Signal processing utilities for audio

This is an audio research project.

Provided is a script to downsample 32 bits audio files to 16 or even 8 bits, but instead of simply truncating to a lower bit definition, the LSB is pulse width modulated by error integration, so that after DAC conversion we still get a resolution higher to the number of bits. The issue is that we generate noise with an LSB amplitude which sounds like correlated white noise. However this noise has only an LSB peak to peak amplitude which is really low (inaudible on 16 bits, comparable to an analog tape hiss on 8 bits). In Theory you get an extra bit depth at SF/4 (SF = Sampling Frequency), and an extra bit depth every time you divide the frequency by 2. So in Theory with 44.1 KHz recording with a 32 bits source downsampled to 16 bits, you have 16 bits precision at 22 KHz, 17 bits at 11 Khz, 18 bits at 5.5 Khz, etc... If I'm not mistaken (still needs to be benched).

The goal is to adapt high definition recordings to CD and to keep the max possible resolution on the CD. In Theory dither does the same, so this lib would be useless. However this could allows reducing the dither level (so the noise) and keep the precision (still needs to be benched). The goal is to see if it's useless or if it does really add precision.

TODO:

• Add unittest
• Add dither to decorrelate the noise.
• Add support to downsample to 24 bits (from 24 bits is now supported)
• Add frequency downsampling (ex: 96 KHz -> 44.1 KHz)

Install:

first install virtualenvwrapper:

• https://virtualenvwrapper.readthedocs.io/en/latest/install.html

Then:

mkvirtualenv --python=/usr/bin/python3 python_sigprocessutils
git clone git@github.com:ygbourhis/python_sigprocessutils.git
cd python_sigprocessutils
pip install -Ur requirements.txt
pip install -e .

Before using in a shell do not forget to activate the virtualenv:

workon python_sigprocessutils

Script usage:

audio_down_sample -i input_file.wav -o output_file.wav -b 16

Or:

audio_down_sample -i input_file.wav -o output_file.wav -b 8

Or:

audio_down_sample -i input_file.wav -o output_file.wav -b 8 --verbose

Library usage examples to downsample a Wave file with python code:

faded = wave.open("Alan_Walker_-_Faded.wav")
downsample = wave.open("downsample_integrated.wav", "w")
downsample.setparams((2, 1, 44100, 0, 'NONE', 'not compressed'))

integrator_r = Integrator()
integrator_l = Integrator()
downsample_l = DownSamplingLSBIntegration(integrator_l)
downsample_r = DownSamplingLSBIntegration(integrator_r)

nb_frames = faded.getnframes()
for i in range(nb_frames):
    paked_input = faded.readframes(1)
    input_g, input_d = struct.unpack('<hh', paked_input)
    output_g = downsample_l.transfert(input_g/256) + 128
    output_d = downsample_r.transfert(input_d/256) + 128
    print(input_g, ':', output_g, '|', input_d, ':', output_d)
    packed_output_g = struct.pack('B', output_g)
    packed_output_d = struct.pack('B', output_d)
    downsample.writeframes(packed_output_g)
    downsample.writeframes(packed_output_d)

OR:

for i in range(nb_frames):
    paked_input = faded.readframes(1)
    input_g, input_d = struct.unpack('<hh', paked_input)
    output_g = downsample_l.transfert(input_g/256) + 128
    output_d = downsample_r.transfert(input_d/256) + 128
    print(input_g, ':', output_g, '|', input_d, ':', output_d)
    packed_output = struct.pack('<BB', output_g, output_d)
    downsample.writeframesraw(packed_output)

OR:

downsample.setnframes(2)
for i in range(nb_frames):
    paked_input = faded.readframes(1)
    input_g, input_d = struct.unpack('<hh', paked_input)
    output_g = downsample_l.transfert(input_g/256) + 128
    output_d = downsample_r.transfert(input_d/256) + 128
    print(input_g, ':', output_g, '|', input_d, ':', output_d)
    packed_output = struct.pack('<BB', output_g, output_d)
    downsample.writeframes(packed_output)

downsample.close()
faded.close()

Useful Links:

• http://www.bravegnu.org/blog/python-wave.html
• https://github.com/sole/snippets/blob/master/audio/generate_noise_test_python/script.py
• http://soledadpenades.com/2009/10/29/fastest-way-to-generate-wav-files-in-python-using-the-wave-module/
• https://soledadpenades.com/posts/2009/fastest-way-to-generate-wav-files-in-python-using-the-wave-module/
• http://fsincere.free.fr/isn/python/cours_python_ch9.php
• https://www.cameronmacleod.com/blog/reading-wave-python