如何在 Avfoundation 中正确更改采样率
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【中文标题】如何在 Avfoundation 中正确更改采样率【英文标题】:How to change sample rate properly in Avfoundation 【发布时间】:2018-10-27 05:00:12 【问题描述】:我已经完成了这个简单的程序。它所做的只是同时记录和回放缓冲区。如果采样率为 44100 赫兹,一切正常,但如果我将采样率更改为 16000 或 8000,它根本不会产生任何声音,或者可能是一些听不见的白噪声。为什么会发生这种情况?
如何以不同的采样率记录?
我尝试过的以下代码:
import UIKit
import AVFoundation
class ViewController: UIViewController
var engine = AVAudioEngine()
let player = AVAudioPlayerNode()
let audiosession = AVAudioSession.sharedInstance()
let newSrc:UnsafeMutablePointer<Float>! = nil
override func viewDidLoad()
super.viewDidLoad()
let audioSession = AVAudioSession.sharedInstance()
print(audioSession.sampleRate) // here it prints 44100 hz. because it still using the internal mic.
do
try audioSession.setCategory(AVAudioSessionCategoryPlayAndRecord, with: .allowBluetooth)
try audioSession.setMode(AVAudioSessionModeDefault)
try audioSession.setActive(true)
catch
print(audioSession.sampleRate) // here it will print 16000 hz if my bluetooth earbuds is connected, if not it will be 44100 hz.
let input = engine.inputNode
let bus = 0
let mixer = AVAudioMixerNode() // creating mixer as it is needed to set audio format
engine.attach(mixer)
engine.attach(player)
engine.connect(input, to: mixer, format: input.outputFormat(forBus: 0))
let inputFormat = input.inputFormat(forBus: bus)
engine.connect(player, to: engine.mainMixerNode, format: input.inputFormat(forBus: 0))
let fmt = AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: 44100.0, channels: 1, interleaved: false)
mixer.installTap(onBus: bus, bufferSize: 1024, format: fmt) (buffer, time) -> Void in
print(buffer.format)
print(buffer.floatChannelData)
print(buffer.format.streamDescription.pointee.mBytesPerFrame)
self.player.scheduleBuffer(buffer)
if self.player.isPlaying
print("true")
engine.prepare()
do
try! engine.start()
player.play()
catch
print(error)
【问题讨论】:
【参考方案1】:作为discussed here,AVAudioEngine
混音器节点和分接头都不会为您进行速率转换。实际上,在您的情况下,混音龙头不是抛出或记录错误,而是静默地(明白吗?)让您保持沉默。
由于您不能使用AVAudioMixerNode
进行速率转换,您可以将其替换为方便的AVAudioConverter
,确保设置AVAudioPlayerNode
的正确输出格式,因为
在播放缓冲区时,有一个隐含的假设,即缓冲区是相同的 采样率作为节点的输出格式。
如果您不这样做,您可能会听到间隙和/或音高偏移的音频。
像这样
let input = engine.inputNode
let bus = 0
let inputFormat = input.inputFormat(forBus: bus)
engine.attach(player)
let fmt = AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: 8000, channels: 1, interleaved: false)!
engine.connect(player, to: engine.mainMixerNode, format: fmt)
let converter = AVAudioConverter(from: inputFormat, to: fmt)!
input.installTap(onBus: bus, bufferSize: 1024, format: inputFormat) (buffer, time) -> Void in
let inputCallback: AVAudioConverterInputBlock = inNumPackets, outStatus in
outStatus.pointee = AVAudioConverterInputStatus.haveData
return buffer
let convertedBuffer = AVAudioPCMBuffer(pcmFormat: fmt, frameCapacity: AVAudioFrameCount(fmt.sampleRate) * buffer.frameLength / AVAudioFrameCount(buffer.format.sampleRate))!
var error: NSError? = nil
let status = converter.convert(to: convertedBuffer, error: &error, withInputFrom: inputCallback)
assert(status != .error)
print(convertedBuffer.format)
print(convertedBuffer.floatChannelData)
print(convertedBuffer.format.streamDescription.pointee.mBytesPerFrame)
self.player.scheduleBuffer(convertedBuffer)
【讨论】:
谢谢。我已经想通了,并通过使用 AVAudioConverter 使其工作。我真的认为这可以在不使用转换器的情况下完成,这就是为什么打开这个线程。关于播放器节点,是的,我听到了音调/移位,然后我的洞察力突然起作用了,我尝试将其设置为播放我用于 AudioConverter 的格式。但是您现在的回答清楚地表明,不使用 Converter 是不可能的,但我的问题是,为什么他们给了我们一个选项?如果我们不能设置录制格式,那为什么还有一个选项呢? 头文件doco说attempts to apply this as the format of the specified output bus. This should only be done when attaching to an output bus which is not connected to another node; an error will result otherwise.
我很高兴它尝试了,但我不知道它什么时候应该工作,也不知道为什么它没有给出错误就失败了。【参考方案2】:
这个解决方案对我有用
let fmt = AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: 44100, channels: 1, interleaved: false)!
inputNode.installTap(onBus: 0, bufferSize: 1024, format: fmt) (buffer: AVAudioPCMBuffer, when: AVAudioTime) in
self.recognitionRequest?.append(buffer)
【讨论】:
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