#Problem

We want to shift the tempo of a song to make a transition into another song

#Question

How do we alter the tempo of a song to match another song?

How can we shift the tempo without making it noticeable to the listener?

#Resources

1. Echonest API
2. Echonest Remix API
3. Tempo change study

1. Mini-abstract and releveance of Echonest API

The Echonest API is an API for song related information. It has methods that allow the user to get song information, and contains classes that allow a song to be altered. Using the pyechonest.track module, you can find the tempo and time signature of the song.

import pyechonest.track as track

t = track.track_from_filename('file.mp3')
t.tempo
t.time_signature

The above commands will yield the tempo and time_signature of a song.

2. Mini-abstract and relevance of Echonest Remix API

The Echonest Remix API contains methods to alter a song. Specifically for our case, it allows the beat of a song to be altered.

Example code taken from https://github.com/echonest/remix/blob/master/examples/stretch/simple_stretch.py

import math
import os
import sys
import dirac
from echonest.remix import audio

audiofile = audio.LocalAudioFile(input_filename)
beats = audiofile.analysis.beats
collect = []

for beat in beats:
  beat_audio = beat.render()
  scaled_beat = dirac.timeScale(beat_audio.data, ratio)
  ts = audio.AudioData(ndarray=scaled_beat, shape=scaled_beat.shape,
      sampleRate=audiofile.sampleRate, numChannels=scaled_beat.shape[1])
  collect.append(ts)
  
out = audio.assemble(collect, numChannels=2)
out.encode(output_filename)

Using the audio and modify modules from echonest.remix, one can alter the time of a beat using the dirac.timeScale method. Using this altered beat, you can re-apply the previous characteristics of the beat and make a new song out of these beats.

3. Mini-abstract and relevance of Tempo change study

This is an article describing the results of a study in human perception of tempo change. This is important for us so we know what threshold we can alter the beat of a song before it becomes too obvious to the listener. The results of this article showed that humans perceive tempo changes when the tempo is altered by approximately 8%.