Noise/distortion after doing filters with vDSP_deq22 (biquad IIR filter)

Question

I'm working on a DSP class (obj-c++) for Novocaine, but my filters only seem to cause noise/distortion on the signal. I've posted my full code and coefficients here: https://gist.github.com/2702844 But it basically boils down to: ``` // 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) ``` See the gist for full code/context. The coefficients: ``` 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 ``` (all divided by `a0`) Since I presumed the coe…

Accepted Answer

Update: I recommend everyone to use my DSP class I released on github: https://github.com/bartolsthoorn/NVDSP It'll probably save you quite some work.

Got it working, woooo! Long live the japanese: http://objective-audio.jp/2008/02/biquad-filter.html

The applyFilter method had to be:

```
- (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);
}
```

Full class: https://github.com/bartolsthoorn/NVDSP

Noise/distortion after doing filters with vDSP_deq22 (biquad IIR filter)

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