PADsynth algorithm

by Nasca Octavian Paul

1) Introduction
2) Sound examples of instruments that use this algorithm (using ZynAddSubFX)
3) Description of the algorithm
    3.1) General descripton (diagrams,etc..)
    3.2) Steps of the algorithm (with pseudocode)
    3.3) Public domain C/C++ code that shows some simple implementations
    3.4) Public domain C++ class that implements the algorithm (ready to use)
4) Tips and suggestions
5) Conclusions

1) Introduction

    This algorithm generates very beautiful sounds, even if its idea is much simpler than other algorithms. It generates a perfectly looped wavetable sample which can be used in instruments. It easily generates sounds of ensembles, choirs, metalic sounds (bells) and many other types of sound.
    I want to make this algorithm known and you are welcome to learn and use this algorithm into your projects or products (non-commercial or commercial). You will not be dissapoined by this algorithm. This page includes some public domain C/C++ sources that can be used in your projects/products.
    This algorithm is implemented in ZynAddSubFX in the PADsynth module and you can download it to hear yourself how beautiful sounds it can generate. Before reading this document, I recomand to listen to all sound examples in the next category. It will give you an idea of what kind of sounds it produces.

2) Sound examples of instruments that use this algorithm

These sound examples are generated by ZynAddSubFX. All instruments's wavetables are generated by this algorithm.
These examples are grouped into 2 categories:
2.1) With FX. In this category, some effects are used. This effects can be reverberation, phaser, etc. Mostly, the only effect was the reverberation.

Bells, Strings and Choir (you must listen this one)
Synth Piano
Saw Synth Piano and Choir
Phased Choir

2.2) Without FX. All instruments in this category the instruments are "dry". No reverberation, no other effects.

Bells and Strings (you must listen this one, too)
Organ and Choir
Saw Piano (this example contains the same notes played many times, but with different bandwidth and harmonic profiles)
Soft Pad

Parameter files of above examples (for ZynAddSubFX).

3) Description of the algorithm

        In order to understand how this algorithm works, you need to be familiar with how I think about the musical instruments. Please read an introduction for the description of the meaning and the importance of Bandwidth of each harmonic and Randomness.

    3.1) General description
   This algorithm generates some large wavetables that can played at diferent speeds to get the desired sound. This algorithm describes only how these wavetables are generated. The result is a perfectly looped wavetable. Unlike other synthesis methods which use Inverse Fast Fourier Transform, this one does not use overlap/add methods and there is only one IFFT for the whole sample.

   The basic steps are:

Make a very large array that represents the amplitude spectrum of the sound (default all values are zero)
Generates the distribution of each harmonic in frequency and add it to the array
Put random phases to each frequency of the spectrum
Do a single Inverse Fourier Transform of the whole spectrum. Here is no need of any overlapping windows, because there is only one single IFFT for the whole sample.

The ouput is the sample which can be used as a wavetable. In the next image, the steps are represented graphically:

            3.1.1) The bandwidth of each harmonic
        I consider one harmonic(overtone) as being composed of many frequencies. These sine components of one harmonic are spread over a certain band of frequencies.
        Higher harmonics has bigger bandwidth. In natural choirs/ensembles the bandwidth is proportional to the harmonic's frequency.
        Here is an example of a spectrum of an instrument generated by ZynAddSubFX:


full spectrum	closeup of the spectrum

        The harmonics becomes wider and wider, until at a certain frequency, where they may merge to a noise band (as in the full spectrum image from above). This is a normal thing and I recomand to not avoid this by limiting the bandwidth of the harmonics.

        3.1.2) The frequency distribution of one harmonic/overtone (or the harmonic profile)
      This describes the function of the spread of the harmonic.
        Here are some examples of how they can be spread:

a) A special case is where there is only a single sine component inside the hamonic. In this case, the harmonic and the "sine component" are the same thing.

Audio example: single harmonic, many harmonics

b) Detuned. In this case there are two sine components which are detuned.

Audio example: single harmonic, many harmonics

c) Evenly spread inside the harmonic (all components has the same amplitude)

       Audio example: single harmonic, many harmonics

       d) Normal (Gaussian) distribution. The sine components amplitude are bell shaped. The largest amplitude is in the center of the band. This distribution gives the most natural sounds (it simulates a very, very large enssemble).

Audio example: single harmonic, many harmonics

        Of course, you can use many others harmonic's profiles. ZynAddSubFX's PADsynth module offers many ways to generate the harmonic profile.
        Also, it's very important that the harmonic must have the same amplitude, regardless of the profile functions/parameters and the bandwidth.

   3.1.3) The phases of the sine components of the harmonics
        This algorithm considers the phases of the sine components of each harmonics as random.

3.2) Steps, input and output of the algorithm (with pseudocode)
   3.2.1) Steps of the basic algorithm
   Input:
        N - wavetable size. It's recomanded to be a power of 2. This is, usually, a big number (like 262144)
        samplerate - the samplerate (eg. 44100)
        f - frequency of the the fundamental note (eg. 440)
        bw - bandwidth of first harmonic in cents (eg. 50 cents); must be greater than zero
        number_harmonics - the number of harmonics. Of course, number_harmonics<(samplerate/f)
        A[1..number_harmonics] - amplitude of the harmonics
   Output:
        smp[0..N-1] - the generated wavetable

   Internal variables:
        freq_amp[0..N/2-1] = {0,0,0,0,...,0}
        freq_phase[0..N/2-1], etc...

