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Convolution problem

deniz gurhan
Note: This was originally posted on 20th August 2013 at http://forums.arm.com

When i obtain high pass filter using arm_conv_partial_f32 function instead of arm_fir_f32 function; i get same output array with opposite signs. This situation is not seen when i use low pass or mid pass filter. Anyone help?
  • Note: This was originally posted on 22nd August 2013 at http://forums.arm.com

    I have a hunch what is happening.  The FIR filter function arm_fir_f32() requires that the filter coefficients be in time reversed order.  The arm_conv_partial_f32() has the coefficients in normal order.  In a high pass filter, the coefficients typically alternate in sign.  If the length of the filter is even then time flipping the filter is equivalent to scaling by -1.


    When i obtain high pass filter using arm_conv_partial_f32 function instead of arm_fir_f32 function; i get same output array with opposite signs. This situation is not seen when i use low pass or mid pass filter. Anyone help?
  • Note: This was originally posted on 22nd August 2013 at http://forums.arm.com

    Of course, stupid of me. It makes sense. Thanks a lot Paul.


    I have a hunch what is happening.  The FIR filter function arm_fir_f32() requires that the filter coefficients be in time reversed order.  The arm_conv_partial_f32() has the coefficients in normal order.  In a high pass filter, the coefficients typically alternate in sign.  If the length of the filter is even then time flipping the filter is equivalent to scaling by -1.