KissFFT forward / inverse is outputting noise, why

Question

I'm trying to use KissFFT natively in a java app, but the forward/inverse of an input signal isn't returning as it should: the signal amplitude is almost non-existent. If I remove the scaling factor (dividing by 2N), the result is harmonic noise. Can anyone spot the bug?

Here is the forward call (copied from GDX, so should be ok!):

JNIEXPORT void JNICALL Java_com_badlogic_gdx_audio_analysis_KissFFT_spectrum(JNIEnv* env, jclass clazz, jlong handle, jshortArray obj_samples, jfloatArray obj_spectrum) {
    short* samples = (short*)env->GetPrimitiveArrayCritical(obj_samples, 0);
    float* spectrum = (float*)env->GetPrimitiveArrayCritical(obj_spectrum, 0);

        KissFFT* fft = (KissFFT*)handle;
        kiss_fftr( fft->forwardConfig, (kiss_fft_scalar*)samples, fft->spectrum );

        int len = fft->numSamples / 2 + 1;
        for( int i = 0; i < len; i++ )
        {
            float re = scale(fft->spectrum[i].r) * fft->numSamples;
            float im = scale(fft->spectrum[i].i) * fft->numSamples;

            if( i > 0 )
                spectrum[i] = sqrtf(re*re + im*im);
            else
                spectrum[i] = sqrtf(re*re + im*im);
        }

    env->ReleasePrimitiveArrayCritical(obj_samples, samples, 0);
    env->ReleasePrimitiveArrayCritical(obj_spectrum, spectrum, 0);

}

And here is the inverse (written by me, probably has an error ;) ):

JNIEXPORT void JNICALL Java_com_badlogic_gdx_audio_analysis_KissFFT_inverse(JNIEnv* env, jclass clazz, jlong handle, jshortArray obj_samples) {
    short* shortSamples = (short*)env->GetPrimitiveArrayCritical(obj_samples, 0);

//@line:108

        KissFFT* fft = (KissFFT*)handle;
        kiss_fft_cpx out[fft->numSamples];

        kiss_fftri( fft->inverseConfig, fft->spectrum, (kiss_fft_scalar*)out );

        for (int i=0; i < fft->numSamples; i++) {
            shortSamples[i] = (out[i].r) / (fft->numSamples*2);
        }

    env->ReleasePrimitiveArrayCritical(obj_samples, shortSamples, 0);

}

Answer 1

Per protectedmember's request, here is a more complete example (on the native side, anyway). I ended up modifying the libgdx code slightly (using v.0.9.4). I also compiled with floating-point rather than fixed, as the latter introduced quantization noise when inversing. This code should work for either (because kiss_fft_scalar adapts to short or float depending on the compiler flag), but have not re-tested with fixed, since I found it's not useful for a fft->ifft series.

I am curious to know if you are successful, do let me know!

(Please note that this code, following libgdx, is under Apache 2.0 license!)

#include <com.badlogic.gdx.audio.analysis.KissFFT.h>

#include <kissfft/kiss_fftr.h>
#include <stdlib.h>
#include <stdio.h>
#include <math.h>

struct KissFFT
{
    kiss_fftr_cfg forwardConfig;
    kiss_fftr_cfg inverseConfig;
    kiss_fft_cpx* spectrum;
    int numSamples;
    int spectrumSize;
};

JNIEXPORT jlong JNICALL Java_com_badlogic_gdx_audio_analysis_KissFFT_create(JNIEnv* env, jclass clazz, jint numSamples) {

    KissFFT* fft = new KissFFT();
    fft->forwardConfig = kiss_fftr_alloc(numSamples,0,NULL,NULL);
    fft->inverseConfig = kiss_fftr_alloc(numSamples,1,NULL,NULL);
    fft->spectrum = (kiss_fft_cpx*)malloc(sizeof(kiss_fft_cpx) * numSamples);
    fft->numSamples = numSamples;
    fft->spectrumSize = numSamples/2+1;
    return (jlong)fft;
}

JNIEXPORT void JNICALL Java_com_badlogic_gdx_audio_analysis_KissFFT_destroy(JNIEnv* env, jclass clazz, jlong handle) {

    KissFFT* fft = (KissFFT*)handle;
    free(fft->forwardConfig);
    free(fft->inverseConfig);
    free(fft->spectrum);
    free(fft);  
}

JNIEXPORT void JNICALL Java_com_badlogic_gdx_audio_analysis_KissFFT_spectrum(JNIEnv* env, jclass clazz, jlong handle, jfloatArray obj_samples, jfloatArray obj_mags) {

    kiss_fft_scalar* samples = (kiss_fft_scalar*)env->GetPrimitiveArrayCritical(obj_samples, 0);
    float* mags = (float*)env->GetPrimitiveArrayCritical(obj_mags, 0);

    KissFFT* fft = (KissFFT*) handle;

    kiss_fftr(fft->forwardConfig, samples, fft->spectrum);

    for(int i=0;i<fft->spectrumSize;i++) {
        mags[i] = hypotf(fft->spectrum[i].r,fft->spectrum[i].i);
    }

    env->ReleasePrimitiveArrayCritical(obj_samples, samples, 0);
    env->ReleasePrimitiveArrayCritical(obj_mags, mags, 0);
}

JNIEXPORT void JNICALL Java_com_badlogic_gdx_audio_analysis_KissFFT_inverse(JNIEnv* env, jclass clazz, jlong handle, jfloatArray obj_samples) {
    kiss_fft_scalar* samples = (kiss_fft_scalar*)env->GetPrimitiveArrayCritical(obj_samples, 0);

    KissFFT* fft = (KissFFT*)handle;

    kiss_fftri( fft->inverseConfig, fft->spectrum, samples );

    for(int i=0;i<fft->numSamples;i++) {
        samples[i] = samples[i] / (float)fft->numSamples;
    }

    env->ReleasePrimitiveArrayCritical(obj_samples, samples, 0);

}

Answer 2

See my answer to a similar question.

or from the kissfft README file:

        Q: Why don't I get the output I expect?
        A: The two most common causes of this are
                1) scaling : is there a constant multiplier between what you got and what you want?
                2) mixed build environment -- all code must be compiled with same preprocessor
                definitions for FIXED_POINT and kiss_fft_scalar

Answer 3

Whoops! Found the problem. I should have been doing the inverse transform right into the short array:

kiss_fftri( fft->inverseConfig, fft->spectrum, shortSamples );