jameh jameh - 10 months ago 98
Python Question

Python Wave byte data

I'm trying to read the data from a .wav file.

import wave
wr = wave.open("~/01 Road.wav", 'r')
# sample width is 2 bytes
# number of channels is 2
wave_data = wr.readframes(1)

This gives:


Which is the "first frame" of the song. These 4 bytes obviously correspond to the (2 channels * 2 byte sample width) bytes per frame, but what does each byte correspond to?

In particular, I'm trying to convert it to a mono amplitude signal.


If you want to understand what the 'frame' is you will have to read the standard of the wave file format. For instance: https://web.archive.org/web/20140221054954/http://home.roadrunner.com/~jgglatt/tech/wave.htm

From that document:

The sample points that are meant to be "played" ie, sent to a Digital to Analog Converter(DAC) simultaneously are collectively called a sample frame. In the example of our stereo waveform, every two sample points makes up another sample frame. This is illustrated below for that stereo example.

sample       sample              sample
frame 0      frame 1             frame N
 _____ _____ _____ _____         _____ _____
| ch1 | ch2 | ch1 | ch2 | . . . | ch1 | ch2 |
|_____|_____|_____|_____|       |_____|_____|
|     | = one sample point

To convert to mono you could do something like this,

import wave

def stereo_to_mono(hex1, hex2):
    """average two hex string samples"""
    return hex((ord(hex1) + ord(hex2))/2)

wr = wave.open('piano2.wav','r')

nchannels, sampwidth, framerate, nframes, comptype, compname =  wr.getparams()

ww = wave.open('piano_mono.wav','wb')

frames = wr.readframes(wr.getnframes()-1)

new_frames = ''

for (s1, s2) in zip(frames[0::2],frames[1::2]):
    new_frames += stereo_to_mono(s1,s2)[2:].zfill(2).decode('hex')


There is no clear-cut way to go from stereo to mono. You could just drop one channel. Above, I am averaging the channels. It all depends on your application.