Xcode iOS: how to find position of one audio file (snippet) inside another? - ios

Question

I have audio files, with different durations. They have common content and unique content. E.g. two files, 70 seconds each, last 10 seconds of the first file is the same as first two seconds of the second file. How can I find the exact position of common content (e.g. 60.0 of the first file)?
Sounds a little bit messy, hope the following image can help https://drive.google.com/file/d/0BzBE2Kfw8uQoUWNTN1RXOEtLVEk/view?usp=sharing
So, I'm looking for the red mark - common content starts at 60.0 sec of the first file.
The problem is that I have files with different durations. Sometimes it's 70 seconds long, sometimes one file is 70 seconds, the other is 80 seconds long, etc. Most likely they have 60.0 seconds of unique content, but I'm not sure (it could be 59.9 of unique content, etc.).
Thus, I assume I need to get a short snippet of the second file from first 10 seconds and find it in the first file:
For example, output: 2.5 sec of the second file = 62.5 from the first file - works for me, as well.
THE MAIN GOAL IS TO PLAY FILE AFTER FILE GAPLESS. If I get the values, I'll be able to do this. Sometimes the values can be: 2.5 = 63.7, that's why I need the exact match.
Can anybody help with the code or at least some information of how to compare two snippets of audio content? Thanks in advance!

Answer 1

Wow, that is quite a problem to solve. And I must confess that i've not done anything exactly like this or have any code based suggestions.
All I will say is that if I were looking to try and solve this problem, then I would try and save the audio file as some kind of uncompressed and fixed size (as in a known number of bytes per second) format.
Then you could take a section of one file and byte match it with another, then you would know how many bytes inwards that snippet occurred. Then, knowing the bytes per ms (sort of frame size), you could work out the exact time position.
It's a bit hair brained, but i've used that technique with images before but at least audio is linear!
Here is an approximate example of how I would go about doing the comparison of a sample within a sound file.
- (int)positionOf:(NSData*)sample inData:(NSData*)soundfile {
// the block size has to be big enough to find something genuinely unique but small enough to ensure it is still fast.
int blockSize = 128;
int position = 0;
int returnPosition = INT32_MAX;
// check to see if the block size exceeds the sample or data file size
if (soundfile.length < blockSize || sample.length < blockSize) {
return returnPosition;
}
// create a byte array of the sample, ready to use to compare with the shifting buffer
char* sampleByteArray = malloc(sample.length);
memcpy(sampleByteArray, sample.bytes, sample.length);
// now loop through the sound file, shifting the window along.
while (position < (soundfile.length - blockSize)) {
char* window = malloc(blockSize);
memcpy(window, soundfile.bytes + position, blockSize);
// check to see if this is a match
if(!memcmp(sampleByteArray, window, blockSize)) {
// these are the same, now to check if the whole sample is the same
if ((position + sample.length) > soundfile.length) {
// the sample won't fit in the remaining soundfile, so it can't be this!
free(window);
break;
}
if(!memcmp(sampleByteArray, soundfile.bytes + position, sample.length)) {
// this is an entire match, position marks the start in bytes of the sample.
free(window);
returnPosition = position;
break;
}
}
free(window);
position++;
}
free(sampleByteArray);
return returnPosition;
}
It compiles, didn't have time to setup the scenario to check your exact case, but i'm quite confident this may help.