7.2. Visualize Sound Signals

We will extend the non blocking code for playing audio to visualize the audio signal in real time while playing it. We start with a simple display of the audio signal in matplotlib. For a mono signal we plot \(x(t)\). For each buffer (chunk) of data in the audio file we plot the amplitude as function of time. For a stereo signal we plot both the left \(x_L(t)\) signal and the right signal \(x_R(t)\).

import soundfile as sf
import pyaudio
import numpy as np
import matplotlib.pyplot as plt
import time

plt.close('all')
plt.ion()

FILE = "../../data/sowhat.wav"

chunk = 1024 * 2
pa = pyaudio.PyAudio()

f = sf.SoundFile(FILE)

is_stereo = f.channels == 2
if is_stereo:
    fig, axs = plt.subplots(nrows=2, ncols=1, sharex=True)
    xleftline, = axs[0].plot(np.arange(chunk), np.zeros(chunk), '-')
    axs[0].set_ylim([-1,1])
    xrightline, = axs[1].plot(np.arange(chunk), np.zeros(chunk), '-')
    axs[1].set_ylim([-1,1])
else:
    fig, axs = plt.subplots(nrows=1, ncols=1)
    xmonoline, = axs[0].plot(np.zeros(chunk), np.zeros(chunk), '-')
    axs[0].set_ylim([-1,1])

plt.show(block=False)


def callback(in_data, frame_count, time_info, status):
    fsx = f.read(chunk, dtype='float32')
    if is_stereo:
        xleft = fsx[:,0]
        xright = fsx[:,1]
        xleftline.set_ydata(xleft)
        xrightline.set_ydata(xright)
    else:
        xmono = fsx
        xmonoline.set_ydata(xmono)
    return (fsx.tobytes(), pyaudio.paContinue)


stream = pa.open(format=pyaudio.paFloat32,
                 channels=f.channels,
                 rate=f.samplerate,
                 frames_per_buffer=chunk,
                 output=True,
                 stream_callback=callback)


stream.start_stream()
start_time = time.time()
duration = 120    # play the first 2 minutes

while stream.is_active() and time.time() - start_time < duration:
    # time.sleep(0.1) # just waiting for the stream to finish
    fig.canvas.draw()
    fig.canvas.flush_events()


stream.stop_stream()
stream.close()

pa.terminate()
plt.close('all')
print('done')