SoundTouch音频处理库源码分析及算法提取(2)

SoundTouch音频处理库初始化流程剖析
定义一个变量SoundTouch m_SoundTouch;

SoundTouch的派生关系
FIFOSamplePipe->FIFOProcessor->SoundTouch (流程[1])
因此首先构造基类FIFOSamplePipe，接着派生出FIFOProcessor，然后才以FIFOProcessor派生出SoundTouch。这里不得不提一下老外的C++水平真的很高，在这里基本上把类的继承发挥到了极致。能够分析这样的代码简直就是一种享受。先看一下基类FIFOSamplePipe，如下定义：
class FIFOSamplePipe
{
public:
// virtual default destructor
virtual ~FIFOSamplePipe() {}

/// Returns a pointer to the beginning of the output samples.
/// This function is provided for accessing the output samples directly.
/// Please be careful for not to corrupt the book-keeping!
///
/// When using this function to output samples, also remember to 'remove' the
/// output samples from the buffer by calling the
/// 'receiveSamples(numSamples)' function
virtual SAMPLETYPE *ptrBegin() = 0;

/// Adds 'numSamples' pcs of samples from the 'samples' memory position to
/// the sample buffer.
virtual void putSamples(const SAMPLETYPE *samples, ///< Pointer to samples.
uint numSamples ///< Number of samples to insert.
) = 0;

// Moves samples from the 'other' pipe instance to this instance.
void moveSamples(FIFOSamplePipe &other ///< Other pipe instance where from the receive the data.
)
{
int oNumSamples = other.numSamples();

putSamples(other.ptrBegin(), oNumSamples);
other.receiveSamples(oNumSamples);
};

/// Output samples from beginning of the sample buffer. Copies requested samples to
/// output buffer and removes them from the sample buffer. If there are less than
/// 'numsample' samples in the buffer, returns all that available.
///
/// /return Number of samples returned.
virtual uint receiveSamples(SAMPLETYPE *output, ///< Buffer where to copy output samples.
uint maxSamples ///< How many samples to receive at max.
) = 0;

/// Adjusts book-keeping so that given number of samples are removed from beginning of the
/// sample buffer without copying them anywhere.
///
/// Used to reduce the number of samples in the buffer when accessing the sample buffer directly
/// with 'ptrBegin' function.
virtual uint receiveSamples(uint maxSamples ///< Remove this many samples from the beginning of

pipe.
) = 0;

/// Returns number of samples currently available.
virtual uint numSamples() const = 0;

// Returns nonzero if there aren't any samples available for outputting.
virtual int isEmpty() const = 0;

/// Clears all the samples.
virtual void clear() = 0;
}

这里没有实现FIFOSamplePipe类的构造函数，因此系统隐性的调用了默认的自动生成的FIFOSamplePipe()。当然他应该没有做任何的初始化，同样也不需要做任何的初始化。通过定义virtual ~FIFOSamplePipe() {}虚析构函数，使得new一个子类，例如：FIFOSamplePipe* a = new FIFOProcessor，当a销毁的时候都会执行子类FIFOProcessor的析构函数，保证不管多少层继承都会一次过全部销毁，这是作为一个基类的特点。类的继承和多态果然是C++最为强悍的一部分，有助于编写重复性很高的类。通过看这个基类的声明，我们可以留意到除了定义大多数虚函数之外，他唯独实现了moveSamples这个函数，也就是子类如果没有override moveSamples，都将调用这个方法。他做的处理也相对来说很简单，根据注释，我们不难理解，正是这个函数实现了各个派生类之间的数据共享传递的接口。
// Moves samples from the 'other' pipe instance to this instance.
moveSamples(FIFOSamplePipe &other ///< Other pipe instance where from the receive the data.
)
{
int oNumSamples = other.numSamples();

