音频单元中可能的输出采样率

Question

我正在尝试为远程 I/O 音频单元的输入流设置不同的采样率（如 32kHz、24kHz 等）。但是输出总是以这些采样率之一播放 - 22.05kHz、33.1kHz、11.0kHz，无论我设置什么。令人奇怪的是，当我叫 AudioUnitGetProperty上kAudioUnitScope_Output了kAudioUnitProperty_SampleRate，它总是返回44.1

- (void)startToneUnit
{
    AudioComponentDescription defaultOutputDescription;
    defaultOutputDescription.componentType = kAudioUnitType_Output;
    defaultOutputDescription.componentSubType = kAudioUnitSubType_RemoteIO;
    defaultOutputDescription.componentManufacturer = kAudioUnitManufacturer_Apple;
    defaultOutputDescription.componentFlags = 0;
    defaultOutputDescription.componentFlagsMask = 0;

        // Get the default playback output unit
    AudioComponent defaultOutput = AudioComponentFindNext(NULL, &defaultOutputDescription);
    NSAssert(defaultOutput, @"Can't find default output");

        // Create a new unit based on this that we'll use for output
    OSErr err = AudioComponentInstanceNew(defaultOutput, &toneUnit);
    NSAssert1(toneUnit, @"Error creating unit: %hd", err);

        // Set our tone rendering function on the unit
    AURenderCallbackStruct input;
    input.inputProc = RenderTone;
    input.inputProcRefCon = (__bridge void *)(self);
    err = AudioUnitSetProperty(toneUnit,
                               kAudioUnitProperty_SetRenderCallback,
                               kAudioUnitScope_Input,
                               0,
                               &input,
                               sizeof(input));
    NSAssert1(err == noErr, @"Error setting callback: %hd", err);

        // Set the format to 32 bit, single channel, floating point, linear PCM
    const int four_bytes_per_float = 4;
    const int eight_bits_per_byte = 8;

    AudioStreamBasicDescription streamFormat;
    streamFormat.mSampleRate = SAMPLE_RATE;
    streamFormat.mFormatID = kAudioFormatLinearPCM;
    streamFormat.mFormatFlags =
    kAudioFormatFlagsNativeFloatPacked;
    streamFormat.mBytesPerPacket = four_bytes_per_float;
    streamFormat.mFramesPerPacket = 1;
    streamFormat.mBytesPerFrame = four_bytes_per_float;
    streamFormat.mChannelsPerFrame = 1;
    streamFormat.mBitsPerChannel = four_bytes_per_float * eight_bits_per_byte;
    err = AudioUnitSetProperty (toneUnit,
                                kAudioUnitProperty_StreamFormat,
                                kAudioUnitScope_Input,
                                0,
                                &streamFormat,
                                sizeof(AudioStreamBasicDescription));

    NSAssert1(err == noErr, @"Error setting stream format: %hd", err);

        // Stop changing parameters on the unit
    err = AudioUnitInitialize(toneUnit);
    NSAssert1(err == noErr, @"Error initializing unit: %hd", err);

        // Start playback
    err = AudioOutputUnitStart(toneUnit);
    NSAssert1(err == noErr, @"Error starting unit: %hd", err);

    Float64 outSampleRate = 0.0;
    UInt32 size = sizeof(Float64);
    AudioUnitGetProperty (toneUnit,
                          kAudioUnitProperty_SampleRate,
                          kAudioUnitScope_Output,
                          0,
                          &outSampleRate,
                          &size);
    NSLog(@"Output sample rate is now at %f Hz", outSampleRate);

   }

音频单元支持的所有可能的输出采样率是多少？对此 Apple 文档的任何参考都将非常有帮助

谢谢

Answer 1

AudioUnit通常，s 可以以您指定的任何采样率运行。RemoteIO作为硬件外观的（或 Mac OS X 上的 HAL）AudioUnits 受到更多限制——变速时钟发生器、可以以任意采样率运行的采样率发生器非常昂贵，通常不适合电话:)

不同的 iOS 硬件、硬件型号、硬件线或版本可能支持不同的采样率。 RemoteIO只接受您要求的费率并将板载转换器设置为最接近您要求的费率。你总是必须做一个AudioUnitGetProperty看看你实际得到了什么。如果您想以不同的速率录制或工作，则需要使用转换器插件。