ASIO 获取音频参数init_asio_static_data函数

要打开音频设备，总得先获取参数吧。
比如UAC设备，在打开MIC时，系统会通过特定类请求设置采样率，这样固件会源源不断地通过此采样率获取音频数据，然后通过USB要同步传输将数据发送给主机。
在ASIO中，获取ASIO设备的参数有几个函数，详情可见：ASIO 函数功能介绍 。

ASIO的测试用例是通过init_asio_static_data()函数来初始化ASIO设备，然后打开设备进行MIC数据采样的，其主要流程如下：

• 通过ASIOGetChannels获取通道数
• 通过ASIOGetBufferSize获取数据缓冲区大小
• 通过ASIOGetSampleRate获取ASIO设备支持的采样率
• 如果采样率过大或过小，则通过ASIOSetSampleRate设置工作时的采样率为44100。
• 通过ASIOOutputReady判断输出通道是否准备好

当然，获取的这些数据是需要保存在内存中，以供在实际获取音频数据时的使用，所以这些参数包存在DriverInfo结构体中。

// internal data storage
typedef struct DriverInfo
{
    // ASIOInit()
    ASIODriverInfo driverInfo;

    // ASIOGetChannels()
    long           inputChannels;
    long           outputChannels;

    // ASIOGetBufferSize()
    long           minSize;
    long           maxSize;
    long           preferredSize;
    long           granularity;

    // ASIOGetSampleRate()
    ASIOSampleRate sampleRate;

    // ASIOOutputReady()
    bool           postOutput;

    // ASIOGetLatencies ()
    long           inputLatency;
    long           outputLatency;

    // ASIOCreateBuffers ()
    long inputBuffers;    // becomes number of actual created input buffers
    long outputBuffers;    // becomes number of actual created output buffers
    ASIOBufferInfo bufferInfos[kMaxInputChannels + kMaxOutputChannels]; // buffer info's

    // ASIOGetChannelInfo()
    ASIOChannelInfo channelInfos[kMaxInputChannels + kMaxOutputChannels]; // channel info's
    // The above two arrays share the same indexing, as the data in them are linked together

    // Information from ASIOGetSamplePosition()
    // data is converted to double floats for easier use, however 64 bit integer can be used, too
    double         nanoSeconds;
    double         samples;
    double         tcSamples;    // time code samples

    // bufferSwitchTimeInfo()
    ASIOTime       tInfo;            // time info state
    unsigned long  sysRefTime;      // system reference time, when bufferSwitch() was called

    // Signal the end of processing in this example
    bool           stopped;
} DriverInfo;

代码流程如下：

long init_asio_static_data (DriverInfo *asioDriverInfo)
{    // collect the informational data of the driver
    // get the number of available channels
    if(ASIOGetChannels(&asioDriverInfo->inputChannels, &asioDriverInfo->outputChannels) == ASE_OK)
    {
        printf ("ASIOGetChannels (inputs: %d, outputs: %d);\n", asioDriverInfo->inputChannels, asioDriverInfo->outputChannels);

        // get the usable buffer sizes
        if(ASIOGetBufferSize(&asioDriverInfo->minSize, &asioDriverInfo->maxSize, &asioDriverInfo->preferredSize, &asioDriverInfo->granularity) == ASE_OK)
        {
            printf ("ASIOGetBufferSize (min: %d, max: %d, preferred: %d, granularity: %d);\n",
                     asioDriverInfo->minSize, asioDriverInfo->maxSize,
                     asioDriverInfo->preferredSize, asioDriverInfo->granularity);

            // get the currently selected sample rate
            if(ASIOGetSampleRate(&asioDriverInfo->sampleRate) == ASE_OK)
            {
                printf ("ASIOGetSampleRate (sampleRate: %f);\n", asioDriverInfo->sampleRate);
                if (asioDriverInfo->sampleRate <= 0.0 || asioDriverInfo->sampleRate > 96000.0)
                {
                    // Driver does not store it's internal sample rate, so set it to a know one.
                    // Usually you should check beforehand, that the selected sample rate is valid
                    // with ASIOCanSampleRate().
                    if(ASIOSetSampleRate(44100.0) == ASE_OK)
                    {
                        if(ASIOGetSampleRate(&asioDriverInfo->sampleRate) == ASE_OK)
                            printf ("ASIOGetSampleRate (sampleRate: %f);\n", asioDriverInfo->sampleRate);
                        else
                            return -6;
                    }
                    else
                        return -5;
                }