putSamples(other.ptrBegin(), oNumSamples);
other.receiveSamples(oNumSamples);
};
在创建SoundTouch类之前，经过(流程[1])的前面两个步骤，他们都隐形的调用了默认的析构函数，由于基类FIFOSamplePipe没有实现构造函数，我们可以默认他不做任何的初始化，然后FIFOProcessor简单的把成员变量FIFOSamplePipe *output;一个指向基类的指针赋值简单做了一下初始化，让他指向NULL。
FIFOProcessor()
{
output = NULL;
}
现在回到SoundTouch的构造函数，在构造完前面两个类之后，他终于可以调用自己的默认构造函数
SoundTouch::SoundTouch()
{
// Initialize rate transposer and tempo changer instances
pRateTransposer = RateTransposer::newInstance();
pTDStretch = TDStretch::newInstance();
setOutPipe(pTDStretch);
rate = tempo = 0;
virtualPitch =
virtualRate =
virtualTempo = 1.0;
calcEffectiveRateAndTempo();
channels = 0;
bSrateSet = FALSE;
}
看一下SoundTouch类的成员变量
class SoundTouch : public FIFOProcessor
{
private:
/// Rate transposer class instance
class RateTransposer *pRateTransposer;

/// Time-stretch class instance
class TDStretch *pTDStretch;

/// Virtual pitch parameter. Effective rate & tempo are calculated from these parameters.
float virtualRate;

/// Virtual pitch parameter. Effective rate & tempo are calculated from these parameters.
float virtualTempo;

/// Virtual pitch parameter. Effective rate & tempo are calculated from these parameters.
float virtualPitch;

/// Flag: Has sample rate been set?
BOOL bSrateSet;

/// Calculates effective rate & tempo valuescfrom 'virtualRate', 'virtualTempo' and
/// 'virtualPitch' parameters.
void calcEffectiveRateAndTempo();

protected :
/// Number of channels
uint channels;

/// Effective 'rate' value calculated from 'virtualRate', 'virtualTempo' and 'virtualPitch'
float rate;

/// Effective 'tempo' value calculated from 'virtualRate', 'virtualTempo' and 'virtualPitch'
float tempo;
...
根据构造函数他实例化pRateTransposer,pTDStretch这两个类。
首先看一下RateTransposer类的成员函数newInstance，通过一个宏定义INTEGER_SAMPLES来new一个定点还是浮点处理的类，马上就可以判断不管RateTransposerInteger还是RateTransposerFloat都应该直接从RateTransposer派生。（假设INTEGER_SAMPLES被定义）他将构造一个RateTransposerInteger。
RateTransposer *RateTransposer::newInstance()
{
#ifdef INTEGER_SAMPLES
return ::new RateTransposerInteger;
#else
return ::new RateTransposerFloat;
#endif
}
看一下RateTransposerInteger类的定义，不出所料果然由RateTransposer派生
class RateTransposer : public FIFOProcessor
{
protected:
...
FIFOSampleBuffer storeBuffer;

/// Buffer for keeping samples between transposing & anti-alias filter
FIFOSampleBuffer tempBuffer;

