Downsampling wav audio file

Question

I have to downsample a wav file from 44100Hz to 16000Hz without using any external Python libraries, so preferably wave and/or audioop. I tried jus

Answer 1

You can use resample in scipy. It's a bit of a headache to do, because there's some type conversion to be done between the bytestring native to python and the arrays needed in scipy. There's another headache, because in the wave module in Python, there is no way to tell if the data is signed or not (only if it's 8 or 16 bits). It might (should) work for both, but I haven't tested it.

Here's a small program which converts (unsigned) 8 and 16 bits mono from 44.1 to 16. If you have stereo, or use other formats, it shouldn't be that difficult to adapt. Edit the input/output names at the start of the code. Never got around to use the command line arguments.

#!/usr/bin/env python
# -*- coding: utf-8 -*-
#
#  downsample.py
#  
#  Copyright 2015 John Coppens 
#  
#  This program is free software; you can redistribute it and/or modify
#  it under the terms of the GNU General Public License as published by
#  the Free Software Foundation; either version 2 of the License, or
#  (at your option) any later version.
#  
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
#  
#  You should have received a copy of the GNU General Public License
#  along with this program; if not, write to the Free Software
#  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
#  MA 02110-1301, USA.
#  
#

inwave = "sine_44k.wav"
outwave = "sine_16k.wav"

import wave
import numpy as np
import scipy.signal as sps

class DownSample():
    def __init__(self):
        self.in_rate = 44100.0
        self.out_rate = 16000.0

    def open_file(self, fname):
        try:
            self.in_wav = wave.open(fname)
        except:
            print("Cannot open wav file (%s)" % fname)
            return False

        if self.in_wav.getframerate() != self.in_rate:
            print("Frame rate is not %d (it's %d)" % \
                  (self.in_rate, self.in_wav.getframerate()))
            return False

        self.in_nframes = self.in_wav.getnframes()
        print("Frames: %d" % self.in_wav.getnframes())

        if self.in_wav.getsampwidth() == 1:
            self.nptype = np.uint8
        elif self.in_wav.getsampwidth() == 2:
            self.nptype = np.uint16

        return True

    def resample(self, fname):
        self.out_wav = wave.open(fname, "w")
        self.out_wav.setframerate(self.out_rate)
        self.out_wav.setnchannels(self.in_wav.getnchannels())
        self.out_wav.setsampwidth (self.in_wav.getsampwidth())
        self.out_wav.setnframes(1)

        print("Nr output channels: %d" % self.out_wav.getnchannels())

        audio = self.in_wav.readframes(self.in_nframes)
        nroutsamples = round(len(audio) * self.out_rate/self.in_rate)
        print("Nr output samples: %d" %  nroutsamples)

        audio_out = sps.resample(np.fromstring(audio, self.nptype), nroutsamples)
        audio_out = audio_out.astype(self.nptype)

        self.out_wav.writeframes(audio_out.copy(order='C'))

        self.out_wav.close()

def main():
    ds = DownSample()
    if not ds.open_file(inwave): return 1
    ds.resample(outwave)
    return 0

if __name__ == '__main__':
    main()