How to subtract one audio wave from another?

Question

How to subtract one audio wave from another? In general and in C# (or if we cannot do it in C# in C/C++)

I have sound wave A and sound wave B (BTW: they are in PCM) I want to subtract B from A

What do I need? Open Source Libs (NOT GPL, but LGPL will be ok) Tutorials on how to do such operation (with or without using libs) Articles on this topic

PS: it’s all about AEC…

Answer 1

If the samples are normalised to the same level, and are stored in a signed format such that the "zero level" is 0 or 0.0, the answer is fairly simple:

S_C = (S_A / 2) - (S_B / 2);

for each sample S_A and S_B in A and B.

If you are using unsigned values for the samples then you will need to do more work: first, you need to convert them to a signed value with a zero centre (eg, if you have 16 bit unsigned samples, subtract 32768 from each), then apply the formula, then convert them back to the unsigned format. Be careful of overflow - here's an example of how to do the conversions for the aforementioned 16 bit samples:

#define PCM_16U_ZERO 32768

short pcm_16u_to_16s(unsigned short u)
{
    /* Ensure that we never overflow a signed integer value */
    return (u < PCM_16U_ZERO) ? (short)u - PCM_16U_ZERO : (short)(u - PCM_16U_ZERO);
}

unsigned short pcm_16s_to_16u(short s)
{
    /* As long as we convert to unsigned before the addition, unsigned arithmetic
       does the right thing */
    return (unsigned short)s + PCM_16U_ZERO;
}

Answer 2

https://stackoverflow.com/questions/1723563/acoustic-echo-cancellation-aec-in-wpf-with-c Asks a similar question and has an accepted answer. The suggested library does have some sort of Echo Cancellation I think.

Unfortunately I haven't used any open source audio libraries. I have used Fmod to process audio before, I don't remember there being any AEC in it, but you can get access to the raw audio as it processes it and run your own code on it.