FFT waterfall chart (rain chart)

2023-01-18

The FFT waterfall chart is the frequency distribution chart of the data over time. The following will introduce how to use MaixPy to draw the waterfall chart.

Drawing method

  • Prepare time domain signals (such as audio data)
rx = I2S(I2S.DEVICE_0)
rx.channel_config(rx.CHANNEL_0, rx.RECEIVER, align_mode = I2S.STANDARD_MODE)
rx.set_sample_rate(sample_rate)
audio = rx.record(sample_points)
  • Perform FFT operation (use FFT operation on the data and get its frequency distribution)
fft_points = 512
fft_res = FFT.run(audio.to_bytes(),fft_points)
fft_amp = FFT.amplitude(fft_res)
  • Draw on image (due to the symmetry of the FFT result, only a part of it needs to be drawn)
hist_x_num = 128
img = image.Image(size=(128,128))
for i in range(hist_x_num):
        img[i] = fft_amp[i]

Detailed API reference I2S-API, FFT-API

Routine

The following example is tested in firmware v0.5.1 MaixDock

Acquire audio data in real time and draw it as an FFT waterfall chart

demo_fft_waterfall.py

effect: