使用vDSP_deq22进行滤波后的噪声/失真(双二阶IIR滤波器)

Question

我正在为Novocaine开发一个DSP类(obj-c++)，但我的滤波器似乎只对信号造成噪声/失真。

我已经在这里发布了我的完整代码和系数：https://gist.github.com/2702844，但它基本上归结为：

// Deinterleaving...
// DSP'ing one channel:
NVDSP *handleDSP = [[NVDSP alloc] init];
[handleDSP setSamplingRate:audioManager.samplingRate];
float cornerFrequency = 6000.0f;
float Q = 0.5f;
[handleDSP setHPF:cornerFrequency Q:Q];
[handleDSP applyFilter:audioData length:numFrames];

// DSP other channel in the same way
// Interleaving and sending to audio output (Novocaine block)

有关完整的代码/上下文，请参阅要点。

系数：

2012-05-15 17:54:18.858 nvdsp[700:16703] b0: 0.472029
2012-05-15 17:54:18.859 nvdsp[700:16703] b1: -0.944059
2012-05-15 17:54:18.860 nvdsp[700:16703] b2: 0.472029
2012-05-15 17:54:18.861 nvdsp[700:16703] a1: -0.748175
2012-05-15 17:54:18.861 nvdsp[700:16703] a2: 0.139942

(全部除以a0)

因为我假设系数的顺序是：{ b0/a0, b1/a0, b2/a0, a1/a0, a2/a0 } (参见：IIR coefficients for peaking EQ, how to pass them to vDSP_deq22?)

是什么导致失真/噪声(滤波器不工作)？

Answer 1

我建议大家使用我在github：https://github.com/bartolsthoorn/NVDSP上发布的类，它可能会帮你节省很多工作。

让它工作了，呼！日本万岁：http://objective-audio.jp/2008/02/biquad-filter.html

applyFilter方法必须是：

- (void) applyFilter: (float *)data frames:(NSUInteger)frames {
    /*
     The first two samples of data being passed to vDSP_deq22 have to be initialized from the previous call. So, you'd want to hold onto a float buffer and feed the tailing two samples after a vDSP_deq22 call back to the front of that array for the next time you call. (Alex Wiltschko)
     */

    // Thanks a lot to: http://objective-audio.jp/2008/02/biquad-filter.html

    // Provide buffer for processing
    float *tInputBuffer = (float*) malloc((frames + 2) * sizeof(float));
    float *tOutputBuffer = (float*) malloc((frames + 2) * sizeof(float));

    // Copy the data
    memcpy(tInputBuffer, gInputKeepBuffer, 2 * sizeof(float));
    memcpy(tOutputBuffer, gOutputKeepBuffer, 2 * sizeof(float));
    memcpy(&(tInputBuffer[2]), data, frames * sizeof(float));

    // Do the processing
    vDSP_deq22(tInputBuffer, 1, coefficients, tOutputBuffer, 1, frames);

    // Copy the data
    memcpy(data, tOutputBuffer, frames * sizeof(float));
    memcpy(gInputKeepBuffer, &(tInputBuffer[frames]), 2 * sizeof(float));
    memcpy(gOutputKeepBuffer, &(tOutputBuffer[frames]), 2 * sizeof(float));

    free(tInputBuffer);
    free(tOutputBuffer);
}

完整类：https://github.com/bartolsthoorn/NVDSP

Answer 2

使用以下命令复制数据：

memcpy(data, tOutputBuffer+2, frames * sizeof(float));

它会起作用的