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a very basic framework for playing music

edited April 2012 in Code Sharing Posts: 93

this is a fairly simple framework for playing musical notes with SFXR. it is based on the drum machine example.

the function note() will play notes. at the moment is is very primitive- all it will do is take a number value, in terms of semitones above (or, for negative values, below) C3, and the waveform to use. it also has, in this example, a kick and hat sound which can be used.

if someone else has time, making it so that it checks a table of frequency translations that is generated at setup(), rather than doing the 0.19 * 2^24*freq every time it plays a new note would be just great. maybe even make it so that you can enter in a sting such as note_c#4 rather than 14.

i will be making a program that allows you to write songs now, and will post that once it is complete. that program will plrobably include options for note length/volume and such.



  • Posts: 202

    Before attacking your precalculation problem let's get some existing things straight, perhaps I don't understand SFXR properly ...

    From your code:

    StartFrequency = 0.19 * 2^(freq/24)

    1. freq seems to be a relative note number rather than a frequency.

      StartFrequency = 0.19 * 2^(rel_note/24)

    2. You say it is relative to C3 which has a frequency of about 130Hz.

      StartFrequency = 0.13 * 2^(rel_note/24)

      (Frequency is given in kHz, right?)

    3. An octave is made up of 12 semitones.

      StartFrequency = 0.13 * 2^(rel_note/12)

    If you're writing a program to make music, you may base your note numbers on MIDI numbers, so A4 (I call THAT a reference, what the hell is C3?) is number 69.

    function freqOfMidiNote(note_number) return 0.44 * 2 ^ ((note_number - 69) / 12) end

    While I'm at it, you can now make a table of frequencies:

    frequencies = {} for i = 0, 100 do frequencies[i] = freqOfMidiNote(i) end
  • edited April 2012 Posts: 93

    The freq is just a note number, not an actual frequency. I gave it at name because i wanted to avoid mixing it up with the function name. SFXR uses a 0-1 scale rather than hertz, which is why the math seems counterintuitive.

    I spent about an hour with a tuner trying to nail down the numbers and I chose C3 because it was the absolute closest to a round value- its frequency in SFXR is 0.1902. To make the equation follow midi scale, simply change the frequency calculation to 0.19 *2((freq-60)/24)

    I'm not having a problem with making a frequency table, persay. I just didn't feel like doing it. :P I am working on a program that enables you to write midi-like songs with this system at the moment.

  • Posts: 384

    Hi folks,

    I too have spent many hours tuning SFXR... :) I found the magic formula to equate keyboard notes with pitches. It's part of my ABCplayer, which reads ABC tunes - the ASCII equivalent of MIDI. Thread with links to code here:

    Have a look and try it out, let me know if you would like to collaborate. Next I plan to work on different timbres an multivoicing using the ABC standard.

  • Posts: 202

    I thought an authoritative answer could be a valid reason to bring this thread back from the dead, be it just for education or further inspiration. I also had to correct myself, it bugged me.

    The core line in SFXR is this:


    Remember that p_base_freq is expressed in a range of 0...1. Examining the code further reveals that fperiod is not exactly the period, but 8 times supersampled. Let me reflect this:


    Also, fperiod is expressed in terms of the sample rate, that means that at a sample rate of 44100 Hz a period value of 44100 means a sound of 1 Hz. It means that fperiod = SR/f, with f = frequency of the sound and SR = sample rate.



    f/SR=8/100*(p_base_freq*p_base_freq+0.001) (1) p_base_freq=sqrt(f/SR*100/8-0.001)

    You can use the formula (1) directly, with SR = 44100, to calculate the correct value to feed into SFXR.

    Quick check: With f = 130 Hz (note C3) the result is p_base_freq = 0.19. KMEB is right. We have a formula for converting a frequency to an SFXR base frequency value.

    Next step: Let's ignore the slight offset of 0.001 and write the square root as a power.

    (2) p_base_freq=(f/SR*100/8)^(1/2)

    Express the frequency using a MIDI note. The basics are:

    f=fN*2^((n-N)/12) with n : MIDI note N : base note for this formula (your choice) fN : frequency of fN use for example: N = 69 (note A4) fN = 440 or as KMEB did: N = 48 (note C3) fN = 130

    Insert f into (2) and simplify:

    p_base_freq=(fN*2^((n-N)/12)/SR*100/8)^(1/2) (3) p_base_freq=(2^((n-N)/24)*(fN/SR*100/8)^(1/2)

    Using KMEB's choice we get (keep in mind that n is the MIDI note number, therefore n-48, whereas KMEB sets C3 to number 0):


    Phew, it's time to say "and that's it".

  • Posts: 384

    Wow, @Codeslinger, I'm impressed you tracked down how sfxr works out that value. I had fudged it by finding the 0.5 sfxr input to be A440 and tuning a semitone increment by ear to a multiplier of 1.0296. Your formula makes more sense, so I'll merge it into my code, if that's ok.

  • Posts: 146

    @Codeslinger very nice analysis, I'm sure that will be helpful for future musical endeavours with codea :)

  • BortelsBortels Mod
    edited May 2012 Posts: 1,557

    Wonderful analysis! I knew if I sat long enough someone would be bothered enough to go reverse-engineer the whole sfxr thing! Thank you, and bravo!

    This needs to go in the wiki.

  • Posts: 384

    Hi @Codeslinger, I don't suppose we could call you back to look at sfxr's SustainTime? :) It's not in seconds, and I had to take the square root to get manageable durations, but that makes it too short...

  • Posts: 202

    I'm afraid to come up with an unsatisfying intermediate analysis.

    The core lines in SFXR are these:

    env_length[0]=(int)(p_env_attack*p_env_attack*100000.0f); env_length[1]=(int)(p_env_sustain*p_env_sustain*100000.0f); env_length[2]=(int)(p_env_decay*p_env_decay*100000.0f);

    If you set attack and decay to zero and sustain to 1, SFXR will generate 100000 samples (precisely, it will generate 100003 samples, but that doesn't matter here). At a rate of 44100 samples per seconds the duration will be a bit more than 2 seconds.

    I just compiled SFXR right out of the Codea Runtime to test it and I was right, at least on my Linux system. The formula here is:

    with d: duration in seconds p_env_sustain = sqrt(d * 0.441) where 0.441 = sample rate / 100000

    This is difficult to recreate with Codea's Example "Sounds Plus" in the advanced pane, even you "purify" the playSound function. Don't know the reason yet.

  • Posts: 384

    Oh wow, @Codeslinger, that's interesting. Thanks for looking into this. So we will have to take into account the sample size...

    On another note (no pun intended) I notice that the formula you and @KMEB worked on is a bit out of tune. I think you need that 0.001 in there.


  • Posts: 384

    Thanks @Codeslinger, it works just right. I found when using it on sounds with attack and
    decay that if I wanted the entire sound to fade away after x seconds that I should subtract the existing attack and decay values from the result of your formula. As long as the attack and delay aren't longer than x.

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