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POLow Pass Audio Filter, calculating RMS on iOS
 

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    <p>I am working on an App for the iPad, and I want to analyze the Audio from a Video that I am playing. This is all going well, using the MTAudioProcessingTap. Currently I have some test code to test/measure the volume of the left and right channel. This is all going pretty well:</p> <pre><code>void process(MTAudioProcessingTapRef tap, CMItemCount numberFrames, MTAudioProcessingTapFlags flags, AudioBufferList *bufferListInOut, CMItemCount *numberFramesOut, MTAudioProcessingTapFlags *flagsOut) { OSStatus err = MTAudioProcessingTapGetSourceAudio(tap, numberFrames, bufferListInOut, flagsOut, NULL, numberFramesOut); if (err) NSLog(@"Error from GetSourceAudio: %ld", err); float leftVolume, rightVolume; for (CMItemCount i = 0; i &lt; bufferListInOut-&gt;mNumberBuffers; i++) { AudioBuffer *pBuffer = &amp;bufferListInOut-&gt;mBuffers[i]; int cSamples = numberFrames * pBuffer-&gt;mNumberChannels; float *pData = (float *)pBuffer-&gt;mData; float rms = 0.0f; for (int j = 0; j &lt; cSamples; j++) { rms += pData[j] * pData[j]; } if (cSamples &gt; 0) { rms = sqrtf(rms / cSamples); } if (0 == i) { leftVolume = rms; } if (1 == i || (0 == i &amp;&amp; 1 == bufferListInOut-&gt;mNumberBuffers)) { rightVolume = rms; } } NSLog(@"Left / Right Volume: %f / %f", leftVolume, rightVolume); } </code></pre> <p>But for the purpose of this App, I want it to just measure the RMS ("intensity") of the range of 0-80Hz (as example). Therefore, I need a Low-Pass filter.</p> <p>I have been Googling for a long time now, but my problem is that I can't find any clear post, tutorial or solution that is obvious. Almost every problem that sounds somewhat like mine has a random piece of code underneath it with crappy or lack of comments so I can't figure out what all the magic numbers are doing there, and what they mean.. </p> <p>Could someone push me in the right direction here? Note that in my case I <strong>do</strong> want to understand the code and not just run off with a working sample.</p> <p>Thanks</p>
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