Audio Reverses Polarity When it Reaches Full Scale

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jcrouse
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2011/04/27 11:05:29 (permalink)

Audio Reverses Polarity When it Reaches Full Scale

When I use my Presonus  FP10 with Sonar 7 Producer , the polarity of the audio signal reverses polarity when it reaches full scale. This problem is not there when I use Cubase LE. Does any one know the difference between these programs that would cause this?

jpeg file of the polarity reversing available
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    dantarbill
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    Re:Audio Reverses Polarity When it Reaches Full Scale 2011/04/27 11:31:40 (permalink)
    jcrouse


    When I use my Presonus  FP10 with Sonar 7 Producer , the polarity of the audio signal reverses polarity when it reaches full scale. This problem is not there when I use Cubase LE. Does any one know the difference between these programs that would cause this?

    jpeg file of the polarity reversing available

    I've seen this before with an M-Audio 1814 interface.  It's likely an integer wrap around problem.  A 16 bit signed integer (or 8 bit or 24 bit for that matter) has a maximum positive number it can represent.  For 16 bits, it's 0x7FFFFFFF (in hex).  When you go one higher (0x80000000), now the highest bit (most significant bit) is set, indicating that you are now looking at a negative number (in 2's complement).

    Logically, the value should just stop at the rail rather than wrapping around.  I would chalk this up as a driver problem that the interface vendor (Presonus) needs to address...except the fact that you say that Cubase doesn't have this problem...which has me scratching my head.

    post edited by dantarbill - 2011/04/27 11:33:12

    Dan Tarbill
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    bitflipper
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    Re:Audio Reverses Polarity When it Reaches Full Scale 2011/04/27 11:51:40 (permalink)
    Dan's right. It doesn't happen when you reach 0db, but rather when you attempt to exceed 0db. Add 1 to the largest number possible using the number of bits in a data word, and it sets the leftmost bit, which is the sign bit that turns the number into a negative value. It will happen on input, too, if you overdrive your interface while recording.

    And it sounds pretty bad. Which is why you want to make damn sure your output never exceeds the maximum value and that you never overdrive your interface's inputs. If you want to be certain it won't ever happen (during playback anyway), set your brickwall limiter to -3db.


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    brundlefly
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    Re:Audio Reverses Polarity When it Reaches Full Scale 2011/04/27 15:21:46 (permalink)
    This problem is not there when I use Cubase LE.



    I agree with others that this would normally be an interface driver/firmware problem. I don't know why Cubase would not exhibit this behavior except that it might have built-in limiting that prevents it from sending out-of-range amplitude values to the interface.
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    bitflipper
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    Re:Audio Reverses Polarity When it Reaches Full Scale 2011/04/27 17:11:16 (permalink)
    I suppose it's possible that Cubase tests for overflows, but I doubt it. That would be extra overhead that just substituted one bad-sounding problem for another. Either way, it's simple user error and easily avoided.

    Or, as brundlefly suggested, the OP may have a limiter (or some other plugin that lowers the level) in the Cubase project but not in the SONAR project. All it would take, if levels were dangerously hot, would be an EQ or a compressor that doesn't have exactly the same settings between projects.


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