iphone - 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)。但输出总是以这些采样率之一播放 -22.05kHz、33.1kHz、11.0kHz，与我设置的无关。令人惊讶的是，当我在 kAudioUnitScope_Output 上为 kAudioUnitProperty_SampleRate 调用 AudioUnitGetProperty 时，它总是返回 44.1kHz

- (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输出采样率是多少？对 Apple 文档的任何引用都将非常有帮助

谢谢

Answer 1

AudioUnit 通常可以以您指定的任何采样率运行。 RemoteIO(或 Mac OS X 上的 HAL)AudioUnit 作为硬件的门面，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，非常昂贵且通常不适合电话:)

不同的 iOS 硬件、硬件型号或硬件系列或版本可能支持不同的采样率。 RemoteIO 只接受您请求的速率并将板载转换器设置为最接近您请求的速率。您始终必须执行 AudioUnitGetProperty 才能查看实际得到的内容。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。