pcm音量处理

基本的操作，作为一个整理

第一种线性音量处理，处理16bit与32bit的情况

static inline int16_t clamp16(int32_t sample)
{
    if ((sample >> 15) ^ (sample >> 31)) {
        sample = 0x7FFF ^ (sample >> 31);
    }
    return sample;
}

static inline int32_t clamp32(int64_t sample)
{
    if ((sample >> 31) ^ (sample >> 63)) {
        sample = 0x7FFFFFFF ^ (sample >> 63);
    }
    return sample;
}

void apply_volume(float volume, void *buf, int sample_size, int bytes)
{
    int16_t *input16 = (int16_t *)buf;
    int32_t *input32 = (int32_t *)buf;
    unsigned int i = 0;

    if (sample_size == 2) {
        for (i = 0; i < bytes / sizeof(int16_t); i++) {
            int32_t samp = (int32_t)(input16[i]);
            input16[i] = clamp16((int32_t)(volume * samp));
        }
    } else if (sample_size == 4) {
        for (i = 0; i < bytes / sizeof(int32_t); i++) {
            int64_t samp = (int64_t)(input32[i]);
            input32[i] = clamp32((int64_t)(volume * samp));
        }
    } else {
        
    }
    return;
}

如果使用db作为音量计算参数，那就是先将db转成对应的线性浮点数

static inline float DbToAmpl(float decibels)
{
	return exp( decibels * 0.115129f); // exp( dB * ln(10) / 20 )
}

static inline float AmplToDb(float amplification)
{
	return 20 * log10(amplification);
}

void apply_volume_by_db(float decibels, void *buf, int sample_size, int bytes)
{
	float vol = DbToAmpl(decibels);
	apply_volume(vol, buf, sample_size, bytes);	
	return;
}

如果需要对音量曲线可以定制，那么可以配置一个db的数据，自定义音频曲线，这里举例为0~100个index

#define AUDIO_VOLUME_INDEX 101
#define VOLUME_MIN_DB (-100)
static float volume_cruve_in_dB[AUDIO_VOLUME_INDEX] = {
    VOLUME_MIN_DB, /*mute*/
    -60,   -53,   -47.5, -43.5, -39,   -36,   -34,   -32,   -30,   -28,    /*1-10*/
    -27,   -26,   -25,   -24,   -23,   -22.2, -21.5, -21,   -20.6, -20.3,  /*11-20*/
    -19.9, -19.5, -19,   -18.7, -18.4, -18.2, -18,   -17.8, -17.5, -17.3,  /*21-30*/
    -17,   -16.8, -16.5, -16.2, -15.9, -15.6, -15.4, -15.2, -14.9, -14.7,  /*31-40*/
    -14.4, -14.1, -13.9, -13.7, -13.5, -13.3, -13.1, -12.9, -12.7, -12.4,  /*41-50*/
    -12.1, -11.8, -11.6, -11.4, -11.2, -11,   -10.8, -10.6, -10.3, -10,    /*51-60*/
    -9.8,  -9.6,  -9.4,  -9.2,  -9,    -8.7,  -8.4,  -8.1,  -7.8,  -7.5,   /*61-70*/
    -7.2,  -6.9,  -6.7,  -6.4,  -6.1,  -5.8,  -5.5,  -5.2,  -5,    -4.8,   /*71-80*/
    -4.7,  -4.5,  -4.3,  -4.1,  -3.8,  -3.6,  -3.3,  -3,    -2.7,  -2.5,   /*81-90*/
    -2.2,  -2,    -1.8,  -1.5,  -1.3,  -1,    -0.8,  -0.5,  -0.3,  0,      /*91-100*/
};

float get_db_by_index(int volume_index)
{
    float db = 0.0;
    if (volume_index >= AUDIO_VOLUME_INDEX) {
        ALOGE("%s, invalid index!\n", __FUNCTION__);
        return VOLUME_MIN_DB;
    }
    if (volume_index >= 0) {
        db = volume_cruve_in_dB[volume_index];
    }

    return db;
}

音量计算时先将16bit转为32bit，再进行音量处理，这种处理是曾经有遇到过小信号文件时系统由于有预衰减信噪比不好，转给32bit可以提升snr

void apply_volume_16to32(float volume, int16_t *in_buf, int32_t *out_buf, int bytes)
{
    int16_t *input16 = (int16_t *)in_buf;
    int32_t *output32 = (int32_t *)out_buf;
    unsigned int i = 0;

    for (i = 0; i < bytes / sizeof(int16_t); i++) {
        int32_t samp = ((int32_t)input16[i]) << 16;
        output32[i] = clamp32((int64_t)(samp * (double)(volume)));
    }

    return;
}