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https://github.com/bartolsthoorn/NVDSP

iOS/OSX DSP for audio (with Novocaine)
https://github.com/bartolsthoorn/NVDSP

audio-filters dsp ios novocaine

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iOS/OSX DSP for audio (with Novocaine)

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README 

        
## Audio Filters on iOS and OSX



Implement high quality audio filters with just a few lines of code and [Novocaine](https://github.com/alexbw/novocaine), or your own audio library of choice.



NVDSP comes with a wide variety of audio filters:



+ All Pass Filter (NVAllpassFilter)

+ Band Pass Filter, 0dB gain (NVBandpassFilter)

+ Band Pass Filter, Q gain (NVBandpassQPeakGainFilter)

+ High Pass Filter (NVHighpassFilter)

+ High Shelving Filter (NVHighShelvingFilter)

+ Low Shelving Filter (NVLowShelvingFilter)

+ Low Pass Filter (NVLowPassFilter)

+ Notch Filter (NVNotchFilter)

+ Peaking EQ Filter (NVPeakingEQFilter)



### Combining it with Novocaine (highpass filter)

To start out I recommend you to get a fresh copy of [Novocaine](https://github.com/alexbw/novocaine) and open Novocaine's excellent example project. Then import NVDSP and the Filters folder and start your filtering journey.

``` objective-c

// ... import Novocaine here ... 

#import "NVDSP/NVDSP.h"

#import "NVDSP/Filters/NVHighpassFilter.h"



// init Novocaine audioManager

audioManager = [Novocaine audioManager];

float samplingRate = audioManager.samplingRate;



// init fileReader which we will later fetch audio from

NSURL *inputFileURL = [[NSBundle mainBundle] URLForResource:@"Trentemoller-Miss-You" withExtension:@"mp3"];



fileReader = [[AudioFileReader alloc] 

                  initWithAudioFileURL:inputFileURL 

                  samplingRate:audioManager.samplingRate

                  numChannels:audioManager.numOutputChannels];



// setup Highpass filter

NVHighpassFilter *HPF;

HPF = [[NVHighpassFilter alloc] initWithSamplingRate:samplingRate];



HPF.cornerFrequency = 2000.0f;

HPF.Q = 0.5f;



// setup audio output block

[fileReader play];

[audioManager setOutputBlock:^(float *outData, UInt32 numFrames, UInt32 numChannels) {

    [fileReader retrieveFreshAudio:outData numFrames:numFrames numChannels:numChannels];

    

    [HPF filterData:outData numFrames:numFrames numChannels:numChannels];

}];

```

Note that NVDSP works with raw audio buffers, so it can also work with other libraries instead of Novocaine.



### More examples

#### Peaking EQ filter

``` objective-c

// import Novocaine.h and NVDSP.h

#import "NVDSP/Filter/NVPeakingEQFilter.h"

NVPeakingEQFilter *PEF = [[NVPeakingEQFilter alloc] initWithSamplingRate:audioManager.samplingRate];

PEF.centerFrequency = 1000.0f;

PEF.Q = 3.0f;

PEF.G = 20.0f;

[PEF filterData:data numFrames:numFrames numChannels:numChannels];

```



#### Lowpass filter

``` objective-c

// import Novocaine.h and NVDSP.h

#import "NVDSP/Filter/NVLowpassFilter.h"

NVLowpassFilter *LPF = [[NVLowpassFilter alloc] initWithSamplingRate:audioManager.samplingRate];

LPF.cornerFrequency = 800.0f;

LPF.Q = 0.8f;

[LPF filterData:data numFrames:numFrames numChannels:numChannels];

```



#### Notch filter

``` objective-c

// import Novocaine.h and NVDSP.h

#import "NVDSP/Filter/NVNotchFilter.h"

NVNotchFilter *NF = [[NVNotchFilter alloc] initWithSamplingRate:audioManager.samplingRate];

NF.centerFrequency = 3000.0f;

NF.Q = 0.8f;

[NF filterData:data numFrames:numFrames numChannels:numChannels];

