我正在使用AudioQueue来播放一些歌曲，我的问题是如何判断已经排队的缓冲区的播放长度？我想一次传输两秒钟的数据，我遇到的问题是我怎么知道实际上对应两秒音乐的字节数（所以我总是可以提前两秒）。
由于
丹尼尔
答案 0 :(得分：3)
这是一个使用音频文件服务来获取比特率/数据包/帧数据以从音乐文件中获取对应于x秒的字节数的类，该示例已经使用mp3和m4a文件进行了测试
标题
#import <Foundation/Foundation.h>
#import <AudioToolbox/AudioToolbox.h>
@interface MusicChunker : NSObject
{
AudioFileID audioFile;
int _sampleRate;
int _totalFrames;
UInt64 _framesPerPacket;
UInt64 _totalPackets;
UInt64 fileDataSize;
AudioFilePacketTableInfo _packetInfo;
int _fileLength;
AudioStreamBasicDescription _fileDataFormat;
NSFileHandle * _fileHandle;
int _packetOffset;
int _totalReadBytes;
int _maxPacketSize;
BOOL firstTime;
BOOL _ism4a;
}
-(id)initWithURL:(NSURL*)url andFileType:(NSString*)ext;
//gets next chunk that corresponds to seconds of audio
-(NSData*)getNextDataChunk:(int)seconds;
@end
实施
#import "MusicChunker.h"
void ReportAudioError(OSStatus statusCode);
@implementation MusicChunker
- (id)init
{
self = [super init];
if (self) {
// Initialization code here.
}
return self;
}
void ReportAudioError(OSStatus statusCode) {
switch (statusCode) {
case noErr:
break;
case kAudioFileUnspecifiedError:
[NSException raise:@"AudioFileUnspecifiedError" format:@"An unspecified error occured."];
break;
case kAudioFileUnsupportedDataFormatError:
[NSException raise:@"AudioFileUnsupportedDataFormatError" format:@"The data format is not supported by the output file type."];
break;
case kAudioFileUnsupportedFileTypeError:
[NSException raise:@"AudioFileUnsupportedFileTypeError" format:@"The file type is not supported."];
break;
case kAudioFileUnsupportedPropertyError:
[NSException raise:@"AudioFileUnsupportedPropertyError" format:@"A file property is not supported."];
break;
case kAudioFilePermissionsError:
[NSException raise:@"AudioFilePermissionsError" format:@"The operation violated the file permissions. For example, an attempt was made to write to a file opened with the kAudioFileReadPermission constant."];
break;
case kAudioFileNotOptimizedError:
[NSException raise:@"AudioFileNotOptimizedError" format:@"The chunks following the audio data chunk are preventing the extension of the audio data chunk. To write more data, you must optimize the file."];
break;
case kAudioFileInvalidChunkError:
[NSException raise:@"AudioFileInvalidChunkError" format:@"Either the chunk does not exist in the file or it is not supported by the file."];
break;
case kAudioFileDoesNotAllow64BitDataSizeError:
[NSException raise:@"AudioFileDoesNotAllow64BitDataSizeError" format:@"The file offset was too large for the file type. The AIFF and WAVE file format types have 32-bit file size limits."];
break;
case kAudioFileInvalidPacketOffsetError:
[NSException raise:@"AudioFileInvalidPacketOffsetError" format:@"A packet offset was past the end of the file, or not at the end of the file when a VBR format was written, or a corrupt packet size was read when the packet table was built."];
break;
case kAudioFileInvalidFileError:
[NSException raise:@"AudioFileInvalidFileError" format:@"The file is malformed, or otherwise not a valid instance of an audio file of its type."];
break;
case kAudioFileOperationNotSupportedError:
[NSException raise:@"AudioFileOperationNotSupportedError" format:@"The operation cannot be performed. For example, setting the kAudioFilePropertyAudioDataByteCount constant to increase the size of the audio data in a file is not a supported operation. Write the data instead."];
break;
case -50:
[NSException raise:@"AudioFileBadParameter" format:@"An invalid parameter was passed, possibly the current packet and/or the inNumberOfPackets."];
break;
default:
[NSException raise:@"AudioFileUknownError" format:@"An unknown error type %@ occured. [%s]", [NSNumber numberWithInteger:statusCode], (char*)&statusCode];
break;
}
}
+ (AudioFileTypeID)hintForFileExtension:(NSString *)fileExtension
{
AudioFileTypeID fileTypeHint = kAudioFileAAC_ADTSType;
if ([fileExtension isEqual:@"mp3"])
{
fileTypeHint = kAudioFileMP3Type;
}
else if ([fileExtension isEqual:@"wav"])
{
fileTypeHint = kAudioFileWAVEType;
}
else if ([fileExtension isEqual:@"aifc"])
{
fileTypeHint = kAudioFileAIFCType;
}
else if ([fileExtension isEqual:@"aiff"])
{
fileTypeHint = kAudioFileAIFFType;
}
else if ([fileExtension isEqual:@"m4a"])
{
fileTypeHint = kAudioFileM4AType;
}
else if ([fileExtension isEqual:@"mp4"])
{
fileTypeHint = kAudioFileMPEG4Type;
}
else if ([fileExtension isEqual:@"caf"])
{
fileTypeHint = kAudioFileCAFType;
}
else if ([fileExtension isEqual:@"aac"])
{
fileTypeHint = kAudioFileAAC_ADTSType;
}
return fileTypeHint;
}
-(id)initWithURL:(NSURL*)url andFileType:(NSString*)ext
{
self = [super init];
if (self) {
// Initialization code here.
