从iPhone上的线性PCM中提取幅度数据

Question

我很难从存储在audio.caf中的iPhone上的线性PCM中提取幅度数据.

我的问题是:

1. 线性PCM将幅度样本存储为16位值.它是否正确？
2. 振幅如何存储在AudioFileReadPacketData()返回的数据包中？录制单声道线性PCM时,不是每个样本(在一个帧中,在一个数据包中)只是一个SInt16阵列？什么是字节顺序(大端与小端)？
3. 线性PCM幅度的每一步在物理上意味着什么？
4. 当在iPhone上记录线性PCM时,中心点0(SInt16)还是32768(UInt16)？最大最小值在物理波形/气压中意味着什么？

还有一个问题:iPhone麦克风无法测量声音/气压波形吗？

我的代码如下:

// get the audio file proxy object for the audio
AudioFileID fileID;
AudioFileOpenURL((CFURLRef)audioURL, kAudioFileReadPermission, kAudioFileCAFType, &fileID);

// get the number of packets of audio data contained in the file
UInt64 totalPacketCount = [self packetCountForAudioFile:fileID];

// get the size of each packet for this audio file
UInt32 maxPacketSizeInBytes = [self packetSizeForAudioFile:fileID];

// setup to extract the audio data
Boolean inUseCache = false;
UInt32 numberOfPacketsToRead = 4410; // 0.1 seconds of data
UInt32 ioNumPackets = numberOfPacketsToRead;
UInt32 ioNumBytes = maxPacketSizeInBytes * ioNumPackets;
char *outBuffer = malloc(ioNumBytes);
memset(outBuffer, 0, ioNumBytes);

SInt16 signedMinAmplitude = -32768;
SInt16 signedCenterpoint = 0;
SInt16 signedMaxAmplitude = 32767;

SInt16 minAmplitude = signedMaxAmplitude;
SInt16 maxAmplitude = signedMinAmplitude;

// process each and every packet
for (UInt64 packetIndex = 0; packetIndex < totalPacketCount; packetIndex = packetIndex + ioNumPackets)
{
   // reset the number of packets to get
   ioNumPackets = numberOfPacketsToRead;

   AudioFileReadPacketData(fileID, inUseCache, &ioNumBytes, NULL, packetIndex, &ioNumPackets, outBuffer);

   for (UInt32 batchPacketIndex = 0; batchPacketIndex < ioNumPackets; batchPacketIndex++)
   {
      SInt16 packetData = outBuffer[batchPacketIndex * maxPacketSizeInBytes];
      SInt16 absoluteValue = abs(packetData);

      if (absoluteValue < minAmplitude) { minAmplitude = absoluteValue; }
      if (absoluteValue > maxAmplitude) { maxAmplitude = absoluteValue; }
   }
}

NSLog(@"minAmplitude: %hi", minAmplitude);
NSLog(@"maxAmplitude: %hi", maxAmplitude);

使用此代码,我几乎总是得到最小值为0,最大值为128!这对我来说毫无意义.

我正在使用AVAudioRecorder录制音频,如下所示:

// specify mono, 44.1 kHz, Linear PCM with Max Quality as recording format
NSDictionary *recordSettings = [[NSDictionary alloc] initWithObjectsAndKeys:
   [NSNumber numberWithFloat: 44100.0], AVSampleRateKey,
   [NSNumber numberWithInt: kAudioFormatLinearPCM], AVFormatIDKey,
   [NSNumber numberWithInt: 1], AVNumberOfChannelsKey,
   [NSNumber numberWithInt: AVAudioQualityMax], AVEncoderAudioQualityKey,
   nil];

// store the sound file in the app doc folder as calibration.caf
NSString *documentsDir = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
NSURL *audioFileURL = [NSURL fileURLWithPath:[documentsDir stringByAppendingPathComponent: @"audio.caf"]];

// create the audio recorder
NSError *createAudioRecorderError = nil;
AVAudioRecorder *newAudioRecorder = [[AVAudioRecorder alloc] initWithURL:audioFileURL settings:recordSettings error:&createAudioRecorderError];
[recordSettings release];

if (newAudioRecorder)
{
   // record the audio
   self.recorder = newAudioRecorder;
   [newAudioRecorder release];

   self.recorder.delegate = self;
   [self.recorder prepareToRecord];
   [self.recorder record];
}
else
{
   NSLog(@"%@", [createAudioRecorderError localizedDescription]);
}

感谢您提供的任何见解.这是我使用Core Audio的第一个项目,所以请随意撕开我的方法!

PS我试图搜索Core Audio列表存档,但请求一直出错:(http://search.lists.apple.com/?q=linear+pcm+amplitude&cmd=Search%21&ul=coreaudio-api)

PPS我看过:

http://en.wikipedia.org/wiki/Sound_pressure

http://en.wikipedia.org/wiki/Linear_PCM

http://wiki.multimedia.cx/index.php?title=PCM

获取声音文件中给定时间的振幅？

http://music.columbia.edu/pipermail/music-dsp/2002-April/048341.html

我还阅读了整个Core Audio Overview和大部分Audio Session Programming Guide,但我的问题仍然存在.

Answer 1

1)os x/iphone文件读取例程允许您确定样本格式,通常为LPCM的SInt8,SInt16,SInt32,Float32,Float64或连续的24位signed int之一

2)对于int格式,MIN_FOR_TYPE表示负相位的最大幅度,MAX_FOR_TYPE表示正的最大幅度.0等于沉默.浮点格式在[-1 ... 1]之间调制,与浮点数一样为零.在读取,写入,记录或使用特定格式时,字节顺序很重要 - 文件可能需要特定格式,并且您通常希望以本机字节顺序操作数据.apple audio file libs中的一些例程允许您传递表示源字节序的标志,而不是手动转换它.CAF有点复杂 - 它就像一个或多个音频文件的元包装器,并支持许多类型.

3)lpcm的幅度表示只是一个强力线性幅度表示(回放不需要转换/解码,幅度步长相等).

4)见#2.这些值与气压无关,它们与0 dBFS有关; 例如,如果您将流直接输出到DAC,则int max(如果浮点数为-1/1)表示单个样本将剪切的级别.

它与每个ADC和组件链一样,在电压方面对输入的处理能力有限制.此外,采样率定义了可捕获的最高频率(最高为采样率的一半).adc可以使用固定或可选择的位深度,但是当选择另一位深度时,最大输入电压通常不会改变.

你在代码级别犯的一个错误:你正在操纵`outBuffer'作为字符 - 而不是SInt16