通过pcm音频数据计算分贝

很多场合我们需要动态显示实时声音分贝,下面列举三种计算分贝的算法。(以双声道为例,也就是一个short类型,最大能量值为32767)

1:计算分贝 音频数据与大小

首先我们分别累加每个采样点的数值,除以采样个数,得到声音平均能量值。
然后再将其做100与32767之间的等比量化。得到1-100的量化值。
通常情况下,人声分布在较低的能量范围,这样就会使量化后的数据大致分布在1-20的较小区间,不能够很敏感的感知变化。
所以我们将其做了5倍的放大,当然计算后大于100的值,我们将其赋值100.

//参数为数据,采样个数
//返回值为分贝
#define VOLUMEMAX   32767
int SimpleCalculate_DB(short* pcmData, int sample)
{
    signed short ret = 0;
    if (sample > 0){
        int sum = 0;
        signed short* pos = (signed short *)pcmData;
        for (int i = 0; i < sample; i++){
            sum += abs(*pos);
            pos++;
        }
        ret = sum * 500.0 / (sample * VOLUMEMAX);
        if (ret >= 100){
            ret = 100;
        }
   }
   return ret;
}

2:计算均方根(RMS) 即能量值

static const float kMaxSquaredLevel = 32768 * 32768;
constexpr float kMinLevel = 30.f;

void Process(const int16_t* data, size_t length) 
{
    float sum_square_ = 0;
    size_t sample_count_ = 0;
    for (size_t i = 0; i < length; ++i) {
        sum_square_ += data[i] * data[i];
    }
    sample_count_ += length;.
    float rms = sum_square_ / (sample_count_ * kMaxSquaredLevel);
    //20log_10(x^0.5) = 10log_10(x)
    rms = 10 * log10(rms);
    if (rms < -kMinLevel)
        rms = -kMinLevel;
    rms = -rms;
    return static_cast<int>(rms + 0.5);
}

3:获取音频数据最大的振幅(即绝对值最大)(0-32767),除以1000,得到(0-32)。从数组中获取相应索引所对应的分贝值。(提取自webrtc)

const int8_t permutation[33] =
    {0,1,2,3,4,4,5,5,5,5,6,6,6,6,6,7,7,7,7,8,8,8,9,9,9,9,9,9,9,9,9,9,9};
    
int16_t WebRtcSpl_MaxAbsValueW16C(const int16_t* vector, size_t length) 
{
    size_t i = 0;
    int absolute = 0, maximum = 0;
    for (i = 0; i < length; i++) {
        absolute = abs((int)vector[i]);
        if (absolute > maximum) {
            maximum = absolute;
        }
    }
    if (maximum > 32767) {
        maximum = 32767;
    }
    return (int16_t)maximum;
}

void ComputeLevel(const int16_t* data, size_t length)
{
    int16_t _absMax = 0;
    int16_t _count = 0;
    int8_t _currentLevel = 0;
    int16_t absValue(0);
    absValue = WebRtcSpl_MaxAbsValueW16(data,length);
    if (absValue > _absMax)
        _absMax = absValue;
    if (_count++ == 10) {
        _count = 0;
        int32_t position = _absMax/1000;
        if ((position == 0) && (_absMax > 250)){
            position = 1;
        }
        _currentLevel = permutation[position];
        _absMax >>= 2;
    }
}

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