Audio playback, creating nested loop for fade in/out

Question

First time poster here.

A quick question about setting up a loop here. I want to set up a for loop for the first 1/3 of the main loop that will increase a value from .00001 or similar to 1. So I can use it to multiply a sample variable so as to create a fade-in in this simple audio file playback routine.

So far it's turning out to be a bit of a head scratcher, any help greatfully recieved.

for(i=0; i < end && !feof(fpin); i+=blockframes) 
{ 
    samples = fread(audioblock, sizeof(short), blocksamples, fpin); 
    frames = samples; 
    for(j=0; j < frames; j++)
    {   
        for (f = 0; f< frames/3 ;f++)
        {
            fade = fade--;
        } 
    output[j] = audioblock[j]/fade;
    }

    fwrite(output,sizeof(short), frames, fpoutput); 
} 

Apologies, So far I've read and re-write the file successfully. My problem is I'm trying to figure out a way to loop the variable 'fade' so it either increases or decreases to 1, so as I can modify the output variable.

I wanted to do this in say 3 stages: 1. From 0 to frames/3 to increace a multiplication factor from .0001 to 1 2. from frames 1/3 to frames 2/3 to do nothing (multiply by 1) and 3. For the factor to decrease again below 1 so as for the output variable to decrease back to the original point.

How can I create a loop that will increase and decrease these values over the outside loop?

Answer 1

I think you thought yourself into a corner, to say so. There's no need to create a nested loop to modify the fade gain, you can simply keep track of it through the loop and adapt it if necessary. Also, your variable naming of frames/samples seems a bit muddy to me, I hope I got that one right.

float fadeTime = 0.5f;
float outputGain = 0.0f;
float gainProgressionPerSample = 1.0f/(fadeTime*SAMPLERATE);
float fadeInStop = fadeTime*SAMPLERATE;
float fadeOutBegin;

for(i=0; i < end && !feof(fpin); i+=blockframes) 
{ 
    samples = fread(audioblock, sizeof(short), blocksamples, fpin); 
    fadeOutBegin = samples - fadeTime*SAMPLERATE;
    for(j=0; j < samples; j++)
    {
       if (j < fadeInStop)   
       {
         outputGain += gainProgressionPerSample;
       }
       // else is not used intentionally to allow fade in/out blending for short tracks
       if (j > fadeOutBegin)
       {
         outputGain -= gainProgressionPerSample;
       }
       output[j] = audioblock[j]*outputGain;
    }

    fwrite(output,sizeof(short), samples, fpoutput); 
}