Function GainCompressor

Engine/lib/openal-soft/core/mastering.cpp:187–276

This is the heart of the feed-forward compressor. It operates in the log * domain (to better match human hearing) and can apply some basic automation * to knee width, attack/release times, make-up/post gain, and clipping * reduction. */

 * reduction.
 */
void GainCompressor(Compressor *Comp, const uint SamplesToDo)
{
    const bool autoKnee{Comp->mAuto.Knee};
    const bool autoAttack{Comp->mAuto.Attack};
    const bool autoRelease{Comp->mAuto.Release};
    const bool autoPostGain{Comp->mAuto.PostGain};
    const bool autoDeclip{Comp->mAuto.Declip};
    const uint lookAhead{Comp->mLookAhead};
    const float threshold{Comp->mThreshold};
    const float slope{Comp->mSlope};
    const float attack{Comp->mAttack};
    const float release{Comp->mRelease};
    const float c_est{Comp->mGainEstimate};
    const float a_adp{Comp->mAdaptCoeff};
    const float *crestFactor{Comp->mCrestFactor};
    float postGain{Comp->mPostGain};
    float knee{Comp->mKnee};
    float t_att{attack};
    float t_rel{release - attack};
    float a_att{std::exp(-1.0f / t_att)};
    float a_rel{std::exp(-1.0f / t_rel)};
    float y_1{Comp->mLastRelease};
    float y_L{Comp->mLastAttack};
    float c_dev{Comp->mLastGainDev};

    ASSUME(SamplesToDo > 0);

    for(float &sideChain : al::span<float>{Comp->mSideChain, SamplesToDo})
    {
        if(autoKnee)
            knee = maxf(0.0f, 2.5f * (c_dev + c_est));
        const float knee_h{0.5f * knee};

        /* This is the gain computer.  It applies a static compression curve
         * to the control signal.
         */
        const float x_over{std::addressof(sideChain)[lookAhead] - threshold};
        const float y_G{
            (x_over <= -knee_h) ? 0.0f :
            (std::fabs(x_over) < knee_h) ? (x_over + knee_h) * (x_over + knee_h) / (2.0f * knee) :
            x_over};

        const float y2_crest{*(crestFactor++)};
        if(autoAttack)
        {
            t_att = 2.0f*attack/y2_crest;
            a_att = std::exp(-1.0f / t_att);
        }
        if(autoRelease)
        {
            t_rel = 2.0f*release/y2_crest - t_att;
            a_rel = std::exp(-1.0f / t_rel);
        }

        /* Gain smoothing (ballistics) is done via a smooth decoupled peak
         * detector.  The attack time is subtracted from the release time
         * above to compensate for the chained operating mode.
         */