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POHow to prevent iPhone 3GS from filtering low frequencies ( < 150Hz )
 

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  1. POHow to prevent iPhone 3GS from filtering low frequencies ( < 150Hz )
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    <p>I'm developing a bass guitar pitch detection app on iphone 3GS. I found I can't get sound data lower than 150Hz with RemoteIO. However bass guitar may generate tones lower than 50hz. According to the report "iPhone 4 Headset Input Frequency Response", <a href="http://blog.faberacoustical.com/2010/iphone/iphone-4-audio-and-frequency-response-limitations/" rel="nofollow noreferrer">http://blog.faberacoustical.com/2010/iphone/iphone-4-audio-and-frequency-response-limitations/</a> There is a sharp drop-off below 150 Hz.</p> <p>Here shows how I setup the AudioUnit.</p> <pre><code>// set audio unit { // create AudioUnit { AudioComponentDescription desc; desc.componentType = kAudioUnitType_Output; desc.componentSubType = kAudioUnitSubType_RemoteIO; desc.componentManufacturer = kAudioUnitManufacturer_Apple; desc.componentFlags = 0; desc.componentFlagsMask = 0; AudioComponent comp = AudioComponentFindNext(NULL, &amp;desc); OSAssert(AudioComponentInstanceNew(comp, &amp;m_AudioUnit)); } //enable input on the remote I/O unit (output is default enabled, but input is not) { UInt32 one = 1; OSAssert(AudioUnitSetProperty(m_AudioUnit, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Input, 1, &amp;one, sizeof(one))); } //set render callback function { AURenderCallbackStruct callbackInfo; callbackInfo.inputProc=staticPerformThru; callbackInfo.inputProcRefCon=this; OSAssert(AudioUnitSetProperty(m_AudioUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Input, 0, &amp;callbackInfo, sizeof(callbackInfo))); } //set in/output format { CAStreamBasicDescription outFormat; outFormat.SetAUCanonical(channels, false); outFormat.mSampleRate = sampleRate; OSAssert(AudioUnitSetProperty(m_AudioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &amp;outFormat, sizeof(outFormat))); OSAssert(AudioUnitSetProperty(m_AudioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, 1, &amp;outFormat, sizeof(outFormat))); } //Initialize remote I/O unit OSStatus r=AudioUnitInitialize(m_AudioUnit); OSAssert(r); } //start audio output OSAssert(AudioOutputUnitStart(m_AudioUnit)); </code></pre> <p>This is the callback function.</p> <pre><code>OSStatus AudioThruWorker::staticPerformThru( void *inRefCon, AudioUnitRenderActionFlags *ioActionFlags, const AudioTimeStamp *inTimeStamp, UInt32 inBusNumber, UInt32 inNumberFrames, AudioBufferList *ioData) { AudioUnitRender(((AudioThruWorker*)inRefCon)-&gt;m_AudioUnit, ioActionFlags, inTimeStamp, 1, inNumberFrames, ioData); //Detect pitch here... return 0; } </code></pre> <p>To identify the root cause,</p> <ol> <li>I modified my callback function to just bypass input data to output.</li> <li>Use Mac to generate white noise</li> <li>Use iRig to redirect the signal from Mac's headset to iPhone3Gs which is running my program.</li> <li>Use iRig to redirect output of iPhone back to Mac.</li> <li>Record data on Mac. <img src="https://i.stack.imgur.com/PU3Ej.png" alt="enter image description here"> The output data spectrum shows in below figure. <img src="https://i.stack.imgur.com/2icS3.png" alt="enter image description here"></li> </ol> <p>You can see the sharp drop at 150Hz. </p> <p>To identify whether the problem is at input side or output side, I changed the callback function to ignore input data and output a white noise. Here is the result. <img src="https://i.stack.imgur.com/NtcxR.png" alt="enter image description here"> Very clear that there is NO drop at 150 Hz. Therefore the problem should at the input side.</p> <p>I thought this is a hardware limitation. HOWEVER I tried the app "Amplitube" on the same device, turn off all effects ,input white noise and analyze output. It has NO drop at 150Hz. Here is the result.<img src="https://i.stack.imgur.com/ZXCmI.png" alt="Amplitube frequency response on iPhone 3gs"> This means the drop-off problem is NOT a hardware limitation. There must be some way software can do to avoid the problem.</p> <p>Does anyone know the secret?</p> <p>Thanks.</p>
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    1. COdid you found the problem after all? I'm curious what happened.
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      1. USValentin Radu
    2. COSorry not yet. I guess it is caused by iOS 4.x limitation. I know iOS 5 has a "measurement mode". However I haven't tested it.
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