[Qt-interest] "Decoding" PCM samples from QAudioInput

[Josiah Bryan]

kurt.korbatits at nokia.com wrote:
> The easy answer here is to have a look at how it is done in the 
> audioinput example.
>  
> It uses signed 16bit samples
> Finds the highest value in the samples ignore sign, thats the max.
>  
> Dont forget to check for clipping.

Thanks, Kurt! That helps a lot with the basics - though I am having 
trouble understanding the example in some areas. Specifically, why is 
the FFT needed on the samples? I thought (from other reading) that FFT 
would be used to extract the frequencies present in the sample. Whereas 
this example seems to be just giving a max volume across the entire 
sample, regardless of frequency - so the presence of the FFT is 
confusing to this simple brain. Any hints? Or even better, a nice 
commented walkthrough of the audioinput example?

Thanks again!

Cheers!
-josiah


> 
> Kurt Korbatits
> Software Engineer
> Nokia - Qt Development Frameworks
> 
>  
> 
> ------------------------------------------------------------------------
>  
> 
> Subject: 	[Qt-interest] "Decoding" PCM samples from QAudioInput
> Date: 	Tue, 6 Oct 2009 14:24:38 +0200
> From: 	ext Josiah Bryan <jbryan at productiveconcepts.com>
> To: 	qt-interest at trolltech.com <qt-interest at trolltech.com>
> 
> 
> 
> Hi All -
> 
> I've got this idea to listen to incoming audio and analyze it for beats 
> inorder to add some "life" to my onscreen animation. I envision a class 
> where I can just connect to a signal like "beatDetected(float strength)" 
> and let the beat algorithm and audio code sit on one side of the wall, 
> and the animation on the other. All well and good.
> 
> So, to start out, I've taken a look at QAudioInput and friends from Qt 
> 4.6. Compiles great, runs fine. I can only get the "audio/pcm" codec to 
> work on windows. According to this block, it's all that my computer 
> supports anyway (or at least, Qt supports):
> 
> 	QList<QAudioDeviceId> list = 
> QAudioDeviceInfo::deviceList(QAudio::AudioInput);
> 	foreach(QAudioDeviceId devid, list)
> 	{
> 		QAudioDeviceInfo deviceInfo(devid);
> 		qDebug() << "Device name: " << deviceInfo.deviceName();
> 		QStringList codecs = deviceInfo.supportedCodecs();
> 		qDebug() << "       Supported Codecs: "<<codecs;
> 	}
> 
> The part I'm having trouble with, and that I'd like to get advice on is 
> this:
> 
> How do I get at the individual sample values in the output buffer?
> 
> Here's what I've tried:
> 
> I've set up my format like this:
> 
> 	format.setFrequency(8000);
> 	format.setChannels(1);
> 	format.setSampleSize(8);
> 
> 	format.setCodec("audio/pcm");
> 	format.setByteOrder(QAudioFormat::LittleEndian);
> 	format.setSampleType(QAudioFormat::UnSignedInt);
> 
> Then I use a QBuffer to wrap a QByteArray, and pass the QBuffer to the 
> QAudioInput::start() method, then set a timer to check the QBuffer every 
> X ms (right now, 50ms).
> 
> Here's where I get stuck:
> 
> With this code, I was just trying to print out an average of the samples 
> in the buffer. I thought since this was one channel and (it seems) 8 
> bits to a sample, then the below code should work to average it out. The 
> sample represents a "volume" of sort ("energy" I guess), so the below 
> code, you could say, should print the avg volume of each 50ms sample.
> 
> 	int sz = byteArray.size();
> 	int sum = 0;
> 	char * data = byteArray.data();
> 	for(int i=0;i<sz;i++)
> 	    sum += (int)data[i];
> 	int avg = sz == 0 ? 0 : sum / sz;
> 	byteArray.clear();
> 	buffer.seek(0);
> 
> 	qDebug() << "SampleIO: bytes: "<<sz<<", avg:"<<avg;
> 
> Now, the more "low level" I get, the worse off my working knowledge gets 
> - so I'm sure I'm making some wrong assumptions and or just plain stupid 
> mistakes up there - but where? The code compiles and runs fine - just 
> the numbers jump positive to negative and back - and they don't seem to 
> respond to extreme environmental sound changes (e.g. a clap or what not.)
> 
> Thanks for your time. Cheers!
> -josiah
> 
> -- 
> 
> -=-=-=-=-=-=-=-=-=-=-=-=-
> Josiah Bryan
> Productive Concepts, Inc.
> jbryan at pciint.com
> (765) 964-6009, ext. 224
> 
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-- 

-=-=-=-=-=-=-=-=-=-=-=-=-
Josiah Bryan
Productive Concepts, Inc.
jbryan at pciint.com
(765) 964-6009, ext. 224