Low Pass filter + sample rate conversion using Avaudioengine iOS - swift

Question

We are working on a project which allows us to record some sounds from a microphone with a 5k Hz sample rate with some Low-Pass filter & HighPass filter.
What we are using
We are using AvaudioEngine for this purpose.
We are using AVAudioConverter for downgrading the sample rate.
We are using AVAudioUnitEQ for the LowPass & HighPass filter.
Code
let bus = 0
let inputNode = engine.inputNode
let equalizer = AVAudioUnitEQ(numberOfBands: 2)
equalizer.bands[0].filterType = .lowPass
equalizer.bands[0].frequency = 3000
equalizer.bands[0].bypass = false
equalizer.bands[1].filterType = .highPass
equalizer.bands[1].frequency = 1000
equalizer.bands[1].bypass = false
engine.attach(equalizer) //Attach equalizer
// Connect nodes
engine.connect(inputNode, to: equalizer, format: inputNode.inputFormat(forBus: 0))
engine.connect(equalizer, to: engine.mainMixerNode, format: inputNode.inputFormat(forBus: 0))
engine.connect(engine.mainMixerNode, to: engine.outputNode, format: inputNode.inputFormat(forBus: 0))
let outputFormat = AVAudioFormat(commonFormat: .pcmFormatInt16,
sampleRate: 5000,
channels: 1,
interleaved: false)!
// Converter to downgrade sample rate
guard let converter: AVAudioConverter = AVAudioConverter(from: inputNode.inputFormat(forBus: 0), to: outputFormat) else {
print("Can't convert in to this format")
return
}
engine.mainMixerNode.installTap(onBus: bus, bufferSize: 2688, format: engine.mainMixerNode.outputFormat(forBus: 0)) { (buffer, time) in
var newBufferAvailable = true
let inputCallback: AVAudioConverterInputBlock = { inNumPackets, outStatus in
if newBufferAvailable {
outStatus.pointee = .haveData
newBufferAvailable = false
return buffer
} else {
outStatus.pointee = .noDataNow
return nil
}
}
let convertedBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat, frameCapacity: AVAudioFrameCount(outputFormat.sampleRate) * buffer.frameLength / AVAudioFrameCount(buffer.format.sampleRate))!
var error: NSError?
let status = converter.convert(to: convertedBuffer, error: &error, withInputFrom: inputCallback)
assert(status != .error)
if status == .haveData {
// Process with converted buffer
}
}
engine.prepare()
do {
try engine.start()
} catch {
print("Can't start the engine: \(error)")
}
Issue
low-pass and high-pass filters are not working.
Alternate Approach
To check code is working or not, we have added a reverb effect instead of lowpass filter.
Reverb effect(Using AVAudioUnitReverb) works with same code.
Can anyone help me where are we doing wrong in applying lowpass filter?

Answer 1

I think the main problem with this code was that the AVAudioConverter was being created before calling engine.prepare() which can and will change the mainMixerNode output format. Aside from that, there was a redundant connection of mainMixerNode to outputNode, along with a probably incorrect format - mainMixerNode is documented to be automatically created and connected to the output node "on demand". The tap also did not need a format.
let bus = 0
let inputNode = engine.inputNode
let equalizer = AVAudioUnitEQ(numberOfBands: 2)
equalizer.bands[0].filterType = .lowPass
equalizer.bands[0].frequency = 3000
equalizer.bands[0].bypass = false
equalizer.bands[1].filterType = .highPass
equalizer.bands[1].frequency = 1000
equalizer.bands[1].bypass = false
engine.attach(equalizer) //Attach equalizer
// Connect nodes
engine.connect(inputNode, to: equalizer, format: inputNode.inputFormat(forBus: 0))
engine.connect(equalizer, to: engine.mainMixerNode, format: inputNode.inputFormat(forBus: 0))
// call before creating converter because this changes the mainMixer's output format
engine.prepare()
let outputFormat = AVAudioFormat(commonFormat: .pcmFormatInt16,
sampleRate: 5000,
channels: 1,
interleaved: false)!
// Downsampling converter
guard let converter: AVAudioConverter = AVAudioConverter(from: engine.mainMixerNode.outputFormat(forBus: 0), to: outputFormat) else {
print("Can't convert in to this format")
return
}
engine.mainMixerNode.installTap(onBus: bus, bufferSize: 2688, format: nil) { (buffer, time) in
var newBufferAvailable = true
let inputCallback: AVAudioConverterInputBlock = { inNumPackets, outStatus in
if newBufferAvailable {
outStatus.pointee = .haveData
newBufferAvailable = false
return buffer
} else {
outStatus.pointee = .noDataNow
return nil
}
}
let convertedBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat, frameCapacity: AVAudioFrameCount(outputFormat.sampleRate) * buffer.frameLength / AVAudioFrameCount(buffer.format.sampleRate))!
var error: NSError?
let status = converter.convert(to: convertedBuffer, error: &error, withInputFrom: inputCallback)
assert(status != .error)
if status == .haveData {
// Process with converted buffer
}
}
do {
try engine.start()
} catch {
print("Can't start the engine: \(error)")
}

Answer 2

override func viewDidLoad() {
super.viewDidLoad()
lableItem.text = "Select Frequency"
setUpDropdown()
navigationItem.title = "High Pass Filter"
do{
try audioSession.setCategory(.playAndRecord, mode: .default, options: [.mixWithOthers, .defaultToSpeaker,.allowBluetoothA2DP,.allowAirPlay,.allowBluetooth])
try audioSession.setActive(true)
} catch{
print(error.localizedDescription)
}
let bus = 0
let inputNode = engine.inputNode
let equalizer = AVAudioUnitEQ(numberOfBands: 2)
equalizer.bands[0].filterType = .highPass
equalizer.bands[0].frequency = 20000.0
equalizer.bands[0].bypass = false
equalizer.bands[1].filterType = .lowPass
equalizer.bands[1].frequency = 1000.0
equalizer.bands[1].bypass = false
engine.attach(equalizer) //Attach equalizer
// Connect nodes
engine.connect(inputNode, to: equalizer, format: inputNode.inputFormat(forBus: 0))
engine.connect(equalizer, to: engine.mainMixerNode, format: inputNode.inputFormat(forBus: 0))
// call before creating converter because this changes the mainMixer's output format
engine.prepare()
let outputFormat = AVAudioFormat(commonFormat: .pcmFormatInt32,
sampleRate: 44100,
channels: 1,
interleaved: false)!
// Downsampling converter
guard let converter: AVAudioConverter = AVAudioConverter(from: engine.mainMixerNode.outputFormat(forBus: 0), to: outputFormat) else {
print("Can't convert in to this format")
return
}
engine.mainMixerNode.installTap(onBus: bus, bufferSize: 2688, format: nil) { (buffer, time) in
var newBufferAvailable = true
let inputCallback: AVAudioConverterInputBlock = { inNumPackets, outStatus in
if newBufferAvailable {
outStatus.pointee = .haveData
newBufferAvailable = false
return buffer
} else {
outStatus.pointee = .noDataNow
return nil
}
}
let convertedBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat, frameCapacity: AVAudioFrameCount(outputFormat.sampleRate) * buffer.frameLength / AVAudioFrameCount(buffer.format.sampleRate))!
var error: NSError?
let status = converter.convert(to: convertedBuffer, error: &error, withInputFrom: inputCallback)
assert(status != .error)
if status == .haveData {
// Process with converted buffer
}
}
do {
try engine.start()
} catch {
print("Can't start the engine: \(error)")
}
}