Friday, January 31, 2014

Generating a spectrogram of The Shepard Tone

Earlier today I saw a tweet from Tim O'Reilly pointing to an interesting article in The Atlantic on audio illusions.  The Shepard Tone illusion sounds like a continuously increasing or decreasing tone which goes on indefinitely... which is impossible because you'd eventually end up at at inaudibly low or high pitch.

Curious as to what was going on I downloaded the YouTube video, extracted the audio and ran off a spectrogram with the open source sox audio tool. I had done this before last year, so I was familiar with the process.

1. Download the YouTube video:
youtube-dl -o shepard_tone.mp4

2. Extract the audio from the video
ffmpeg -i shepard_tone.mp4 -f mp3 shepard_tone.mp3

3. Convert to mono (the YouTube video has stereo audio)
sox shepard_tone.mp3 shepard_tone_mono.mp3 remix 1,2

4. Create the spectrogram
sox shepard_tone_mono.mp3 -n spectrogram -o spectrum.png

One small problem: most of the action is down at the lower frequencies. So I resampled the audio and regenerated the spectrogram so that the interesting bits were more visible.

5. sox shepard_tone_mono.mp3 -r 4k -o shepard_tone_4k.wav

6. sox shepard_tone_4k.wav -n spectrogram -o spectrum.png

Tools used :