我目前正在申请作为我的计算机科学学士学位的一部分。该应用程序将关联iPhone硬件（加速度计，gps）和正在播放的音乐的数据。
该项目仍处于起步阶段，仅仅工作了2个月。
我现在的那一刻，以及我需要帮助的地方，是从itunes库中的歌曲中读取PCM样本，然后使用音频单元播放它们。 目前我想要实现的实现如下：从iTunes中选择一首随机歌曲，并在需要时从中读取样本，并将其存储在缓冲区中，我们称之为sampleBuffer。稍后在消费者模型中，音频单元（具有混音器和remoteIO输出）具有回调，我只需将sampleBuffer中所需数量的样本复制到回调中指定的缓冲区中。我通过扬声器听到的东西并不是我所期待的;我可以认识到它正在播放这首歌，但似乎它被错误地解码并且它有很多噪音！我附上了一个显示前半秒钟（24576个样本@ 44.1kHz）的图像，这不像一个看起来很小的输出。 在我进入列表之前，我检查过文件没有损坏，类似地我已经为缓冲区编写了测试用例（所以我知道缓冲区不会改变样本），虽然这可能不是最好的方法（有些人会主张去音频队列路线），我想对样本进行各种操作，以及在完成之前更改歌曲，重新排列播放的歌曲等等。此外，音频中可能有一些不正确的设置但是，显示样本的图形（显示样本被错误地解码）直接从缓冲区获取，因此我现在只想解决为什么从磁盘读取和解码不能正常工作。现在我只是想通过工作得到一个游戏。 无法发布图片，因为新的stackoverflow是图片的链接：http://i.stack.imgur.com/RHjlv.jpg
清单：
这是我设置audioReadSettigns的地方，它将用于AVAssetReaderAudioMixOutput
// Set the read settings
audioReadSettings = [[NSMutableDictionary alloc] init];
[audioReadSettings setValue:[NSNumber numberWithInt:kAudioFormatLinearPCM]
forKey:AVFormatIDKey];
[audioReadSettings setValue:[NSNumber numberWithInt:16] forKey:AVLinearPCMBitDepthKey];
[audioReadSettings setValue:[NSNumber numberWithBool:NO] forKey:AVLinearPCMIsBigEndianKey];
[audioReadSettings setValue:[NSNumber numberWithBool:NO] forKey:AVLinearPCMIsFloatKey];
[audioReadSettings setValue:[NSNumber numberWithBool:NO] forKey:AVLinearPCMIsNonInterleaved];
[audioReadSettings setValue:[NSNumber numberWithFloat:44100.0] forKey:AVSampleRateKey];
现在，下面的代码清单是一个接收带有歌曲的persistant_id的NSString的方法：
-(BOOL)setNextSongID:(NSString*)persistand_id {
assert(persistand_id != nil);
MPMediaItem *song = [self getMediaItemForPersistantID:persistand_id];
NSURL *assetUrl = [song valueForProperty:MPMediaItemPropertyAssetURL];
AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:assetUrl
options:[NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES]
forKey:AVURLAssetPreferPreciseDurationAndTimingKey]];
NSError *assetError = nil;
assetReader = [[AVAssetReader assetReaderWithAsset:songAsset error:&assetError] retain];
if (assetError) {
NSLog(@"error: %@", assetError);
return NO;
}
CMTimeRange timeRange = CMTimeRangeMake(kCMTimeZero, songAsset.duration);
[assetReader setTimeRange:timeRange];
track = [[songAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
assetReaderOutput = [AVAssetReaderAudioMixOutput assetReaderAudioMixOutputWithAudioTracks:[NSArray arrayWithObject:track]
audioSettings:audioReadSettings];
if (![assetReader canAddOutput:assetReaderOutput]) {
NSLog(@"cant add reader output... die!");
return NO;
}
[assetReader addOutput:assetReaderOutput];
[assetReader startReading];
// just getting some basic information about the track to print
NSArray *formatDesc = ((AVAssetTrack*)[[assetReaderOutput audioTracks] objectAtIndex:0]).formatDescriptions;
for (unsigned int i = 0; i < [formatDesc count]; ++i) {
CMAudioFormatDescriptionRef item = (CMAudioFormatDescriptionRef)[formatDesc objectAtIndex:i];
const CAStreamBasicDescription *asDesc = (CAStreamBasicDescription*)CMAudioFormatDescriptionGetStreamBasicDescription(item);
if (asDesc) {
// get data
numChannels = asDesc->mChannelsPerFrame;
sampleRate = asDesc->mSampleRate;
asDesc->Print();
}
}
[self copyEnoughSamplesToBufferForLength:24000];
return YES;
}
以下介绍函数 - （void）copyEnoughSamplesToBufferForLength：
-(void)copyEnoughSamplesToBufferForLength:(UInt32)samples_count {
[w_lock lock];
int stillToCopy = 0;
if (sampleBuffer->numSamples() < samples_count) {
stillToCopy = samples_count;
}
NSAutoreleasePool *apool = [[NSAutoreleasePool alloc] init];
CMSampleBufferRef sampleBufferRef;
SInt16 *dataBuffer = (SInt16*)malloc(8192 * sizeof(SInt16));
int a = 0;
while (stillToCopy > 0) {
sampleBufferRef = [assetReaderOutput copyNextSampleBuffer];
if (!sampleBufferRef) {
// end of song or no more samples
return;
}
CMBlockBufferRef blockBuffer = CMSampleBufferGetDataBuffer(sampleBufferRef);
CMItemCount numSamplesInBuffer = CMSampleBufferGetNumSamples(sampleBufferRef);
