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Can someone explain up sampling please ?

Discussion in 'Archived Threads 2001-2004' started by LiorM, Apr 19, 2002.

  1. LiorM

    LiorM Agent

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    The one thing I just can understand is,

    The up sampling process , takes the digital signal and doubles it’s sampling frequency

    For grater details and clarity.

    Now, how the hell you create something that was never exists at the beginning.

    I worked as sound engineer at the 90’s and the first thing I’ve learn was that you can

    Create something that does not exist.

    If you don’t have a bass track, you can add as much as you want 60Hz, you still Won’t get it :).

    So if you have 48Khz of data , you can double it , tripple it , or do what ever you like with it , and you will still come out with the same data sampled at the beginning ...

    Am I wrong here or what ?

    Shark
     
  2. Bob McElfresh

    Bob McElfresh Producer

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    For a simple example, say you have 48 thousand numbers like this:

    500

    800

    400

    600

    ...

    This represents 1 second worth of digital samples at 48 Khz (48,000 times per second).

    Now you want a 96 Khz up-sample. So you simply do this:

    500

    650

    800

    600

    400

    500

    600

    ...

    So you can simply "fill in" a number to smooth things out.

    Then you send this set of 96,000 numbers per second to a D/A converter that expects this many samples per second.

    In reality, I think they do some other formula's than simple average between the numbers. But the concept is the same.

    Does this help?

    Does that help?
     
  3. LiorM

    LiorM Agent

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    Well, that clears things up, but I still

    Can’t get use to the idea that some algorithm

    Adds bits to the original material.

    Who knows, I need to hear then I decide what's

    Better.
     
  4. ling_w

    ling_w Second Unit

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    Read below
     
  5. ling_w

    ling_w Second Unit

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    A linear interpolation would produce a floawed result.

    e.g.

    100

    200

    250

    250

    200

    100

    upsampled 2X would produce

    100

    150

    200

    225

    250

    250

    250

    225

    200

    150

    100

    The green interpolation is in error because it would simulate a clipped signal, in addition to a connect the dots look by the red interpolations.

    Various upsamplers uses psychoaccoustic models to do their interpolation, while even some real old units, like the ancient Wadia, used their French Curve or spline algorithm to do their 64x oversampling.

    A properly done 2x upsampling would probably look more like:

    100

    156

    200

    231

    250

    256

    250

    231

    200

    156

    100

    A higher order upsampling would become much more complicated. 64x requires so much computational power because in order to fill the 63 empty spots between 2 samples, one would need to string up a 64th order polynomial equation consisting 32 samples before and 32 after the current sample.
     
  6. Bob McElfresh

    Bob McElfresh Producer

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