plot audio data in gnuplot

Question

how could I convert an audio file such as a aiff into a svg using gnuplot? I used sox (sound exchange) to convert an .aiff into a .dat, which I can load now in gnuplot.

Answer 1

To draw lines between the points, use

plot "test.dat" with lines

Or to keep the point markers as well as the lines, use

plot "test.dat" with linespoints

So your example becomes

set terminal svg    
set output "test.svg"
plot "test.dat" with lines

Further tips:

Don't consider every sample:

With large files you may also find it useful to plot only every nth sample with "every n". This will make the plot much faster to generate and will also yield a smaller (but less detailed) svg file.

e.g.

plot "test.dat" every 100 with lines

Ignore .dat file header:

If your sox-produced .dat file has some lines of introductory metadata, such as

; Sample Rate 44100
; Channels 2

you can add the following to have gnuplot consider those lines comments and ignore them.

set datafile commentschars ";"

This will save you having to pre-process your .dat file in order to remove those lines before gnuplot chokes on them.

Plot both left and right channels of stereo audio:

If you're working with a stereo file, you probably want to see both channels.

We can use "multiplot" to lay out the following two plots (of left then right channel) one above the other on a shared x-axis, as many sound-editing programs do.

set multiplot layout 2,1
plot "test.dat" using 1:2 with lines
plot ""         using 1:3 with lines

The 1:2 and 1:3 instruct gnuplot which columns of the dat file to use as x and y sources. I'm assuming your stereo .dat file produced by sox looks as mine does, with columns for - 1: time since beginning of first sample - 2: normalized sample value of left channel - 3: normalized sample value of right channel

example snippet:

   10.840113       0.20101929      0.17840576 
   10.840136       0.26062012      0.14831543 
   10.840159       0.23779297      0.13146973 

Putting it together: Here's a script which puts all of the above together. If you don't have a stereo data file to try this with, you'll want to remove the plot of 1:3 and the multiplot setting.

#!/usr/bin/env gnuplot
set datafile commentschars ";"

set terminal svg
set output "test.svg"

set multiplot layout 2,1
plot "test.dat" using 1:2 every 100 with lines
plot ""         using 1:3 every 100 with lines
unset multiplot

Prettification

Finally, I've tweaked the script for presentation (borrowing heavily from the excellent "gnuplot in action" book by Philipp K. Janert):

#!/usr/bin/env gnuplot
set datafile commentschars ";"

set terminal svg
set output "test.svg"

set multiplot layout 2,1

set ylabel "sample value"
set bmargin 0
set format x ""
set ytics -0.8,0.2
set key bottom
plot "test.dat" using 1:2 every 100 with lines lc rgbcolor "#a0a0b0" title "left channel"

set xlabel "time (s)"
set bmargin
set tmargin 0
set format x "%g"
set ytics -1.0,0.2,0.8
set key top

plot ""         using 1:3 every 100 with lines lc rgbcolor "#a0a0b0" title "right channel"
unset multiplot

Here's an example output (albeit png):

How to make a .dat file

For anyone following along at home, you can use sox to generate a .dat file from an audio file with the following command:

sox input.wav output.dat

Big file warning: Converting even just 10 seconds of stereo audio at 40kHz will produce a 25Mb output file.