Generating spectrograms with arecord, sox and ffmpeg/avconv

The following command records a WAV file using ALSA (Device 2.0). In this example, Sox is piped into the process — which is actually unnecessary in this case — but could still be used to apply various filters to the resulting test.wav file:

arecord -D hw:2,0 -r 32000 -f S16_LE -c 1 -t wav | sox -t wav -c 1 -L -b 16 -r 32000 - test.wav

Now, using the newly created test.wav file, a spectrogram can be generated:

sox test.wav -n spectrogram -o image.png

The following command creates a WAV file using Alsa (Device 2.0). In this case, Sox is additionally piped, which is completely unnecessary here, however, Sox could still add various filters to the resulting test.wav change:

arecord -D hw:2,0 -r 32000 -f S16_LE -c 1 -t wav | sox -t wav -c 1 -L -b 16 -r 32000 - test.wav

Using the file test.wav that has just been created, a spectrogram is now generated:

sox test.wav -n spectrogram -o image.png

A similar spectrogram image can be created directly from a video file using avconv:

avconv -i test.avi -lavfi showspectrumpic=s=hd480:legend=0,format=yuv420p out.png

The following instruction creates a spectrogram from a video test.avi. This is saved as a video under the name out.avi.

Instead of avconv, ffmpeg could also have been used. The Debian version does not currently support all parameters here.

Alternatives with MPlayer and Sox

You can also generate a spectrogram from a video using mplayer, or a combination of mplayer and sox.

First, extract a WAV file using mplayer:

mplayer test.mp4 -ao pcm:file=/dev/stdout -vo null > test.wav

Then generate the spectrogram:

sox test.wav -n spectrogram -o test.png

Spectrograms created this way can be directly compared. However, even with seemingly identical WAV files, this comparison can be tricky.

To make differences more visible, creating a spectrogram diff is recommended.

First, create a diff WAV file:

sox -m -v 1 source1.wav -v -1 source2.wav diff.wav

Then generate the spectrogram diff:

sox diff.wav -n spectrogram -o diff.png