pyCrossfade

pyCrossfade is born out of a personal effort to create a customizable and beat-matched crossfade functionality.

December 2024: ✨added CLI and Docker image✨

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CLI

Since the creation of this project, Python3 and dependencies got updated and stopped working with the pyCrossfade codebase.

Docker Image

I've created a Docker Image on ghcr.io to help users getting the correct dependecy versions.

docker pull ghcr.io/oguzhan-yilmaz/pycrossfade:latest

CLI Setup (for Linux)

To use the Docker image as a CLI, you'd need to:

• attach your audios/ directory to the container
• attach a directory for persisting pyCrossfade annotations
• set some Env Vars for easier use.

This can be long and ugly, so the best thing to do is create a alias command.

Change the MY_AUDIO_DIRECTORY and add the following bash snippet to your .bashrc :

PYCROSSFADE_DIR="$HOME/.pycrossfade"
PYCROSSFADE_ANNOTATIONS_DIR="$PYCROSSFADE_DIR/annotations"

MY_AUDIO_DIRECTORY="!!CHANGEME!!"

# create the alias command
alias pycrossfade="mkdir -p $PYCROSSFADE_DIR \
  && mkdir -p $PYCROSSFADE_ANNOTATIONS_DIR \
  && docker run --rm -it \
      -v "$MY_AUDIO_DIRECTORY:/app/audios" \
      -v $PYCROSSFADE_ANNOTATIONS_DIR:/app/pycrossfade_annotations \
      -e ANNOTATIONS_DIRECTORY=/app/pycrossfade_annotations \
      -e BASE_AUDIO_DIRECTORY=/app/audios/ \
      ghcr.io/oguzhan-yilmaz/pycrossfade:latest"

CLI Usage

pycrossfade

pycrossfade crossfade --help

CLI Commands

• crossfade: Crossfade between two songs
• crossfade-many: Crossfade between min. of 3 songs
• song: Process song and print metadata
• extract: Extract BPM, ReplayGain, Key/Scale etc.
• mark-downbeats: Play a beep sound on each downbeat
• cut-song: Cut a song between two downbeats

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Older pyCrossfade

Before I added the CLI and Docker feature, I created the v0.1.0 tag. Access below:

• https://github.com/oguzhan-yilmaz/pyCrossfade/releases/tag/v0.1.0
• Older Scripted Usage documentation

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About This Project

This project's main goal is to create seamless crossfade transitions between music files. This requires some DJ'ing abilities such as bpm changing, beat-matching and equalizer manipulation.

Some Definitions on Music Domain

• Beat In music and music theory, the beat is the basic unit of time, the pulse or regularly repeating event. The beat is often defined as the rhythm listeners would tap their toes to when listening to a piece of music.

• Bar (Measure) In musical notation, a bar (or measure) is a segment of time corresponding to a specific number of beats, usually 4.

• Downbeat The downbeat is the first beat of the bar, i.e. number 1.

About Madmom's Beat Tracking

Madmom's Beat Tracking takes a long time to run, 45-150 seconds depending on the music file. It gives a numpy array as output, so when madmom finishes calculating, pyCrossfade saves/caches the said numpy array in a text file named after the song, under the folder pycrossfade_annotations. This makes pyCrossfade robust while working with same songs by avoiding heavy calculations every time.

BPM Matching

The creation of a transition requires two songs, called master and slave songs. Master song is the currently playing track and slave song refers to the next track.

Master and slave tracks can be in different BPM's or speeds, so before applying crossfade, we have to gradually increase/decrease to master track's speed to match slave's speed. Let's say master song has 90 bpm, and slave song has 135 bpm. This makes slave song 1.5x faster than master song. If we were to suddenly increase the speed 1.5x that would be harsh on the listeners ear.

Gradually Time Stretching On Downbeats

Before applying crossfade, to match the bpm's of two songs, master song's speed is gradually increased on given number of downbeats. This ensures the listening experience quality. This works linearly as can be seen in the table below.

Example

from pycrossfade.transition import crop_audio_and_dbeats \
                                   time_stretch_gradually_in_downbeats

from pycrossfade.song import Song
from pycrossfade.utils import save_audio

my_song = Song('some/path/to/a/song.mp3')

final_factor =  1.10 # times faster

# returns a new Song obj. cropped from my_song's between given parameter downbeats(or bars).
sample = crop_audio_and_dbeats(my_song, 50, 60) # sample of 10 bars

# increases the sample song's speed gradually
sample_but_faster_every_beat = time_stretch_gradually_in_downbeats(sample, final_factor)

save_audio(sample_but_faster_every_beat, 'some/output/path.wav', file_format='wav', bit_rate=320)

bars	1	2	3	4	5	6	7	8	9	10	Final Factor
Time Stretching Factor	1.01x	1.02x	1.03x	1.04x	1.05x	1.06x	1.07x	1.08x	1.09x	1.10x	1.10x

Overview of the Transition

A simple visualization of all the processes would be like this:

master song | bpm matching | crossfade | slave song

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pyCrossfade's Approach To Perfect Beat Matching

Human ear can catch minimal errors easily thus making beat-matching is extremely important for any transition. Beat-matching would be easy if every beat had regular timing, but producers are doing their best to humanize their songs, not playing every beat in regular timing to get nonrobotic rhythms.

A Visual Explanation

Here, I cut two songs between their 30th and 50th downbeats, resulting in the same amount of downbeats.

Red lines are denoting every bar, or it's delimiter downbeats.

This is the second song with 20 bars.

First song's waveform is blue and it's bars denoted with red lines. Second song is shown with colors of orange and green.

When we put them on top of each other, we can see that their beats(red and green lines) is not matched, resulting in clashing of drums - or distorted audio.

Even though they have same amount of bars, resulting plot shows that second song is shorter. This is beacuse they have different BPMs - or speeds.

If every song had regular beat timing, then beat-matching would be easy as just time stretching the other song to match their speeds. However, because of humanizing, every bar can be different in length. For this reason, pyCrossfade applies beat matching on the level of bars.

Beat Matching on the level of every bar

pyCrossfade lets you define every transition's length in bars, lets take it as K bars. Then it gets master song's last K bars, and slave song's first K bars, and applies beat matching on each bar. This is ensures the created transition is perfectly beat-matched even if the songs are humanized or not.

Possible Improvements

• Better(maybe Non-Linear) EQ Filtering
• Volume Balancing with Replay Gain
• Developing a better Crossfade EQ Filtering