extAudioFile data... am I getting the right stuff?

I am using the EXTAudioFileReadTest app provided in the Core Audio SDK documentation and I'm trying to get all of the floating point values from the mData buffer so that I can draw a waveform with them.

Currently, I'm printing out the floating point values for single channel and this is what I'm getting in the console:

2010-09-10 19:22:43.360 ExtAudioFileReadTest[71828:a0f] 0.127136

2010-09-10 19:22:43.360 ExtAudioFileReadTest[71828:a0f] -0.057033

2010-09-10 19:22:43.360 ExtAudioFileReadTest[71828:a0f] -0.146455

2010-09-10 19:22:43.360 ExtAudioFileReadTest[71828:a0f] 0.090759

2010-09-10 19:22:43.360 ExtAudioFileReadTest[71828:a0f] 0.240837

2010-09-10 19:22:43.360 ExtAudioFileReadTest[71828:a0f] -0.072719

2010-09-10 19:22:43.361 ExtAudioFileReadTest[71828:a0f] -0.258782

2010-09-10 19:22:43.361 ExtAudioFileReadTest[71828:a0f] -0.063972

2010-09-10 19:22:43.361 ExtAudioFileReadTest[71828:a0f] 0.088692

2010-09-10 19:22:43.361 ExtAudioFileReadTest[71828:a0f] 0.153571

2010-09-10 19:22:43.361 ExtAudioFileReadTest[71828:a0f] 0.080644

2010-09-10 19:22:43.383 ExtAudioFileReadTest[71828:a0f] -0.087060

2010-09-10 19:22:43.383 ExtAudioFileReadTest[71828:a0f] 0.196455

2010-09-10 19:22:43.383 ExtAudioFileReadTest[71828:a0f] 0.167777

2010-09-10 19:22:43.383 ExtAudioFileReadTest[71828:a0f] -0.192430

2010-09-10 19:22:43.383 ExtAudioFileReadTest[71828:a0f] -0.209936

2010-09-10 19:22:43.383 ExtAudioFileReadTest[71828:a0f] 0.012049

2010-09-10 19:22:43.383 ExtAudioFileReadTest[71828:a0f] 0.110493

2010-09-10 19:22:43.384 ExtAudioFileReadTest[71828:a0f] 0.150715

2010-09-10 19:22:43.384 ExtAudioFileReadTest[71828:a0f] 0.016413

2010-09-10 19:22:43.384 ExtAudioFileReadTest[71828:a0f] -0.056843

2010-09-10 19:22:43.384 ExtAudioFileReadTest[71828:a0f] 0.206117

2010-09-10 19:22:43.384 ExtAudioFileReadTest[71828:a0f] 0.020673

2010-09-10 19:22:43.384 ExtAudioFileReadTest[71828:a0f] -0.561129

2010-09-10 19:22:43.384 ExtAudioFileReadTest[71828:a0f] -0.184265

2010-09-10 19:22:43.384 ExtAudioFileReadTest[71828:a0f] 0.620910

2010-09-10 19:22:43.385 ExtAudioFileReadTest[开发者_如何学Python71828:a0f] 0.309018

2010-09-10 19:22:43.385 ExtAudioFileReadTest[71828:a0f] -0.371634

2010-09-10 19:22:43.385 ExtAudioFileReadTest[71828:a0f] -0.238362

2010-09-10 19:22:43.385 ExtAudioFileReadTest[71828:a0f] 0.125136

2010-09-10 19:22:43.385 ExtAudioFileReadTest[71828:a0f] 0.139757

2010-09-10 19:22:43.385 ExtAudioFileReadTest[71828:a0f] -0.023419

2010-09-10 19:22:43.385 ExtAudioFileReadTest[71828:a0f] -0.142903

2010-09-10 19:22:43.385 ExtAudioFileReadTest[71828:a0f] 0.041068

2010-09-10 19:22:43.386 ExtAudioFileReadTest[71828:a0f] 0.252621

2010-09-10 19:22:43.386 ExtAudioFileReadTest[71828:a0f] -0.002240

2010-09-10 19:22:43.386 ExtAudioFileReadTest[71828:a0f] -0.261686

2010-09-10 19:22:43.386 ExtAudioFileReadTest[71828:a0f] -0.105053

2010-09-10 19:22:43.386 ExtAudioFileReadTest[71828:a0f] 0.072798

2010-09-10 19:22:43.386 ExtAudioFileReadTest[71828:a0f] 0.141572

2010-09-10 19:22:43.386 ExtAudioFileReadTest[71828:a0f] 0.110190

I guess i'm confused.. .as I was expecting to find some type of voltage sample and not a number between -1 and 1. What do these values actually mean? What would be a good formula for converting these values to some upper/lower limit that would be between 0 and 1?

Thanks in advance. I've been searching all over the place and can't find this information anywhere... especially not in the documentation.

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this is normal.

the common floating point representation of audio samples modulate from [-1...1]; where the values -1 and 1 represent one sample at 0 dBFS.

a continuous stream of floating point values '0.0' represents a silent signal.

a signal with no DC offset will have equal weight in the positive and negative domains.

if you'd like to convert these values to [0...1], use the formula:

result = (0.5 * inputSample) + 0.5;

but you may have to use a more sophisticated algorithm if the signal extends beyond [-1...1].

in most cases, you should keep it at [-1...1] if you want to store it in floating point.

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I think i've figured this out. I just graphed those points above in excel and it seems to look like a waveform. I didn't realize it was giving the negative voltages as well... but it makes sense.