Making music, part 3: Making noise

Last week we parsed some music. That post was in a bit of a hurry, so we had to leave off a fair few important pieces (like ties and slurs for one; chords for a rather bigger one). We’ll get to them soon, but for now we want to actually get something playing back.

The main part of the story this time around will be John Clement’s rsound package for Racket. With the new package system (and an adequately new version of Racket), installation is rather straightforward:

raco pkg install rsound

This gives us a nice variety of methods for making music. For a quick overview, here’s an older version of the documentation: rsound docs. I’m not sure where / if the new package manager has pre-built docs available, but installing it will build the documentation as well. Still, the API is essentially the same.

The first thing we need is to make a sound. We can use the network and sine-wave macros to generate a simple tone:

(network ()
  [out (sine-wave 440)])

(440 Hz is perhaps the easiest tone on a piano keyboard to remember, it’s the A above middle C.)

If we want to play that back, we can do so with signal->rsound to make a note and play to play it back:

(play
 (signal->rsound
  44100
  (network ()
    [out (sine-wave 440)])))

44100 is the number of frames to generate. Given the default parameters, that will be a one second long sound. Go ahead and give that a whirl if you want something a bit painful. The default volume is… intense.

The last piece we need is the math that turns a piano key into a frequency. Luckily, the math has already been done for us:

f(n) = ( \sqrt[12]{2}\ )^{n-49} \cdot 440\ Hz

Combine the previous code and that function and we have a nice way to make single pitches:

(define (note->rsound 1/2s-above-c)
  (define 12th-root-of-2 (expt 2 (/ 1 12)))
  (define n (+ 1/2s-above-c 40))
  (define freq (* (expt 12th-root-of-2 (- n 49)) 440))
  (rs:network ()
              [out (rs:sine-wave freq)]))

You may have noticed the rs: prefix on rsound functions. That’s because it exports at least one function (silence) that conflicts with what we’ve already written. So by using the prefix-in form of require way we have a namespace of sorts on the rsound functions:

(require (prefix-in rs: rsound))

After single notes, there’s a function signal-+s which we can use to add two signals together. This will let us build chords of those notes we were already making:

(define (chord->rsound notes)
  (rs:signal-+s
   (for/list ([each (in-list notes)])
     (note->rsound each))))

Unfortunately, that doesn’t quite work at the moment (since we don’t have the parser working for chords yet), but we can still try it out:

> (play (signal->rsound 44100 (chord->rsound '(40 44 47))))

With those being the notes on a standard piano corresponding to middle C and the E and G above it, that should be a nice C Major chord in all it’s pure tone glory. It’s ugly, but it’s definitely a chord.

With that, all that’s left is pulling apart a song. We’ll go ahead and use our good friend match again, since we’ve already seen how well it deals with structs:

(define (abc->rsound current-note)
  (define current-rsound
    (match current-note
      [(or #f '())
       (rs:silence 1)]
      [(note _ length pitch)
       (rs:signal->rsound (exact-floor (* length 44100)) (note->rsound pitch))]
      [(silence _ length)
       (rs:silence (exact-floor (* length 44100)))]))

  (define rest-rsound
    (match current-note
      [(or #f '())
       (rs:silence 1)]
      [(item (list-rest next-note rest-notes) _)
       (set-item-next! current-note rest-notes)
       (abc->rsound next-note)]
      [(item next-note _)
       (abc->rsound next-note)]))

  (rs:rs-append*
   (list current-rsound
         (rs:silence 10)
         rest-rsound)))

Hopefully that’s pretty straight forward. We have three parts. In the first internal define, we’re making a sound for the first note. It’s either a note or silence, either way we create the correct number of frames, completely ignoring the item-next field (setting it to _).

In the second part, we ignore the first part and only look at the item-next. This lets us know if we have #f at the end of a song, and empty list if we’ve already played through a repeat enough times, a list of repeated options (which we need to advance with set-item-next!1, or finally a single note. In all cases but the first, we recursively generate that tone; if there’s no next, we generate a single frame of silence (easier than special casing for the last part of the function).

Finally, we put it all together. I put another 10 frames of silence here as a completely randomly chosen break between notes. Without it, repeated notes tend to run together. With it, we get some nice spacing. I’m going to have to tune it a little bit, but for the moment it works great.

So now we can turn a song into an rsound, but how about playing it back? Easy:

(define (abc-play-file filename)
  (rs:play
   (abc->rsound
    (abc-parse/playback
     (call-with-input-file filename abc-lex)))))

Here’s a nice example:

Yes, it’s a bit really fuzzy. That’s mostly because of the compression involved in making it an mp3. Also, the notes are pure sine waves. In reality, musical notes are a bit more complicated than that. We’ll get to that soon, I hope.

How did I generate the file? Glad you asked. The rsound library pretty much does this for us with the rs-write function:

(define (abc->wav abc-filename wav-filename)
  (rs:rs-write
   (abc->rsound
    (abc-parse/playback
     (call-with-input-file abc-filename abc-lex)))
   wav-filename))

For this particular file, I used Audacity to make it into an MP3, but that shouldn’t strictly speaking be necessary.

And there you have it. We have audio playback. There’s still a whole host of things to do, but I think this is pretty good progress.

As always, you can see the source code for this project on GitHub: jpverkamp/abc. I’d love to see some bug reports; I know there are some issues. 😄


  1. yes, unfortunately this means songs can only be played once at the moment [return]
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