F.add_subplot(111,projection='3d') 
Spec, t, freq, im = a.specgram(data,Fs=2)

I've got this far and have no clue what to do next. I wanna change already existing plot into 3d. I have no code of changing it to 3d, due to lack of my knowledge.

Is it possible to convert 2d plot to 3d ? If so how ? Am i better off constructing a new plot with data from specgram?

The desired outcome would be something like the following: Thanks for any replies.

Answer 1

Here are 3D & 2D spectrogram plots of an example signal from scipy that you can find at the end of this page.

from matplotlib import mlab
import matplotlib.pyplot as plt
import numpy as np

# Fixing random state for reproducibility
np.random.seed(666)

title = ('2 Vrms sine wave with modulated frequency around 3kHz, '
         'corrupted by white noise of exponentially decreasing '
         'magnitude sampled at 10 kHz.')

fs = 10e3
N = 1e5
amp = 2 * np.sqrt(2)
noise_power = 0.01 * fs / 2
t = np.arange(N) / float(fs)
mod = 500*np.cos(2*np.pi*0.25*t)
carrier = amp * np.sin(2*np.pi*3e3*t + mod)
noise = np.random.normal(scale=np.sqrt(noise_power), size=t.shape)
noise *= np.exp(-t/5)
y = carrier + noise

def specgram3d(y, srate=44100, ax=None, title=None):
  if not ax:
    ax = plt.axes(projection='3d')
  ax.set_title(title, loc='center', wrap=True)
  spec, freqs, t = mlab.specgram(y, Fs=srate)
  X, Y, Z = t[None, :], freqs[:, None],  20.0 * np.log10(spec)
  ax.plot_surface(X, Y, Z, cmap='viridis')
  ax.set_xlabel('time (s)')
  ax.set_ylabel('frequencies (Hz)')
  ax.set_zlabel('amplitude (dB)')
  ax.set_zlim(-140, 0)
  return X, Y, Z

def specgram2d(y, srate=44100, ax=None, title=None):
  if not ax:
    ax = plt.axes()
  ax.set_title(title, loc='center', wrap=True)
  spec, freqs, t, im = ax.specgram(y, Fs=fs, scale='dB', vmax=0)
  ax.set_xlabel('time (s)')
  ax.set_ylabel('frequencies (Hz)')
  cbar = plt.colorbar(im, ax=ax)
  cbar.set_label('Amplitude (dB)')
  cbar.minorticks_on()
  return spec, freqs, t, im

fig1, ax1 = plt.subplots()
specgram2d(y, srate=fs, title=title, ax=ax1)

fig2, ax2 = plt.subplots(subplot_kw={'projection': '3d'})
specgram3d(y, srate=fs, title=title, ax=ax2)
  
plt.show()

BONUS:

You can listen to the signal by creating a wav file using scipy:

from scipy.io import wavfile
wavfile.write('sig.wav', int(fs), y)