How to compute magnitude from raw pcm data?

507 views Asked by Peter At 03 December 2013 at 18:55

My Android application periodically captures raw pcm audio data from the speaker. The packet size is 128 bytes.

When there is no audio, all the bytes in the packet contain a value of -128. When there is audio, the max value for a byte is +127.

Here is my pseudo-code to compute the amplitude (between 0.0 and 1.0).

 long sum = 0;
 for(int i=0;i<wave.length;i++) {
    sum += ((int) wave[i] + 128);
 }

 float amp = ((float) sum)/((float) 256 * wave.length);

Using this algorithm, it appears the amplitude does not change much whether I speak softly or loudly. Makes me think this algorithm is not right.

I am wondering if my understanding of amplitude calculation is not correct. I would appreciate it if someone can suggest me a better algorithm.

For Android folks, I am receiving data via Visualizer.OnDataCaptureListener interface and its onWaveFormData method.

Thank you in advance for your help.

TechQA.