/// Output sample buffer
FIFOSampleBuffer outputBuffer;
...
上诉两个类他们和基类之间存在这样的关系：
FIFOSamplePipe->FIFOProcessor->RateTransposer->RateTransposerInteger
这里的构造过程不同的是：RateTransposer::RateTransposer() : FIFOProcessor(&outputBuffer)
RateTransposer构造函数指明了父类FIFOProcessor的构造形式FIFOProcessor(&outputBuffer)
FIFOProcessor(FIFOSamplePipe *pOutput ///< Output pipe.
)
{
output = pOutput;
}
RateTransposer把类成员变量outputBuffer作为传递函数参数，这里可能大家就会很奇怪，代码里面根本还没有实例化RateTransposer类，他怎么可能存在一个FIFOSampleBuffer outputBuffer;其实正是体现了c++的多态性，这里传入的实际上是一个__vfptr数组，这个数组就是指向实例化各个派生类的这个变量的指针数组。这下子明白了。__vfptr[0]不一定有值，但是__vfptr肯定是一个存在的值。构造完FIFOProcessor，此时要构造RateTransposer，他有三个FIFOSampleBuffer类定义。
...
class FIFOSampleBuffer : public FIFOSamplePipe
...
与基类的继承关系
FIFOSamplePipe->FIFOSampleBuffer
/// Constructor
FIFOSampleBuffer(int numChannels = 2 ///< Number of channels, 1=mono, 2=stereo.
///< Default is stereo.
);
他没有定义不带参的构造函数，因此这个带参数的构造函数将以默认的方式给调用
FIFOSampleBuffer::FIFOSampleBuffer(int numChannels)
{
assert(numChannels > 0);
sizeInBytes = 0; // reasonable initial value
buffer = NULL;
bufferUnaligned = NULL;
samplesInBuffer = 0;
bufferPos = 0;
channels = (uint)numChannels;
ensureCapacity(32); // allocate initial capacity
}
FIFOSampleBuffer的构造函数将被调用三次。
现在终于可以执行RateTransposer的构造函数
// Constructor
RateTransposer::RateTransposer() : FIFOProcessor(&outputBuffer)
{
numChannels = 2;
bUseAAFilter = TRUE;
fRate = 0;

// Instantiates the anti-alias filter with default tap length
// of 32
pAAFilter = new AAFilter(32);
}
首先看一下AAFilter的相关定义
class AAFilter
{
protected:
class FIRFilter *pFIR;
/// Low-pass filter cut-off frequency, negative = invalid
double cutoffFreq;
/// num of filter taps
uint length;
/// Calculate the FIR coefficients realizing the given cutoff-frequency
void calculateCoeffs();
public:
AAFilter(uint length);
~AAFilter();
/// Sets new anti-alias filter cut-off edge frequency, scaled to sampling
/// frequency (nyquist frequency = 0.5). The filter will cut off the
/// frequencies than that.
void setCutoffFreq(double newCutoffFreq);
/// Sets number of FIR filter taps, i.e. ~filter complexity
void setLength(uint newLength);
uint getLength() const;
/// Applies the filter to the given sequence of samples.
/// Note : The amount of outputted samples is by value of 'filter length'
/// smaller than the amount of input samples.
uint evaluate(SAMPLETYPE *dest,
const SAMPLETYPE *src,
uint numSamples,
uint numChannels) const;
};
在其构造函数中初始化了一个指向class FIRFilter的指针
AAFilter::AAFilter(uint len)
{
pFIR = FIRFilter::newInstance();
cutoffFreq = 0.5;
setLength(len);
}
首先我们看看FIRFilter类成员函数newInstance()，嘿嘿，在这里我们发现了一个非常有用的函数detectCPUextensions();通过这个函数我们可以判断cpu到底支持什么类型的多媒体指令集。根据注释我们也可以很快理解。detectCPUextensions收藏了。他的实现就在Cpu_detect_x86_win.cpp的实现中。美中不足的是，他只能检测x86结构体系的CPU。可能我多想了。根据本人电脑的配置（采用的赛扬cpu），所以只支持mmx指令。
FIRFilter * FIRFilter::newInstance()
{
uint uExtensions;
uExtensions = detectCPUextensions();
// Check if MMX/SSE/3DNow! instruction set extensions supported by CPU
#ifdef ALLOW_MMX
// MMX routines available only with integer sample types
if (uExtensions & SUPPORT_MMX)
{
return ::new FIRFilterMMX;
}
else
#endif // ALLOW_MMX
#ifdef ALLOW_SSE
if (uExtensions & SUPPORT_SSE)
{
// SSE support
return ::new FIRFilterSSE;
}
else
#endif // ALLOW_SSE
#ifdef ALLOW_3DNOW
if (uExtensions & SUPPORT_3DNOW)
{
// 3DNow! support
return ::new FIRFilter3DNow;
}
else
#endif // ALLOW_3DNOW
{
// ISA optimizations not supported, use plain C version
return ::new FIRFilter;
}
}
为此他将通过这个判断构造返回一个FIRFilterMMX类
if (uExtensions & SUPPORT_MMX)
{
return ::new FIRFilterMMX;
}
查看FIRFilterMMX的类定义class FIRFilterMMX : public FIRFilter，他从FIRFilter派生。成员函数uint FIRFilterMMX::evaluateFilterStereo引起了我的高度注意，主要的算法采用MMX指令集来完成某些声音计算。这个就是我们需要的Rate的核心算法。不同指令集的实现，可以参考FIRFilter3DNow，FIRFilterSSE，默认是FIRFilter的evaluateFilterStereo函数的实现。
// mmx-optimized version of the filter routine for stereo sound
uint FIRFilterMMX::evaluateFilterStereo(short *dest, const short *src, uint numSamples) const
{
// Create stack copies of the needed member variables for asm routines :
uint i, j;
__m64 *pVdest = (__m64*)dest;