                // check wether the driver requires the ASIOOutputReady() optimization
                // (can be used by the driver to reduce output latency by one block)
                if(ASIOOutputReady() == ASE_OK)
                    asioDriverInfo->postOutput = true;
                else
                    asioDriverInfo->postOutput = false;
                printf ("ASIOOutputReady(); - %s\n", asioDriverInfo->postOutput ? "Supported" : "Not supported");

                return 0;
            }
            return -3;
        }
        return -2;
    }
    return -1;
}

运行结果如下：

asioVersion:   0
driverVersion: 1
Name:          Sample ASIO
ErrorMessage:  No ASIO Driver Error
ASIOGetChannels (inputs: 16, outputs: 16);
ASIOGetBufferSize (min: 256, max: 256, preferred: 256, granularity: 0);
ASIOGetSampleRate (sampleRate: 44100.000000);
ASIOOutputReady(); - Not supported

获取通道数量ASIOGetChannels

ASIOError ASIOGetChannels(long *numInputChannels, long *numOutputChannels)
{
    if(!theAsioDriver)
    {
        *numInputChannels = *numOutputChannels = 0;
        return ASE_NotPresent;
    }
    return theAsioDriver->getChannels(numInputChannels, numOutputChannels);
}

由COM实始化：

ASIOError AsioSample::getChannels (long *numInputChannels, long *numOutputChannels)
{
    *numInputChannels = kNumInputs;
    *numOutputChannels = kNumOutputs;
    return ASE_OK;
}
enum
{
    kBlockFrames = 256,
    kNumInputs = 16,
    kNumOutputs = 16
};

获取缓冲区参数ASIOGetBufferSize

ASIOError ASIOGetBufferSize(long *minSize, long *maxSize, long *preferredSize, long *granularity)
{
    if(!theAsioDriver)
    {
        *minSize = *maxSize = *preferredSize = *granularity = 0;
        return ASE_NotPresent;
    }
    return theAsioDriver->getBufferSize(minSize, maxSize, preferredSize, granularity);
}

由COM初始化

ASIOError AsioSample::getBufferSize (long *minSize, long *maxSize,
    long *preferredSize, long *granularity)
{
    *minSize = *maxSize = *preferredSize = blockFrames;        // allow this size only
    *granularity = 0;
    return ASE_OK;
}

颗粒度granularity=0，而blockFrames=256，由构造函数实现

AsioSample::AsioSample (LPUNKNOWN pUnk, HRESULT *phr)
    : CUnknown("ASIOSAMPLE", pUnk, phr)
{

    blockFrames = kBlockFrames;
    inputLatency = blockFrames;        // typically
    outputLatency = blockFrames * 2;
}
enum
{
    kBlockFrames = 256,
    kNumInputs = 16,
    kNumOutputs = 16
};

获取采样率ASIOGetSampleRate

ASIOError ASIOGetSampleRate(ASIOSampleRate *currentRate)
{
    if(!theAsioDriver)
        return ASE_NotPresent;
    return theAsioDriver->getSampleRate(currentRate);
}

而COM初始化为

ASIOError AsioSample::getSampleRate (ASIOSampleRate *sampleRate)
{
    *sampleRate = this->sampleRate;
    return ASE_OK;
}

构造函数将将采样率设为:

AsioSample::AsioSample (LPUNKNOWN pUnk, HRESULT *phr)
    : CUnknown("ASIOSAMPLE", pUnk, phr)
{
    ...
    sampleRate = 44100.
}

模拟仿真判断，使用ASIOOutputReady判断

ASIOError ASIOOutputReady(void)
{
    if(!theAsioDriver)
        return ASE_NotPresent;
    return theAsioDriver->outputReady();
}
ASIOError AsioSample::outputReady ()
{
    return ASE_NotPresent;
}
enum {
    ASE_OK = 0,             // This value will be returned whenever the call succeeded
    ASE_SUCCESS = 0x3f4847a0,    // unique success return value for ASIOFuture calls
    ASE_NotPresent = -1000, // hardware input or output is not present or available
    ASE_HWMalfunction,      // hardware is malfunctioning (can be returned by any ASIO function)
    ASE_InvalidParameter,   // input parameter invalid
    ASE_InvalidMode,        // hardware is in a bad mode or used in a bad mode
    ASE_SPNotAdvancing,     // hardware is not running when sample position is inquired
    ASE_NoClock,            // sample clock or rate cannot be determined or is not present
    ASE_NoMemory            // not enough memory for completing the request
};

这里表示的是软件模拟的，即硬件输出输入不存在，故处于测试态。所以执行

    asioDriverInfo->postOutput = false;

并输出：

ASIOOutputReady(); - Not supported