how to add echo effect on audio file using objective-c - iphone

Question

I am developing an application in which I want to add echo effect in recorded audio files using objective-c.
I am using DIRAC to add other effect e.g. man to women, slow, fast.
now I have to make Robot voice of recorded voice. for robot voice I need to add echo effect
Please help me to do this

Answer 1

Echo is pretty simple. You need a delay line, and little multiplication. Assuming one channel and audio already represented in floating point, a delay line would look something like this (in C-like pseudo-code):
int LENGTH = samplerate * seconds; //seconds is the desired length of the delay in seconds
float buffer[ LENGTH ];
int readIndex = 0, writeIndex = LENGTH - 1;
float delayLine.readNext( float x ) {
float ret = buffer[readIndex];
++readIndex;
if( readIndex >= LENGTH )
readIndex = 0;
return ret;
}
void delayLine.writeNext( float x ) {
buffer[ writeIndex ] = x;
++writeIndex;
if( writeIndex >= LENGTH )
writeIndex = 0;
}
Don't forget to initialize the buffer to all zeros.
So that's your delay line. Basic usage would be this:
float singleDelay( float x ) {
delayLine.writeNext(x);
return delayLine.readNext( x );
}
But you won't hear much difference: it'll just come out later. If you want to hear a single echo, you'll need something like this:
float singleEcho( float x, float g ) {
delayLine.writeNext(x);
return x + g * delayLine.readNext( x );
}
where g is some constant, usually between zero and one.
Now say you want a stream of echos: "HELLO... Hello... hello... h..." like that. You just need to do a bit more work:
float echo( float x, float g ) {
float ret = x + g * delayLine.readNext( x );
delayLine.writeNext( ret );
return ret;
}
Notice how the output of the whole thing is getting fed back into the delay line this time, rather than the input. In this case, it's very important that |g| < 1.
You may run into issues of denormals here. I can't recall if that's an issue on iOS, but I don't think so.