AudioBufferList audioBufferList;
CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(sampleBufferRef,
NULL,
&audioBufferList,
sizeof(audioBufferList),
NULL,
NULL,
0,
&blockBuffer);
int data_length = floorf(numSamplesInBuffer * 1.0f);
int j = 0;
for (int bufferCount=0; bufferCount < audioBufferList.mNumberBuffers; bufferCount++) {
SInt16* samples = (SInt16 *)audioBufferList.mBuffers[bufferCount].mData;
for (int i=0; i < numSamplesInBuffer; i++) {
dataBuffer[j] = samples[i];
j++;
}
}
CFRelease(sampleBufferRef);
sampleBuffer->putSamples(dataBuffer, j);
stillToCopy = stillToCopy - data_length;
}
free(dataBuffer);
[w_lock unlock];
[apool release];
}
现在，sampleBuffer将错误地解码样本。任何人都可以帮助我为什么会这样？这适用于我的iTunes资料库中的不同文件（mp3，aac，wav等）。 任何帮助将不胜感激，此外，如果您需要我的代码的任何其他列表，或者输出听起来像什么，我将根据请求附加它。过去一周我一直坐在这里试图调试它并且没有在网上找到任何帮助 - 每个人似乎都在按照我的方式进行操作，但似乎只有我有这个问题。
感谢您提供任何帮助！
彼得
答案 0 :(得分：10)
目前，我还在开展一个项目，该项目涉及从iTunes资料库中将音频样本提取到AudioUnit中。
包含audiounit渲染回调，供您参考。输入格式设置为SInt16StereoStreamFormat。
我使用了Michael Tyson的循环缓冲区实现 - TPCircularBuffer作为缓冲存储。非常容易使用和理解!!!谢谢迈克尔！
- (void) loadBuffer:(NSURL *)assetURL_
{
if (nil != self.iPodAssetReader) {
[iTunesOperationQueue cancelAllOperations];
[self cleanUpBuffer];
}
NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithInt:kAudioFormatLinearPCM], AVFormatIDKey,
[NSNumber numberWithFloat:44100.0], AVSampleRateKey,
[NSNumber numberWithInt:16], AVLinearPCMBitDepthKey,
[NSNumber numberWithBool:NO], AVLinearPCMIsNonInterleaved,
[NSNumber numberWithBool:NO], AVLinearPCMIsFloatKey,
[NSNumber numberWithBool:NO], AVLinearPCMIsBigEndianKey,
nil];
AVURLAsset *asset = [AVURLAsset URLAssetWithURL:assetURL_ options:nil];
if (asset == nil) {
NSLog(@"asset is not defined!");
return;
}
NSLog(@"Total Asset Duration: %f", CMTimeGetSeconds(asset.duration));
NSError *assetError = nil;
self.iPodAssetReader = [AVAssetReader assetReaderWithAsset:asset error:&assetError];
if (assetError) {
NSLog (@"error: %@", assetError);
return;
}
AVAssetReaderOutput *readerOutput = [AVAssetReaderAudioMixOutput assetReaderAudioMixOutputWithAudioTracks:asset.tracks audioSettings:outputSettings];
if (! [iPodAssetReader canAddOutput: readerOutput]) {
NSLog (@"can't add reader output... die!");
return;
}
// add output reader to reader
[iPodAssetReader addOutput: readerOutput];
if (! [iPodAssetReader startReading]) {
NSLog(@"Unable to start reading!");
return;
}
// Init circular buffer
TPCircularBufferInit(&playbackState.circularBuffer, kTotalBufferSize);
__block NSBlockOperation * feediPodBufferOperation = [NSBlockOperation blockOperationWithBlock:^{
while (![feediPodBufferOperation isCancelled] && iPodAssetReader.status != AVAssetReaderStatusCompleted) {
if (iPodAssetReader.status == AVAssetReaderStatusReading) {
// Check if the available buffer space is enough to hold at least one cycle of the sample data
if (kTotalBufferSize - playbackState.circularBuffer.fillCount >= 32768) {
CMSampleBufferRef nextBuffer = [readerOutput copyNextSampleBuffer];
if (nextBuffer) {
AudioBufferList abl;
CMBlockBufferRef blockBuffer;
CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(nextBuffer, NULL, &abl, sizeof(abl), NULL, NULL, kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment, &blockBuffer);
UInt64 size = CMSampleBufferGetTotalSampleSize(nextBuffer);
int bytesCopied = TPCircularBufferProduceBytes(&playbackState.circularBuffer, abl.mBuffers[0].mData, size);
if (!playbackState.bufferIsReady && bytesCopied > 0) {
playbackState.bufferIsReady = YES;
}
CFRelease(nextBuffer);
CFRelease(blockBuffer);
}
else {
break;
}
}
}
}
NSLog(@"iPod Buffer Reading Finished");
}];
[iTunesOperationQueue addOperation:feediPodBufferOperation];
}
static OSStatus ipodRenderCallback (
void *inRefCon, // A pointer to a struct containing the complete audio data
// to play, as well as state information such as the
// first sample to play on this invocation of the callback.