if (length < 2) return 0;

for (i = 0; i < (numSamples - length) / 2; i ++)
{
__m64 accu1;
__m64 accu2;
const __m64 *pVsrc = (const __m64*)src;
const __m64 *pVfilter = (const __m64*)filterCoeffsAlign;

accu1 = accu2 = _mm_setzero_si64();
for (j = 0; j < lengthDiv8 * 2; j ++)
{
__m64 temp1, temp2;

temp1 = _mm_unpacklo_pi16(pVsrc[0], pVsrc[1]); // = l2 l0 r2 r0
temp2 = _mm_unpackhi_pi16(pVsrc[0], pVsrc[1]); // = l3 l1 r3 r1

accu1 = _mm_add_pi32(accu1, _mm_madd_pi16(temp1, pVfilter[0])); // += l2*f2+l0*f0

r2*f2+r0*f0
accu1 = _mm_add_pi32(accu1, _mm_madd_pi16(temp2, pVfilter[1])); // += l3*f3+l1*f1

r3*f3+r1*f1

temp1 = _mm_unpacklo_pi16(pVsrc[1], pVsrc[2]); // = l4 l2 r4 r2

accu2 = _mm_add_pi32(accu2, _mm_madd_pi16(temp2, pVfilter[0])); // += l3*f2+l1*f0

r3*f2+r1*f0
accu2 = _mm_add_pi32(accu2, _mm_madd_pi16(temp1, pVfilter[1])); // += l4*f3+l2*f1

r4*f3+r2*f1

// accu1 += l2*f2+l0*f0 r2*f2+r0*f0
// += l3*f3+l1*f1 r3*f3+r1*f1

// accu2 += l3*f2+l1*f0 r3*f2+r1*f0
// l4*f3+l2*f1 r4*f3+r2*f1

pVfilter += 2;
pVsrc += 2;
}
// accu >>= resultDivFactor
accu1 = _mm_srai_pi32(accu1, resultDivFactor);
accu2 = _mm_srai_pi32(accu2, resultDivFactor);

// pack 2*2*32bits => 4*16 bits
pVdest[0] = _mm_packs_pi32(accu1, accu2);
src += 4;
pVdest ++;
}

_m_empty(); // clear emms state

return (numSamples & 0xfffffffe) - length;
}
因此，如果把SoundTouch移植到arm等没有多媒体指令集的CPU时,应使用FIRFilter的evaluateFilterStere函数。执行完这里，终于可以真正意义上构造我们的RateTransposerInteger()。在构造函数中：
RateTransposerInteger::RateTransposerInteger() : RateTransposer()
{
// Notice: use local function calling syntax for sake of clarity,
// to indicate the fact that C++ constructor can't call virtual functions.
RateTransposerInteger::resetRegisters();
RateTransposerInteger::setRate(1.0f);
}进行了一些必要的初始化。至此pRateTransposer = RateTransposer::newInstance();实例化完毕。至于pTDStretch = TDStretch::newInstance();下回分晓。