Resampling

  • Author or source: ed.corm@liam
  • Type: linear interpolated aliased resampling of a wave file
  • Created: 2004-04-07 09:39:12
notes
som resampling stuff. the code is heavily used in MSynth, but do not lough about ;-)

perhaps, prefiltering would reduce aliasing.
code
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signed short* pSample = ...;
unsigned int sampleLength = ...;

// stretch sample to length of one bar...
float playPosDelta = sampleLength / ( ( 240.0f / bpm ) * samplingRate );

// requires for position calculation...
float playpos1 = 0.0f;
unsigned int iter = 0;

// required for interpolation...
unsigned int i1, i2;

float* pDest = ....;
float* pDestStop = pDest + len;
for( float *pt=pDest;pt<pDestStop;++pt )
{
    // linear interpolation...
    i1 = (unsigned int)playpos;
    i2 = i1 + 1;
    (*pt) = ((pSample[i2]-pSample[i1]) * (playpos - i1) + pSample[i1]);

    // position calculation preventing float sumation error...
    playpos1 = (++iter) * playposIncrement;
}
...

Comments

  • Date: 2004-02-15 12:01:50
  • By: Gog
Linear interpolation normally introduces a lot of artefacts. An easy way to improve upon this is to use the hermite interpolator instead. The improvement is _dramatic_!

i1 = (unsigned int)playpos;
i2 = i1 + 1;

would this be better as:

i1 = (unsigned int) floor(playpos);
i2 = (unsigned int) ceil(i1 + playposIncrement);

?

if you are actually decimating rather than interpolating (which is what would give aliasing), then the second interpolation point in the input could potentially be more than i1 + 1.

no, sorry it wouldn't :%|

yes, a more sophisticated interpolation would
improve the sound and prefiltering would
terminate the aliasing. but everything with
hi runtime overhead.