ios – 有人可以解释这个代码如何使用Accelerate Framework将卷转换成分贝？

我正在使用EZAudio构建一个iOS应用程序.它的委托返回一个float **缓冲区,其中包含指示检测到的卷的浮点值.这个委托是不断调用的,它的工作完成了一个不同的线程.

What I am trying to do is to take the float value from EZAudio and convert it into decibels.

EZAudioDelegate

这是我的简化EZAudio Delegate for getting Microphone Data：

- (void)microphone:(EZMicrophone *)microphone hasAudioReceived:(float **)buffer withBufferSize:(UInt32)bufferSize withNumberOfChannels:(UInt32)numberOfChannels {
    /*
     *  Returns a float array called buffer that contains the stereo signal data
     *  buffer[0] is the left audio channel
     *  buffer[1] is the right audio channel
     */

    // Using a separate audio thread to not block the main UI thread
    dispatch_async(dispatch_get_main_queue(),^{

        float decibels = [self getDecibelsFromVolume:buffer withBufferSize:bufferSize];

        NSLog(@"Decibels: %f",decibels);

    });

}

问题

The problem is that after implementing solutions from the links below,I do not understand how it works. If someone Could explain how it converts volume to decibels I would be very grateful

> How to convert audio input to DB? #85
> How to change the buffer size to increase FFT window? #50
> Changing Buffer Size #84

代码

该解决方案使用Accelerate Framework中的以下方法将卷转换为分贝：

> vDSP_vsq
> vDSP_meanv
> vDSP_vdbcon

以下是EZAudio Delegate所调用的getDecibelsFromVolume方法.它从委托中传递了float ** buffer和bufferSize.

- (float)getDecibelsFromVolume:(float**)buffer withBufferSize:(UInt32)bufferSize {

    // Decibel Calculation.

    float one = 1.0;
    float meanVal = 0.0;
    float tiny = 0.1;
    float lastdbValue = 0.0;

    vDSP_vsq(buffer[0],1,buffer[0],bufferSize);

    vDSP_meanv(buffer[0],&meanVal,bufferSize);

    vDSP_vdbcon(&meanVal,&one,0);


    // Exponential moving average to dB level to only get continous sounds.

    float currentdb = 1.0 - (fabs(meanVal) / 100);

    if (lastdbValue == INFINITY || lastdbValue == -INFINITY || isnan(lastdbValue)) {
        lastdbValue = 0.0;
    }

    float dbValue = ((1.0 - tiny) * lastdbValue) + tiny * currentdb;

    lastdbValue = dbValue;

    return dbValue;
}

解决方法

我将解释如何使用代码计算信号的dB值,然后显示它与vDSP示例的关系.

首先,计算一个数据块的RMS和

double sumSquared = 0;
for (int i = 0 ; i < numSamples ; i++)
{
   sumSquared += samples[i]*samples[i];
}
double rms = sumSquared/numSamples;

有关RMS的更多信息

接下来将RMS值转换为dB

double dBvalue = 20*log10(rms);

这与示例代码有关

vDSP_vsq(buffer[0],bufferSize);

该行循环遍历缓冲区,并计算缓冲区中所有元素的平方.如果缓冲区在调用之前包含值[1,2,3,4],则在调用后将包含值[1,4,9,16]

vDSP_meanv(buffer[0],对缓冲区中的值进行求和,然后返回总和除以元素数.所以对于输入缓冲区[1,16]计算总和30,除以4并返回结果7.5.
 
  
 
vDSP_vdbcon(&meanVal,0);
 
 
该行将meanVal转换为分贝.在这里调用矢量化函数真的没有意义,因为它只能在单个元素上运行.然而,它正在做的是将参数插入到以下公式中：
 
  
 
meanVal = n*log10(meanVal/one)
 
 
其中n为10或20,具体取决于最后一个参数.在这种情况下,它是10. 10用于功率测量,20是用于振幅.我认为20对你来说会更有意义.
 
 
最后一点代码看起来正在做一些简单的平滑的结果,使仪表有点小的弹性.