我在尝试从存储在audio.caf中的iPhone的线性PCM中提取幅度数据时遇到了困难。
我的问题是：
- 线性PCM将幅度样本存储为16位值。这是正确的吗？
- AudioFileReadPacketData()返回的数据包中如何存储幅度？当录制单声道线性PCM时，每个样本（在一个帧中，在一个数据包中）不就是一个SInt16数组吗？字节顺序是大端还是小端？
- 线性PCM幅度中的每个步骤在物理上意味着什么？
- 当在iPhone上录制线性PCM时，中心点是0（SInt16）还是32768（UInt16）？在物理波形/气压中，最大最小值代表什么？
还有一个额外的问题：iPhone麦克风无法测量哪些声音/气压波形？
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);
使用这段代码，我几乎总是得到0和128的最小值和最大值！这对我来说毫无意义。
我正在使用AVAudioRecorder记录音频，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]);
}
感谢您能提供任何帮助。这是我使用核心音频的第一个项目，所以请随意批评我的方法！
附注：我试图搜索核心音频列表档案，但请求一直出错：(http://search.lists.apple.com/?q=linear+pcm+amplitude&cmd=Search%21&ul=coreaudio-api)
附注2：我已经查看了:
http://en.wikipedia.org/wiki/Sound_pressure
http://en.wikipedia.org/wiki/Linear_PCM
http://wiki.multimedia.cx/index.php?title=PCM
http://music.columbia.edu/pipermail/music-dsp/2002-April/048341.html
我已经阅读了整个Core Audio概述和大部分音频会话编程指南，但我的问题仍然存在。