This code implements a novel effect for transitioning between audio signals that we call "audio transport". As an interpolation parameter is changed, the pitches in one signal slide to the pitches in the other, producing a portamento, or musical glide. The assignment of pitches in one sound to pitches in the other is accomplished by solving a 1-dimensional optimal transport problem.
The effect is described in this paper which won "Best Student Paper" at the 22nd International Conference on Digital Audio Effects. Audio examples can be found here, or click on the image below for a video demonstration:
This code provides the static effect, meaning it provides an executable that takes two audio files as input and combines the two using the effect to produce and output audio file. The code demonstrated in the video that uses the effect live uses the functions provided by this library on portaudio streams. The live code is super hacky and you could probably make something much better, which is why I haven't included it here. But if you really want it, I can send it to you. All of the important stuff is in this library.
Using the static effect does not require writing any more code but it does require several dependencies. You will need fftw3, ffmpeg and audiorw.
Install fftw3 and ffmpeg via your system package manager. This code has been tested on versions 3.3.8 and 4.2.3 respectively on a machine running Arch Linux. Hopefully things don't break on your system but if they do, please make a pull request :-) !
Then install the audiorw library with cmake:
git clone https://github.com/sportdeath/audiorw
mkdir audiorw/build
cd audiorw/build
cmake ..
make
sudo make install
Once the dependencies are installed, install audio_transport with cmake:
git clone https://github.com/sportdeath/audio_transport
mkdir audio_transport/build
cd audio_transport/build
cmake .. -D BUILD_EXAMPLES=ON
make
Then, to apply the effect use the transport binary. For example, below we tranform piano audio into guitar audio. The output starts by sounding like the piano audio for the first 20% of the duration. Then between 20% and 70% of the duration, the piano is transformed into a guitar. In the last 30% the audio is simply the guitar.
./transport piano.wav guitar.mp3 20 70 out.flac
You can also apply the effect to a single file with the glide binary. The input file serves as one input to the "transport" and the output of the effect is fed back into the second input. This slurs all of the frequencies in the input like the glide/lag/portamento knob found on some synthesizers ... however it works on any audio input.
In this example we apply the glide effect to a piano with a time constant of 1 millisecond:
./glide piano.wav 1 piano_glide.ogg
If you want to use the audio transport functions provided by this library in another project (e.g. to make a live effect) then this library only requires fftw3. Once you have it, install audio_transport with cmake:
git clone https://github.com/sportdeath/audio_transport
mkdir audio_transport/build
cd audio_transport/build
cmake ..
sudo make install
Then in your own project, include these headers:
#include <audio_transport/spectral.hpp>
#include <audio_transport/audio_transport.hpp>
spectral.hpp provides functions that turn vectors of audio into spectral objects, incapsulating time and frequency as well as their reassigned counterparts which are necessary for the effect. It also provides the inverse.
audio_tranport.hpp provides an interpolate function that takes windows of audio (that are in the spectral format) and combines them according the effect.