Python: Convert mono wave file to stereo

Question

I am looking to convert a wave file from mono to stereo in Python. At the end there should be two identical channels right and left having the mono channel information. My code does not work. I get only the left channel with the input information, the right channel is empty. Any suggestions?

import struct, wave
import numpy as np

def make_stereo(file1, output):
    def everyOther (v, offset=0):
        return [v[i] for i in range(offset, len(v), 2)]
    ifile = wave.open(file1)
    print ifile.getparams()
    # (1, 2, 44100, 2013900, 'NONE', 'not compressed')
    (nchannels, sampwidth, framerate, nframes, comptype, compname) = ifile.getparams()
    frames = ifile.readframes(nframes * nchannels)
    ifile.close()
    out = struct.unpack_from("%dh" % nframes * nchannels, frames)
    # Convert 2 channels to numpy arrays
    if nchannels == 2:
        left = np.array(list(everyOther(out, 0)))
        right = np.array(list(everyOther(out, 1)))
    else:
        left = np.array(out)
        right = left
    ofile = wave.open(output, 'w')
    ofile.setparams((2, sampwidth, framerate, nframes, comptype, compname))
    ofile.writeframes(left.tostring())
    # ofile.writeframes(right.tostring())
    ofile.close()

make_stereo("Input.wav", "Output.wav")

Answer 1

Using numpy for this is the proverbial elephant gun to shoot a fly, except, perhaps, if you are having performance problems.

array is easy to reason about and work with. I'm not at my audio computer, but a translation of your program to use array should look something like this:

import wave, array

def make_stereo(file1, output):
    ifile = wave.open(file1)
    print ifile.getparams()
    # (1, 2, 44100, 2013900, 'NONE', 'not compressed')
    (nchannels, sampwidth, framerate, nframes, comptype, compname) = ifile.getparams()
    assert comptype == 'NONE'  # Compressed not supported yet
    array_type = {1:'B', 2: 'h', 4: 'l'}[sampwidth]
    left_channel = array.array(array_type, ifile.readframes(nframes))[::nchannels]
    ifile.close()

    stereo = 2 * left_channel
    stereo[0::2] = stereo[1::2] = left_channel

    ofile = wave.open(output, 'w')
    ofile.setparams((2, sampwidth, framerate, nframes, comptype, compname))
    ofile.writeframes(stereo.tostring())
    ofile.close()

make_stereo("Input.wav", "Output.wav")