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Commit d3fa051b authored by Antti S. Lankila's avatar Antti S. Lankila
Browse files

Add noise-shaped dithering support 

Used rectangular probability distribution function before by accident,
depite calculating the high-passed triangular probability density function
I intended to use. This commit rectifies the mistake and also adds
noise-shaped error feedback similar to JACK.
parent e080c01b

libs/audioflinger/AudioFlinger.cpp

+2 −2
Original line number Diff line number Diff line
@@ -3399,7 +3399,7 @@ bool AudioFlinger::RecordThread::threadLoop() 
                    // ditherAndClamp() works as long as all buffers returned by mActiveTrack->getNextBuffer()

                    // are 32 bit aligned which should be always true.

                    if (mChannelCount == 2 && mReqChannelCount == 1) {

                        AudioMixer::ditherAndClamp(mRsmpOutBuffer, mRsmpOutBuffer, framesOut);

                        AudioMixer::ditherAndClamp(&dither, mRsmpOutBuffer, mRsmpOutBuffer, framesOut);

                        // the resampler always outputs stereo samples: do post stereo to mono conversion

                        int16_t *src = (int16_t *)mRsmpOutBuffer;

                        int16_t *dst = buffer.i16;
@@ -3408,7 +3408,7 @@ bool AudioFlinger::RecordThread::threadLoop() 
                            src += 2;

                        }

                    } else {

                        AudioMixer::ditherAndClamp((int32_t *)buffer.raw, mRsmpOutBuffer, framesOut);

                        AudioMixer::ditherAndClamp(&dither, (int32_t *)buffer.raw, mRsmpOutBuffer, framesOut);

                    }



                }

libs/audioflinger/AudioFlinger.h

+2 −0
Original line number Diff line number Diff line
@@ -43,6 +43,7 @@ 


#include "AudioBufferProvider.h"

#include "AudioDSP.h"

#include "AudioMixer.h"



namespace android {
@@ -766,6 +767,7 @@ private: 
                int                                 mReqChannelCount;

                uint32_t                            mReqSampleRate;

                ssize_t                             mBytesRead;

                dither_t                            dither;

    };



    class RecordHandle : public android::BnAudioRecord {

libs/audioflinger/AudioMixer.cpp

+46 −19
Original line number Diff line number Diff line
@@ -38,10 +38,10 @@ static inline int16_t clamp16(int32_t sample) 
    return sample;

}



inline static int32_t prng() {

    static int32_t seed = 1;

    seed = (seed * 12345) + 1103515245;

    return int32_t(seed & 0xfff);

inline static uint32_t prng() {

    static uint32_t seed = 22222;

    seed = (seed * 196314165) + 907633515;

    return seed >> 20;

}



// ----------------------------------------------------------------------------
@@ -621,24 +621,51 @@ void AudioMixer::track__16BitsMono(track_t* t, int32_t* out, size_t frameCount, 
    t->in = in;

}



void AudioMixer::ditherAndClamp(int32_t* out, int32_t const *sums, size_t c)

int32_t AudioMixer::lipshitz(int32_t* state, int32_t input)

{

#define COEFF(x) int32_t(x * 4096.0f + 0.5f)

    int32_t output =

          COEFF(-2.033f) * input

        + COEFF(+2.165f) * state[0]

        + COEFF(-1.959f) * state[1]

        + COEFF(+1.590f) * state[2]

        + COEFF(-0.6149f) * state[3];

#undef COEFF



    state[3] = state[2];

    state[2] = state[1];

    state[1] = state[0];

    state[0] = input;



    return output >> 12;

}





void AudioMixer::ditherAndClamp(dither_t* state, int32_t* out, int32_t const *sums, size_t c)

{

    int32_t oldDitherValue = prng();

    for (size_t i=0 ; i<c ; i++) {

        int32_t l = *sums++;

        int32_t r = *sums++;