```



#### Bandpass filter

There are two types of bandpass filters:



    * 0 dB gain bandpass filter (NVBandpassFilter.h)

    * Peak gain Q bandpass filter (NVBandpassQPeakGainFilter.h)



``` objective-c

// import Novocaine.h and NVDSP.h

#import "NVDSP/Filter/NVBandpassFilter.h"

NVBandpassFilter *BPF = [[NVBandpassFilter alloc] initWithSamplingRate:audioManager.samplingRate];

BPF.centerFrequency = 2500.0f;

BPF.Q = 0.9f;

[BPF filterData:data numFrames:numFrames numChannels:numChannels];

```



#### Measure dB level (ranging from -51.0f to 0.0f)

``` objective-c

// import Novocaine.h and NVDSP.h

#import "NVDSP/Utilities/NVSoundLevelMeter.h"

NVSoundLevelMeter *SLM = [[NVSoundLevelMeter alloc] init];

float dB = [SLM getdBLevel:outData numFrames:numFrames numChannels:numChannels];

NSLog(@"dB level: %f", dB);

// NSLogging in an output loop will most likely cause hickups/clicky noises, but it does log the dB level!

// To get a proper dB value, you have to call the getdBLevel method a few times (it has memory of previous values)

// You call this inside the input or outputBlock: [audioManager setOutputBlock:^...

```



#### Applying overall gain. 

All sample values (typically -1.0f .. 1.0f when not clipping) are multiplied by the gain value.

``` objective-c

// import Novocaine.h and NVDSP.h

NVDSP *generalDSP = [[NVDSP alloc] init];

[generalDSP applyGain:outData length:numFrames*numChannels gain:0.8];

```



#### Convert stereo (left/right) to mono

This converts a left and right buffer into a mono signal. It takes the average of the samples.

```objective-c

// Deinterleave stereo buffer into seperate left and right

float *left = (float *)malloc((numFrames + 2) * sizeof(float));

float *right = (float *)malloc((numFrames + 2) * sizeof(float));

[generalDSP deinterleave:data left:left right:right length:numFrames];



// Convert left and right to a mono 2 channel buffer

[generalDSP mono:data left:left right:right length:numFrames];



// Free buffers

free(left);

free(right);

```



### Clipping

Multiple peaking EQs with high gains can cause clipping. Clipping is basically sample data that exceeds the maximum or minimum value of 1.0f or -1.0f respectively. Clipping will cause really loud and dirty noises, like a bad overdrive effect. You can use the method `counterClipping` to prevent clipping (it will reduce the sound level).



``` objective-c

// import Novocaine.h and NVDSP.h

#import "NVDSP/Utilities/NVClippingDetection.h"

NVClippingDetection *CDT = [[NVClippingDetection alloc] init];

// ... possible clipped outData ...//

[CDT counterClipping:outData numFrames:numFrames numChannels:numChannels];

// ... outData is now safe ...//



// or get the amount of clipped samples:

 - (float) getClippedSamples:(float *)data numFrames:(UInt32)numFrames numChannels:(UInt32)numChannels;

// or get the percentage of clipped samples:

 - (float) getClippedPercentage:(float*)data numFrames:(UInt32)numFrames numChannels:(UInt32)numChannels;

// or get the maximum value of a clipped sample that was found

 - (float) getClippingSample:(float *)data numFrames:(UInt32)numFrames numChannels:(UInt32)numChannels;

```



### Example project

See `/Examples/NVDSPExample` for a simple iOS XcodeProject example. Please note the Novocaine bundled with it might be outdated.



### A thing to note

The NVDSP class is written in C++, so the classes that use it will have to be Objective-C++. Change all the files that use NVDSP from MyClass.m to MyClass.mm.



### Thanks to

Alex Wiltschko - Creator of [Novocaine](http://alexbw.github.com/novocaine/)



Yasoshima - Writer of [this article](http://objective-audio.jp/2008/02/biquad-filter.html), revealing how vDSP_deq22 works. (and google translate, I don't speak Japanese)



hrnt - Helpful on IRC #iphonedev (freenode)