AVAudioSession、AudioStreamBasicDescription 和 RemoteIO 设备默认值

Question

我正在尝试编写一个可以进行数字信号处理的应用程序，并希望使其尽可能轻巧。让我困惑一段时间的一件事就是各种设备的默认值可能是什么，这样我就可以避免在从缓冲区接收数据之前发生不需要的转换。我遇到了以下链接http://club15cc.com/code-snippets/ios-2/get-the-default-output-stream-format-for-an-audio-unit-in-ios，它让我走上了我认为正确的道路。

我已扩展链接中的代码以在获取 ASBD(AudioStreamBasicDescription) 内容之前创建和激活 AVAudioSession，然后可以使用 AudioSession 请求各种“首选”设置以查看它们有什么影响。我还将用于列出 ASBD 值的 Apple 代码与上述链接中的代码相结合。

下面的代码被放入通过选择单视图应用程序模板生成的 ViewController.m 文件中。请注意，您需要将 AudioToolbox.framework 和 CoreAudio.framework 添加到项目的链接框架和库中。

#import "ViewController.h"
@import AVFoundation;
@import AudioUnit;

@interface ViewController ()

@end

@implementation ViewController

- (void) printASBD:(AudioStreamBasicDescription) asbd {
    char formatIDString[5];
    UInt32 formatID = CFSwapInt32HostToBig (asbd.mFormatID);
    bcopy (&formatID, formatIDString, 4);
    formatIDString[4] = '\0';

    NSLog (@"  Sample Rate:         %10.0f",  asbd.mSampleRate);
    NSLog (@"  Format ID:           %10s",    formatIDString);
    NSLog (@"  Format Flags:        %10X",    (unsigned int)asbd.mFormatFlags);
    NSLog (@"  Bytes per Packet:    %10d",    (unsigned int)asbd.mBytesPerPacket);
    NSLog (@"  Frames per Packet:   %10d",    (unsigned int)asbd.mFramesPerPacket);
    NSLog (@"  Bytes per Frame:     %10d",    (unsigned int)asbd.mBytesPerFrame);
    NSLog (@"  Channels per Frame:  %10d",    (unsigned int)asbd.mChannelsPerFrame);
    NSLog (@"  Bits per Channel:    %10d",    (unsigned int)asbd.mBitsPerChannel);
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSError *error = nil;
    AVAudioSession *audioSession = [AVAudioSession sharedInstance];

    // Get a reference to the AudioSession and activate it
    [audioSession setCategory:AVAudioSessionCategoryPlayAndRecord error:&error];
    [audioSession setActive:YES error:&error];


    // Then get RemoteIO AudioUnit and use it to get the content of the default AudioStreamBasicDescription
    AudioUnit remoteIOUnit;

    AudioComponentDescription audioComponentDesc = {0};
    audioComponentDesc.componentType = kAudioUnitType_Output;
    audioComponentDesc.componentSubType = kAudioUnitSubType_RemoteIO;
    audioComponentDesc.componentManufacturer = kAudioUnitManufacturer_Apple;

    // Get component
    AudioComponent audioComponent = AudioComponentFindNext(NULL, &audioComponentDesc);
    AudioComponentInstanceNew(audioComponent, &remoteIOUnit);

    // Read the stream format
    size_t asbdSize = sizeof(AudioStreamBasicDescription);
    AudioStreamBasicDescription asbd = {0};
    AudioUnitGetProperty(remoteIOUnit,
                         kAudioUnitProperty_StreamFormat,
                         kAudioUnitScope_Output,
                         0,
                         (void *)&asbd,
                         &asbdSize);

    [self printASBD:asbd];
}

@end

我很想知道人们从其他实际硬件中获得的结果。请注意，代码已构建并部署到 IOS 7.1

Answer 1

格式标志是：

kAudioFormatFlagIsFloat                  = (1 << 0),    // 0x1
kAudioFormatFlagIsBigEndian              = (1 << 1),    // 0x2
kAudioFormatFlagIsSignedInteger          = (1 << 2),    // 0x4
kAudioFormatFlagIsPacked                 = (1 << 3),    // 0x8
kAudioFormatFlagIsAlignedHigh            = (1 << 4),    // 0x10
kAudioFormatFlagIsNonInterleaved         = (1 << 5),    // 0x20
kAudioFormatFlagIsNonMixable             = (1 << 6),    // 0x40
kAudioFormatFlagsAreAllClear             = (1 << 31),

我在 iPad 4 上得到的结果如下：

Sample Rate:                  0
Format ID:                 lpcm
Format Flags:                29
Bytes per Packet:             4
Frames per Packet:            1
Bytes per Frame:              4
Channels per Frame:           2
Bits per Channel:            32

我猜 lpcm（线性脉冲编码调制）并不奇怪，格式标志 = x'29'kAudioFormatFlagIsFloat | kAudioFormatFlagIsPacked 以及每通道 32 位似乎表明预期的 8.24“固定浮点”。