stereo ping pong delay c++ - c++

Question

I have to create a stereo ping pong delay with these parameters.
• Delay Time (0 – 3000 milliseconds)
• Feedback (0 – 0.99)
• Wet / Dry Mix (0 – 1.0)
I have managed to implement the stereo in/out and the 3 parameters, but struggling with how to implement the ping pong. I have this code in the process block, but it only replays the left and right in the opposite channels once. Is there a simple way to loop this to reply over and over and not just once or have is this not the best way to implement ping pong. Any help would be great!
//ping pong implementation
for (int i = 0; i < buffer.getNumSamples(); i++)
{
// Reduce the amplitude of each sample in the block for the
// left and right channels
//channelDataLeft[i] = channelDataLeft[i] * 0.5;
// channelDataRight[i] = channelDataRight[i] * 0.25;
if (i % 2 == 1) //if i is odd this will play
{
// Calculate the next output sample (current input sample + delayed version)
float outputSampleLeft = (channelDataLeft[i] + (mix * delayDataLeft[readIndex]));
float outputSampleRight = (channelDataRight[i] + (mix * delayDataRight[readIndex]));
// Write the current input into the delay buffer along with the delayed sample
delayDataLeft[writeIndex] = channelDataLeft[i] + (delayDataLeft[readIndex] * feedback);
delayDataRight[writeIndex] = channelDataRight[i] + (delayDataRight[readIndex] * feedback);
// Increment read and write index, check to see if it's greater than buffer length
// if yes, wrap back around to zero
if (++readIndex >= delayBufferLength)
readIndex = 0;
if (++writeIndex >= delayBufferLength)
writeIndex = 0;
// Assign output sample computed above to the output buffer
channelDataLeft[i] = outputSampleLeft;
channelDataRight[i] = outputSampleRight;
}
else //if i is even then this will play
{
// Calculate the next output sample (current input sample + delayed version swapped around from if)
float outputSampleLeft = (channelDataLeft[i] + (mix * delayDataRight[readIndex]));
float outputSampleRight = (channelDataRight[i] + (mix * delayDataLeft[readIndex]));
// Write the current input into the delay buffer along with the delayed sample
delayDataLeft[writeIndex] = channelDataLeft[i] + (delayDataLeft[readIndex] * feedback);
delayDataRight[writeIndex] = channelDataRight[i] + (delayDataRight[readIndex] * feedback);
// Increment read and write index, check to see if it's greater than buffer length
// if yes, wrap back around to zero
if (++readIndex >= delayBufferLength)
readIndex = 0;
if (++writeIndex >= delayBufferLength)
writeIndex = 0;
// Assign output sample computed above to the output buffer
channelDataLeft[i] = outputSampleLeft;
channelDataRight[i] = outputSampleRight;
}
}

Answer 1

Not really sure why you have the modulo one and different behavior based on sample index. A ping-pong delay should have two delay buffers, one for each channel. The input of one stereo channel plus the feedback of the opposite channel's delay buffer should be be fed into each delay.
Here is a good image of the audio signal graph of it:
Here is some pseudo-code of the logic:
float wetDryMix = 0.5f;
float wetFactor = wetDryMix;
float dryFactor = 1.0f - wetDryMix;
float feedback = 0.6f;
int sampleRate = 44100;
int sampleCount = sampleRate * 10;
float[] leftInSamples = new float[sampleCount];
float[] rightInSamples = new float[sampleCount];
float[] leftOutSamples = new float[sampleCount];
float[] rightOutSamples = new float[sampleCount];
int delayBufferSize = sampleRate * 3;
float[] delayBufferLeft = new float[delayBufferSize];
float[] delayBufferRight = new float[delayBufferSize];
int delaySamples = sampleRate / 2;
int delayReadIndex = 0;
int delayWriteIndex = delaySamples;
for(int sampleIndex = 0; sampleIndex < sampleCount; sampleIndex++) {
//Read samples in from input
leftChannel = leftInSamples[sampleIndex];
rightChannel = rightInSamples[sampleIndex];
//Make sure delay ring buffer indices are within range
delayReadIndex = delayReadIndex % delayBufferSize;
delayWriteIndex = delayWriteIndex % delayBufferSize;
//Get the current output of delay ring buffer
float delayOutLeft = delayBufferLeft[delayReadIndex];
float delayOutRight = delayBufferRight[delayReadIndex];
//Calculate what is put into delay buffer. It is the current input signal plus the delay output attenuated by the feedback factor
//Notice that the right delay output is fed into the left delay and vice versa
//In this version sound from each stereo channel will ping pong back and forth
float delayInputLeft = leftChannel + delayOutRight * feedback;
float delayInputRight = rightChannel + delayOutLeft * feedback;
//Alternatively you could use a mono signal that is pushed to one delay channel along with the current feedback delay
//This will ping-pong a mixed mono signal between channels
//float delayInputLeft = leftChannel + rightChannel + delayOutRight * feedback;
//float delayInputRight = delayOutLeft * feedback;
//Push the calculated delay value into the delay ring buffers
delayBufferLeft[delayWriteIndex] = delayInputLeft;
delayBufferRight[delayWriteIndex] = delayInputRight;
//Calculate resulting output by mixing the dry input signal with the current delayed output
float outputLeft = leftChannel * dryFactor + delayOutLeft * wetFactor;
float outputRight = rightChannel * dryFactor + delayOutRight * wetFactor;
leftOutSamples[sampleIndex] = outputLeft;
rightOutSamples[sampleIndex] = outputRight;
//Increment ring buffer indices
delayReadIndex++;
delayWriteIndex++;
}