   Functions:
        RND() returns a random value between 0 and 1
        IFFT() is the inverse fourier transform
        normalize_sample() normalizes samples

        profile(fi,bwi){
            x=fi/bwi;
            return exp(-x*x)/bwi;
        };

   Steps:
        FOR nh = 1 to number_harmonics
           bw_Hz=(pow(2,bw/1200)-1.0)*f*nh;
       bwi=bw_Hz/(2.0*samplerate);
            fi=f*nh/samplerate;
            FOR i=0 to N/2-1
                hprofile=profile((i/N)-fi,bwi);
                freq_amp[i]=freq_amp[i]+hprofile*A[nh];
            ENDFOR
        ENDFOR

        FOR i=0 to N/2-1
            freq_phase[i]=RND()*2*PI;
        ENDFOR

        smp=IFFT(N,freq_amp,freq_phase);
        normalize_sample(N,smp);

        OUTPUT smp

   3.2.2) The extended algorithm
        The differences between the extended algorithm and the basic algorithm are minor:
       There is an additional parameter:
            bwscale: that specify how much the bandwidth of the harmonic increase according to it's frequency. Also, there is defined a function called relF(N) who returns the relative frequency of the N'th overtone. It allows to generate detuned harmonics or even metallic sounds (like bells).
       The difference between the basic algorithm is at the computation of bw_Hz and fi:
           bw_Hz=(pow(2.0,bw/1200.0)-1.0)*f*pow(relF(nh),bwscale);
            fi=f*relF(nh)/samplerate;
       If the relF(N) function returns N and the bwscale is equal to 1, this algorithm will be equivalent to the basic algorithm.

   3.2.3) Example Graph of freq_amp array:
        Graphs of the (basic algorithm) freq_amp array for N=262144, f=500 Hz, bw=100 cents, samplerate=44.1 Khz, and A[] where A[n]=1.0/sqrt(n)


Graph of the full array	Closeup of the graph of the array

   3.2.4) Example of the output of this algorithm (c_basic implementation)
               c_basic_sample.ogg - this is the resulting smp array converted to ogg vorbis.

3.3) Public domain C/C++ code that shows a simple implementation
   I wrote some C/C++ implementations of the basic algorithm and the extended algorithm.
   The "c_basic" directory contains the basic algorithm, "c_extended" contains the extended algorithm and the "c_simple_choir" is the implementation of the basic algorithm to make a simple choir.
   These implementations are wrote to be easy to understood and they are not optimised for speed. You can test on Linux by running the ./compile.sh scripts. It's recomanded to have snd installed, to make possible to hear the results as wav file. Of course, you can import the results into your instruments, because the waves are perfectly looped (set the first loop point to 0 and the second to the end of the wav).
   I put the source code under public domain, but it depends on FFTW3 library, so, if you want to use into your products, you must use your IFFT routines to avoid licensing issues of the FFTW library.

    3.4) Public domain C++ class that implements the algorithm (ready to use)
   To be easy to use this algorithm into your projects or products, I made a ready-to-use C++ class. Only thing that you have to do, is to provide it an IFFT routine. Please read the header file for details.

Download C/C++ code from here

4) Tips and suggestions

Keep in mind that the resulting wavetables are perfectly looped
When using the wavetables into instruments, on each NoteOn, start from a random position and not from the start. This avoids hearing the same sound on each keystroke
You can use the same wavetable for generating stereo sounds, by playing the same wavetable at different positions for left and right. The best is to make the difference between left right as N/2
Generate different wavetables for different pitches and use the one who is closest to the desired pitch
Upsample or downsample the harmonics's amplitude array before running the algorithm, according to the fundamental frequency. In this case we need to set a parameter "base_frequency" which represents the frequency where the array is left unchanged.
Example: we have A_orig[]={1,2,1,3,0,0,1,0} and base_frequency is equal to 440 Hz
Here are some cases:
- A[] for 440 Hz: is the same as A_orig[]
- A[] for 220 Hz: is the A_orig[] upsampled by factor of 2
  so: A[]={1, 1, 1.5, 2, 1.5, 1, 2, 3, 1.5, 0, 0, 0, 0.5, 1, 0.5, 0}
  (the original A_orig amplitudes are shown as bold)
- A[] for 880 Hz: the A_orig[] is downsampled by a factor of 2
  so: A[]={1.5, 2, 0, 0.5}
- A[] for F Hz: the A_orig[] is scaled by a factor of 440/F.
Even if this ideea is very simple, the resulting sounds are very natural, because it keeps the spectrum constant according to the harmonic's frequency and not to harmonic's number. This folows the point 4 from the document where I described some principles regarding synthesis.

5) Conclusions
I hope that this algorithm will be implemented in many software/hardware synthesizers. Use it, spread it, write about it, make beautiful instruments with it. If your synthesizer uses plenty of samples, you can use this algorithm to generate many ready-to-use samples.

This algorithm, this page, the images, the implementations from this page, the audio examples, the parameter files from this page
are released under Public Domain by Nasca Octavian Paul.
e-mail: zynaddsubfx AT yahoo DOT com