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TI DSP's Q float system

Discussion in 'Electronic Basics' started by Robert, Oct 8, 2005.

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  1. Robert

    Robert Guest

    Hi all,

    I am new to TI DSP. And I am using 320C55X series. I found their float
    system use something like Q13.3, 13 bits integer and 3 bits fraction.

    I search in TI's document(instruction set) and did not find any thing
    about it.

    Is there any instruction related to it and is it just software
    implemented?

    Any suggestions will be appreciated!
    Best regards,
    Davy
     
  2. Tim Wescott

    Tim Wescott Guest

    1. It's not floating point, it's fixed.

    2. This would be a good question for comp.dsp.

    3. It's mostly implemented as a convention, although there's a C
    library that supports it. In assembly you get to set the radix any darn
    way you want. The MAC instruction has a built-in shift, but I don't
    know if it's used for these operations (that's tool chain specific).

    4. If you can't find stuff in TI's DSP documentation join the club.
    It's usually there, but sometimes it seems like it'd be quicker to build
    your own chip starting from sand than to find it in their documentation.
     
  3. Rich Grise

    Rich Grise Guest

  4. steve

    steve Guest

    I think you are referring to TI's fixed point math library

    http://focus.ti.com/docs/toolsw/folders/print/sprc087.html

    is sort of a bridge between writing pure fixed point code with all the
    headaches and using a pure software floating point library (slow but
    easy to write).

    Its about 4-5 times slower then pure fixed point (if I remember
    correctly), but much faster then floating point emulation (20-100
    slower then fixed point depending on the algorithm your implementing).
    It still requires to figure out what the dynamic range is for your
    signals and choose an appropriate "Q" factor.

    Just my opinion but for me, its the worse of both worlds, slow and
    still have to worry about dynamic ranges, but others think is the best
    of both worlds, fast and little scaling worries.
     
  5. Robert

    Robert Guest

    Hi Steve,

    I just want to use the image lib provide by TI C55x. And it use "Q", as
    you said it is a math lib, I see.

    Thanks!
    Davy
     
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