Mid/Side Encoding/Decoding

[willdatrill2007]

Is it possible to do Mid/Side Encoding or Decoding? I'm trying to figure out how to invert the phase on one audio channel.

[pschatzmann]

I always had it on my todo list to extend the CallbackStream so that we can easily implement some update methods on the data in addition to the read and write callback methods.
I just committed the related code with a corresponding test example. The update is not only working on the write but also for readBytes...

[willdatrill2007]

Ok, Great. Will check it out.

[willdatrill2007]

Will this invert the phase on both channels? If yes, how do you invert phase on only one channel?

[pschatzmann]

Yes, it's on both channels. If you only need one channel you can process only every other sample: e.g all samples with an even or with an odd index if you have a stereo signal.

for (int j = 0; j < sample_count; j++) {
  if (j % 2==0) data16[j] = -data16[j]; // invert left channel
}

[willdatrill2007]

Got it. Thank you so much.

[willdatrill2007]

I had to change the code for the declaration of the "invert" callback to this and it started to work. I figured it out as I was operating the i2s at 32bit.

size_t sample_count = bytes / sizeof(int32_t);
int32_t* data32 = (int32_t*)data;

Will be testing more.

[willdatrill2007]

auto mid = [](uint8_t* data, size_t bytes) {
size_t sample_count = bytes / sizeof(int32_t);
int32_t* out_1 = (int32_t*)data;
for (int k = 0; k < sample_count; k += 2) { // loop over every two samples
int32_t left = out_1[k]; // get the left channel sample
int32_t right = out_1[k + 1]; // get the right channel sample
int32_t mono = (left + right) / 2; // calculate the average of left and right
out_1[k] = mono; // overwrite the left channel with the mono value
out_1[k + 1] = mono; // overwrite the right channel with the mono value
}
return bytes;
};

auto side = [](uint8_t* data, size_t bytes) {
size_t sample_count = bytes / sizeof(int32_t);
int32_t* out_1 = (int32_t*)data;
for (int k = 0; k < sample_count; k += 2) { // loop over every two samples
int32_t left = -out_1[k]; // get the left channel sample
int32_t right = out_1[k + 1]; // get the right channel sample
int32_t side = (left + right) / 2; // calculate the average of left and right
out_1[k] = side; // overwrite the left channel with the side value
out_1[k + 1] = side; // overwrite the right channel with the side value
out_1[k] = -out_1[k]; // invert one channel phase
}
return bytes;
};

I made some progress with my project but when I try to use two callbacks at the same time, one labelled "mid" and the other "side". I sent each to a "VolumeStream" and assign two copiers in the loop but the audio sounds distorted, The volume controls do work. Any assistance would be greatly appreciated.

[pschatzmann]

Why would you do this? You can easily do the processing in one call back. if you want to use 2 CallbackStreams, you would need to put them in a chain!

[willdatrill2007]

Got it. I realized that I can merge the two into one this morning and it's working great so far @ 96000hz, 32bit, buffer count at 3 and buffer size at 1024. Will keep you informed as I go along. Thanks again for all the assistance.