Help with Additive Synthesis

[Wiebo]

Hi, i've been trying to combine a number of sine waves together for the past few days without any luck. No processes i've found on the internet seem to work for me.

I have code which creates different sine waves at different frequencies, but it always only plays the final frequency, so any pointers on what I should do with this code would be appreciated.

for(int i = 0; i < file.getNumSamples(); i++)
        {
            wave = sin(PIX2 * i * (f/SR));
            buffer[a] = wave * gain;
            output = (buffer[a] + output)/2;
 
            file.setSample(output, i);
        }

[RedSon]

From what I know about signals and modulation from my amateur radio books when your sine waves have an additive effect on each other all you will have at the end is one complete wave form that modulates at (what seems like) random intervals but still retains the same amplitude. Were you expecting something different?

[Wiebo]

From what I know about signals and modulation from my amateur radio books when your sine waves have an additive effect on each other all you will have at the end is one complete wave form that modulates at (what seems like) random intervals but still retains the same amplitude. Were you expecting something different?

I did expect that to happen but i'm not getting that, all I would get at the end is a sine wave at say 320Hz.

The problem is that the sine waves don't seem to combine

[JosAH]

for(int i = 0; i < file.getNumSamples(); i++)
        {
            wave = sin(PIX2 * i * (f/SR));
            buffer[a] = wave * gain;
            output = (buffer[a] + output)/2;
 
            file.setSample(output, i);
        }

I don't understand this piece of code: in the body of your loop you only calculate
several samples of one sine, i.e. care to elaborate on what 'a' is and what that
buffer is supposed to do there?

kind regards,

Jos

[Wiebo]

I don't understand this piece of code: in the body of your loop you only calculate
several samples of one sine, i.e. care to elaborate on what 'a' is and what that
buffer is supposed to do there?

kind regards,

Jos

'a' refers to an integer, which is declared previously as part of another while loop to allow the program to run through the above piece of code for each instance of a sine wave. The buffer is merely there to hold the samples before it is then added to 'output'.

Hope that helps

[JosAH]

'a' refers to an integer, which is declared previously as part of another while loop to allow the program to run through the above piece of code for each instance of a sine wave. The buffer is merely there to hold the samples before it is then added to 'output'.

Hope that helps

'buffer[a]' is completely invariant to that loop, i.e. 'a' doesn't change in that
loop so you could've used a single variable 'buffer' (or whatever) for the purpose.

Also the entire loop body only depends on 'i' as I can see it; I think you have
to supply that outer loop too; from just this snippet I can't tell you any more.

kind regards,

Jos

[Wiebo]

'buffer[a]' is completely invariant to that loop, i.e. 'a' doesn't change in that
loop so you could've used a single variable 'buffer' (or whatever) for the purpose.

Also the entire loop body only depends on 'i' as I can see it; I think you have
to supply that outer loop too; from just this snippet I can't tell you any more.

kind regards,

Jos

Heres the whole part of the code, including both loops.

int main()
{
    //Create ne wav file
    CWavFile file;
    file.newWav(SR, 16, 1000, 1);
    float f = 10;        //frequency
    float PIX2 = PI*2;
    float wave;
    float gain;
    int g = 0;
    float buffer[34];    //no. voices
 
for(int a = 0; a < 34; a++)
    {
        gain = 1;
 
        for(int i = 0; i < file.getNumSamples(); i++)
        {
            wave = sin(PIX2 * i * (f/SR));
            buffer[a] = wave * gain;
            output = (buffer[a] + output)/2;
 
            file.setSample(output, i);
        }
 
        g++;
        gain = gain/g;
        f = f + 10;
}

I hope that you can help

[JosAH]

            file.setSample(output, i);

Doesn't that line simply set sample # i with the value of 'output'? If so, the
behaviour can be exaplained: you set thousand values 34 times. The last
thousand values (samples) end up in the file if 'setSample' is defined the
way I just guessed.

Maybe you should define 34*1000 = 34000 samples and write sample
numbers 1000*a+i.

kind regards,

Jos

[Wiebo]

Doesn't that line simply set sample # i with the value of 'output'? If so, the
behaviour can be exaplained: you set thousand values 34 times. The last
thousand values (samples) end up in the file if 'setSample' is defined the
way I just guessed.

That is correct, although I thought that having the following line would keep adding the current value of the sine wave 'a' into output and it would build up until all 34 waves are together as one sound.

output = (buffer[a] + output)/2;

Maybe you should define 34*1000 = 34000 samples and write sample
numbers 1000*a+i.

kind regards,

Jos

This part I don't quite understand, the number of samples is 44100 a second, and at the moment it is a 10 second sine wave, and this can change.

[JosAH]

That is correct, although I thought that having the following line would keep adding the current value of the sine wave 'a' into output and it would build up until all 34 waves are together as one sound.

output = (buffer[a] + output)/2;

No it doesn't work that way because the line above the line you just showed
you assigned the variable 'buffer[a]' a new value. More important is that you
haven't initialized the variable 'output' as far as I can see.

kind regards,

Jos