CODEWITHSUNDEEP
c#c++audiokinectwav
virgula24
virgula24

Reputation: 533

Kinect Audio PCM Values

Im using kinect to extract audio and classifie its features, but i have a question. On http://msdn.microsoft.com/en-us/library/hh855698.aspx it says the audio.start method Opens an audio data stream (16-bit PCM format, sampled at 16 kHz) and starts capturing audio data streamed out of a sensor. The problem is that i dont know how pcm data is represented and i dont know if the method returns pcm true values or not. Because using the sdk examples i get values like 200, 56, 17 and i think audio values are more like -3*10^-5 . So does anyone know how do i get the true PCM values? Or am i doing something wrong?

Thanks

Upvotes: 1

Views: 527

Answers (1)

Matt Curtis
Matt Curtis

Reputation: 23624

I wouldn't expect any particular values. 16-bit PCM means it's a series of 16-bit integers, so -3*10-5 (-0.00003) isn't representable.

I would guess it's encoded with 16-bit signed integers (like a WAV file) which have a range of -32768 to 32767. If you're being very quiet the values will probably be close to 0. If you make a lot of noise you will see some higher values too.

Check out this diagram (from Wikipedia's article on PCM) which shows a sine wave encoded as PCM using 4-bit unsigned integers, which have a range of 0 to 15.

See how that 4-bit sine wave oscillates around 7? That's it's equilibrium. If it was a signed 4-bit integer (which has a range of -8 to 7) it would have the same shape, but its equilibrium would be 0 - the values would be shifted by -8 so it would oscillate around 0.

You can measure the distance from the equilibrium to the highest or lowest points of the sine wave to get its amplitude, or broadly, it's volume (which is why if you're quiet you will mostly see values near 0 in your signed 16-bit data). This is probably the easiest sort of feature detection you can do. You can find plenty of good explanations on the web about this, for example http://scienceaid.co.uk/physics/waves/sound.html.

You could save it to a file and play it back with something like Audacity if you're not sure. Fiddle with the input settings and you'll soon figure out the format.

Upvotes: 1

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