AudioUnitRenderActionFlags *ioActionFlags, // Unused here. When generating audio, use ioActionFlags to indicate silence
// between sounds; for silence, also memset the ioData buffers to 0.
const AudioTimeStamp *inTimeStamp, // Unused here.
UInt32 inBusNumber, // The mixer unit input bus that is requesting some new
// frames of audio data to play.
UInt32 inNumberFrames, // The number of frames of audio to provide to the buffer(s)
// pointed to by the ioData parameter.
AudioBufferList *ioData // On output, the audio data to play. The callback's primary
// responsibility is to fill the buffer(s) in the
// AudioBufferList.
)
{
Audio* audioObject = (Audio*)inRefCon;
AudioSampleType *outSample = (AudioSampleType *)ioData->mBuffers[0].mData;
// Zero-out all the output samples first
memset(outSample, 0, inNumberFrames * kUnitSize * 2);
if ( audioObject.playingiPod && audioObject.bufferIsReady) {
// Pull audio from circular buffer
int32_t availableBytes;
AudioSampleType *bufferTail = TPCircularBufferTail(&audioObject.circularBuffer, &availableBytes);
memcpy(outSample, bufferTail, MIN(availableBytes, inNumberFrames * kUnitSize * 2) );
TPCircularBufferConsume(&audioObject.circularBuffer, MIN(availableBytes, inNumberFrames * kUnitSize * 2) );
audioObject.currentSampleNum += MIN(availableBytes / (kUnitSize * 2), inNumberFrames);
if (availableBytes <= inNumberFrames * kUnitSize * 2) {
// Buffer is running out or playback is finished
audioObject.bufferIsReady = NO;
audioObject.playingiPod = NO;
audioObject.currentSampleNum = 0;
if ([[audioObject delegate] respondsToSelector:@selector(playbackDidFinish)]) {
[[audioObject delegate] performSelector:@selector(playbackDidFinish)];
}
}
}
return noErr;
}
- (void) setupSInt16StereoStreamFormat {
// The AudioUnitSampleType data type is the recommended type for sample data in audio
// units. This obtains the byte size of the type for use in filling in the ASBD.
size_t bytesPerSample = sizeof (AudioSampleType);
// Fill the application audio format struct's fields to define a linear PCM,
// stereo, noninterleaved stream at the hardware sample rate.
SInt16StereoStreamFormat.mFormatID = kAudioFormatLinearPCM;
SInt16StereoStreamFormat.mFormatFlags = kAudioFormatFlagsCanonical;
SInt16StereoStreamFormat.mBytesPerPacket = 2 * bytesPerSample; // *** kAudioFormatFlagsCanonical <- implicit interleaved data => (left sample + right sample) per Packet
SInt16StereoStreamFormat.mFramesPerPacket = 1;
SInt16StereoStreamFormat.mBytesPerFrame = SInt16StereoStreamFormat.mBytesPerPacket * SInt16StereoStreamFormat.mFramesPerPacket;
SInt16StereoStreamFormat.mChannelsPerFrame = 2; // 2 indicates stereo
SInt16StereoStreamFormat.mBitsPerChannel = 8 * bytesPerSample;
SInt16StereoStreamFormat.mSampleRate = graphSampleRate;
NSLog (@"The stereo stream format for the \"iPod\" mixer input bus:");
[self printASBD: SInt16StereoStreamFormat];
}
答案 1 :(得分：2)
我想这有点晚了，但你可以尝试这个库：
https://bitbucket.org/artgillespie/tslibraryimport
使用此功能将音频保存到文件后，您可以使用MixerHost中的渲染回调处理数据。
答案 2 :(得分：0)
如果我是你，我会使用kAudioUnitSubType_AudioFilePlayer播放文件并使用单位渲染回调访问其样本。
或者
使用ExtAudioFileRef将样本直接提取到缓冲区。