Python and the Tell-Tale Heart

I was browsing through SciPy documentation this evening and ran across a function in scipy.misc called electrocardiogram. What?!

It’s an actual electrocardiogram, sampled at 360 Hz. Presumably it’s included as convenient example data. Here’s a plot of the first five seconds.

I wrote a little code using it to turn the ECG into an audio file.

from numpy import int16, iinfo
from scipy.io.wavfile import write
from scipy.misc import electrocardiogram

def to_integer(signal):
    # Take samples in [-1, 1] then scale to 16-bit integers
    m = iinfo(int16).max
    M = max(abs(signal))
    return int16(signal*m/M)

ecg = electrocardiogram()
write("heartbeat.wav", 360, to_integer(ecg))

I had to turn the volume way up to hear it, and that made me think of Edgar Allan Poe’s story The Tell-Tale Heart.

I may be doing something wrong. According to the documentation for the write function, I shouldn’t need to convert the signal to integers. I should just be able to leave the signal as floating point and normalize it to [-1, 1] by dividing by the largest absolute value in the signal. But when I do that, the output file will not play.

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