How to properly decode .wav with Python

Question

I am coding a basic frequency analisys of WAVE audio files, but I have trouble when it comes to convertion from WAVE frames to integer.

Here is the relevant part of my code:

import wave
track = wave.open('/some_path/my_audio.wav', 'r')

byt_depth = track.getsampwidth() #Byte depth of the file in BYTES
frame_rate = track.getframerate()
buf_size = 512

def byt_sum (word):
#convert a string of n bytes into an int in [0;8**n-1]
    return sum( (256**k)*word[k] for k in range(len(word)) )

raw_buf = track.readframes(buf_size)
'''
One frame is a string of n bytes, where n = byt_depth.
For instance, with a 24bits-encoded file, track.readframe(1) could be:
b'\xff\xfe\xfe'.
raw_buf[n] returns an int in [0;255]
'''

sample_buf = [byt_sum(raw_buf[byt_depth*k:byt_depth*(k+1)])
              - 2**(8*byt_depth-1) for k in range(buf_size)]

Problem is: when I plot sample_buf for a single sine signal, I get an alternative, wrecked sine signal. I can't figure out why the signal overlaps udpside-down.

Any idea?

P.S.: Since I'm French, my English is quite hesitating. Feel free to edit if there are ugly mistakes.

Literalism

What resource are you using to produce the graph of the wrecked signal? – Tenebrae

Answer 1

It might be because you need to use an unsigned value for representing the 16bit samples. See https://en.wikipedia.org/wiki/Pulse-code_modulation

Try to add 32767 to each sample.

Also you should use the python struct module to decode the buffer.

import struct
buff_size = 512
# 'H' is for unsigned 16 bit integer, try 'h' also
sample_buff = struct.unpack('H'*buf_size, raw_buf)

Tamarind

Well, it does work for 16bit-encoded audio, but - in an ideal world - this script is supposed to work for any arbitrary byte depth. (I'm not familiar with the struct module, how should I replace the h for 24bit int ?). Thank you anyway! – Literalism

@Literalism : To learn about it (if you already didn't) just import struct and ask help(struct) in the interpreter. All depths are supported except 24-bit, i.e. sampwidth=3. But it's not a problem, you unpack it into 32 bits. Pad the sample with extra zero ('\x00') and remove the zero from the integer after struct.unpack()ing by shifting 8 bits. – Ley

Answer 2

The easiest way is to use a library that does the decoding for you. There are several Python libraries available, my favorite is the soundfile module:

import soundfile as sf
signal, samplerate = sf.read('/some_path/my_audio.wav')

Seeseebeck

This module has been flagged as spam – Lucent

Thanks for the comment @MathieuRodic! The URL has been changed from pysoundfile.readthedocs.io to python-soundfile.readthedocs.io. I have updated the link above. – Seeseebeck