//OSStatus theErr = noErr;
if([ext isEqualToString:@"mp3"])
{
_ism4a=FALSE;
}
else
_ism4a=TRUE;
firstTime=TRUE;
_packetOffset=0;
AudioFileTypeID hint=[MusicChunker hintForFileExtension:ext];
OSStatus theErr = AudioFileOpenURL((CFURLRef)url, kAudioFileReadPermission, hint, &audioFile);
if(theErr)
{
ReportAudioError(theErr);
}
UInt32 thePropertySize;// = sizeof(theFileFormat);
thePropertySize = sizeof(fileDataSize);
theErr = AudioFileGetProperty(audioFile, kAudioFilePropertyAudioDataByteCount, &thePropertySize, &fileDataSize);
if(theErr)
{
ReportAudioError(theErr);
}
theErr = AudioFileGetProperty(audioFile, kAudioFilePropertyAudioDataPacketCount, &thePropertySize, &_totalPackets);
if(theErr)
{
ReportAudioError(theErr);
}
/*
UInt32 size;
size= sizeof(_packetInfo);
theErr= AudioFileGetProperty(audioFile, kAudioFilePropertyPacketTableInfo, &size, &_packetInfo);
g(@"Key %@", key );
}
if(theErr)
{
ReportAudioError(theErr);
}
*/
UInt32 size;
size=sizeof(_maxPacketSize);
theErr=AudioFileGetProperty(audioFile, kAudioFilePropertyMaximumPacketSize , &size, &_maxPacketSize);
size = sizeof( _fileDataFormat );
theErr=AudioFileGetProperty( audioFile, kAudioFilePropertyDataFormat, &size, &_fileDataFormat );
_framesPerPacket=_fileDataFormat.mFramesPerPacket;
_totalFrames=_fileDataFormat.mFramesPerPacket*_totalPackets;
_fileHandle=[[NSFileHandle fileHandleForReadingFromURL:url error:nil] retain];
_fileLength=[_fileHandle seekToEndOfFile];
_sampleRate=_fileDataFormat.mSampleRate;
_totalReadBytes=0;
/*
AudioFramePacketTranslation tran;//= .mFrame = 0, .mPacket = packetCount - 1, .mFrameOffsetInPacket = 0 };
tran.mFrame=0;
tran.mFrameOffsetInPacket=0;
tran.mPacket=1;
UInt32 size=sizeof(tran);
theErr=AudioFileGetProperty(audioFile, kAudioFilePropertyPacketToFrame, &size, &tran);
*/
/*
AudioBytePacketTranslation bt;
bt.mPacket=4;
bt.mByteOffsetInPacket=0;
size=sizeof(bt);
theErr=AudioFileGetProperty(audioFile, kAudioFilePropertyPacketToByte, &size, &bt);
*/
}
return self;
}
//gets next chunk that corresponds to seconds of audio
-(NSData*)getNextDataChunk:(int)seconds
{
//NSLog(@"%d, total packets",_totalPackets);
if(_packetOffset>=_totalPackets)
return nil;
//sampleRate * seconds = number of wanted frames
int framesWanted= _sampleRate*seconds;
NSData *header=nil;
int wantedPackets= framesWanted/_framesPerPacket;
if(firstTime && _ism4a)
{
firstTime=false;
//when we have a header that was stripped off, we grab it from the original file
int totallen= [_fileHandle seekToEndOfFile];
int dif=totallen-fileDataSize;
[_fileHandle seekToFileOffset:0];
header= [_fileHandle readDataOfLength:dif];
}
int packetOffset=_packetOffset+wantedPackets;
//bound condition
if(packetOffset>_totalPackets)
{
packetOffset=_totalPackets;
}
UInt32 outBytes;
UInt32 packetCount = wantedPackets;
int x=packetCount * _maxPacketSize;
void *data = (void *)malloc(x);
OSStatus theErr=AudioFileReadPackets(audioFile, false, &outBytes, NULL, _packetOffset, &packetCount, data);
if(theErr)
{
ReportAudioError(theErr);
}
//calculate bytes to read
int bytesRead=outBytes;
//update read bytes
_totalReadBytes+=bytesRead;
// NSLog(@"total bytes read %d", _totalReadBytes);
_packetOffset=packetOffset;
NSData *subdata=[[NSData dataWithBytes:data length:outBytes] retain];
free(data);
if(header)
{
NSMutableData *data=[[NSMutableData alloc]init];
[data appendData:header];
[data appendData:subdata];
[subdata release];
return [data autorelease];
}
return [subdata autorelease];
}
@end
答案 1 :(得分：1)
如果歌曲采用任意压缩格式，并且您只需要2秒钟的剪辑，则可能必须先将歌曲转换为原始PCM样本或WAV数据（AVAssetReader等）。然后，您可以以已知的采样率计算样本。例如采样率为44.1k的88200帧将为2秒。