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I tried generating a single tone of 220Hz in Ableton Live using the Operator instrument To make sure it was at that frequency I placed a Spectrum device after the Operator instrument.

It surprised me that besides the peak, "noise" (the smaller peaks) is seen in the higher frequency range. Block size = 2048

Changing the block size of the Spectrum device to 16384 increased that "noise": Block size = 16384

What is causing this? Is that noise actually present in the sound, or is this just a quirk of the Spectrum device?

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I want to preface this by saying that I could be totally wrong and invite anyone to correct me as this is just my best guess from one internet academic to another. But what i think is going on is this:

Any time digital audio exists it is created with both a sample rate and bit depth. The sample rate controls how high a frequency is accurately produced (how far to the right on your spectrogram) and the bit depth controls the level of detail in the sound.
Now this is where i might be a bit off but a common misconception is that the bit depth controls the accuracy of the actual waves reproduction. Aka the higher the bit depth, the less jagged the wave is. But because the sampling theorem states that any sampled wave can result in only one possible wave shape (along with skipping a lot of technical explanation and such stuff) the bit depth can basically be summed up as controlling the noise floor of the sound.

I think that noise is coming from the internal bit depth of Operator. I also think it starts to slightly ramp up in the higher frequencies of the spectrogram because aliasing, which is noise created by the sample rate of the audio. You should be able to compare this by looking at the noise floor that is created when you play the 8 or 16 bit sine wave options in Operators waveshape selector.

Two other things to note is that a noise floor of -150Db is super quiet and you would never be able to hear it next to your sound so its nothing to worry about. Also in operator you can change the amount of aliasing noise created by messing with the tone knob next to the master volume control (although you won't be able to see its effect on such low volume high frequencies).

  • -150dB is in fact lower than the self-noise of most professional analog audio equipment. -110 to -120dB is typically considered a good noise floor, and -150 is 8 times quieter than that. – KeithS Jan 24 '14 at 23:58
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This is digital noise-> everything smaler than the resolution of a system is noise!

  • + 100% Clear Sinuses are imposible since. Every System causes distirtion! – Tobias Schmidt Jan 23 '14 at 12:41
  • Thank you for the answer. Could you maybe elaborate on the resolution part? Is the block size equal to the resolution? In what unit? Bits? Hz? I'm really a beginner at this, sorry. – Saaru Lindestøkke Jan 23 '14 at 12:44
  • The smallest resolution for a 16 bit Sample is 0000000000000001 <- binary code – Tobias Schmidt Jan 24 '14 at 12:06
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    Sice DAWs work witch floating comma and variable bit depth and noise shaping/dithering/jittering its very hard to give a specific answer. -> It is like if you look at sand: from a bit of distance the resolution of our eyes is so small that wee do not see the small stones that make up sand -> so we only see one surface -> which is kinda a noise – Tobias Schmidt Jan 24 '14 at 12:08

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