        /* Apply dither to output. This is the high-passed triangular

         * probability density function, discussed in "A Theory of

         * Nonsubtractive Dither", by Robert A. Wannamaker et al. */

        int32_t ditherValue = prng();

        int32_t dithering = oldDitherValue - ditherValue;

        oldDitherValue = ditherValue;



        int32_t nl = (l + ditherValue) >> 12;

        int32_t nr = (r + ditherValue) >> 12;

        l = clamp16(nl);

        r = clamp16(nr);

        /* Noise-shaped dither function. */



        /* High-passed Triangular PDF according to

         * "A Theory of Nonsubtractive Dither" by Robert Wannamaker et al.

         * Other software seems to prefer (prng() + prng()) >> 1 as the

         * random source, which they highpass, but that distribution is not

         * triangular. */

        int32_t newDither = prng();

        int32_t dithering = newDither - state->oldDither;

        state->oldDither = newDither;



        l += lipshitz(state->lipshitzL, state->errorL) + dithering;

        r += lipshitz(state->lipshitzR, state->errorR) + dithering;

        state->errorL = l & 0xfff;

        state->errorR = r & 0xfff;



        l = clamp16(l >> 12);

        r = clamp16(r >> 12);



        *out++ = (r<<16) | (l & 0xFFFF);

    }

}
@@ -721,7 +748,7 @@ void AudioMixer::process__genericNoResampling(state_t* state, void* output, Audi 
        }



        dsp.process(outTemp, BLOCKSIZE);

        ditherAndClamp(out, outTemp, BLOCKSIZE);

        ditherAndClamp(&state->dither, out, outTemp, BLOCKSIZE);

        out += BLOCKSIZE;

        numFrames -= BLOCKSIZE;

    } while (numFrames);
@@ -778,7 +805,7 @@ void AudioMixer::process__genericResampling(state_t* state, void* output, AudioD 
    }



    dsp.process(outTemp, numFrames);

    ditherAndClamp(out, outTemp, numFrames);

    ditherAndClamp(&state->dither, out, outTemp, numFrames);

}



// ----------------------------------------------------------------------------

libs/audioflinger/AudioMixer.h

+13 −2
Original line number Diff line number Diff line
@@ -32,6 +32,13 @@ namespace android { 
#define LIKELY( exp )       (__builtin_expect( (exp) != 0, true  ))

#define UNLIKELY( exp )     (__builtin_expect( (exp) != 0, false ))



struct dither_t {

    /* Dithering variables */

    int32_t         errorL, errorR;

    int32_t         lipshitzL[4], lipshitzR[4];

    int32_t         oldDither;

};



// ----------------------------------------------------------------------------



class AudioMixer
@@ -86,7 +93,7 @@ public: 


    uint32_t    trackNames() const { return mTrackNames; }



    static void ditherAndClamp(int32_t* out, int32_t const *sums, size_t c);

    static void ditherAndClamp(dither_t* dither, int32_t* out, const int32_t* sums, size_t c);



private:

    enum {
@@ -162,7 +169,10 @@ private: 
        int32_t         *outputTemp;

        int32_t         *resampleTemp;

        int32_t         reserved[2];

        track_t         tracks[32]; __attribute__((aligned(32)));

        track_t         tracks[32];



        dither_t        dither;

        __attribute__((aligned(32)));

    };



    int             mActiveTrack;
@@ -174,6 +184,7 @@ private: 


    void invalidateState(uint32_t mask);



    static int32_t lipshitz(int32_t* state, int32_t input);

    static void track__genericResample(track_t* t, int32_t* out, size_t numFrames, int32_t* temp, AudioDSP& dsp);

    static void track__nop(track_t* t, int32_t* out, size_t numFrames, int32_t* temp, AudioDSP& dsp);

    static void volumeRampStereo(track_t* t, int32_t* out, size_t frameCount, int32_t* temp, AudioDSP& dsp);