Ios Avassetrader以低质量播放MPMediaItem？

在结合了几个问题的答案后，我成功地使用Avassetrader从MPMediaItem中获取了原始数据，如和和一个nice。我也可以使用FMOD播放这些原始数据，但随后出现了一个问题

结果音频的质量似乎低于原始曲目。虽然AVAssetTrack formatDescription告诉我数据中有两个通道，但结果听起来是单声道的。它听起来也有点低沉（不那么脆），好像比特率降低了

是我做错了什么，还是Avassetrader故意降低了MPMediaItem数据的质量（因为盗版）

---

初始化AvasseTrader和AvasseTraderTrackOutput

//准备AVAsset和AVAssetReaderOutput等
MPMediaItem*mediaItem=。。。；
NSURL*ipodAudioUrl=[mediaItem valueForProperty:MPMediaItemPropertyAsetUrl]；
AVURLAsset*asset=[[AVURLAsset alloc]initWithURL:ipodAudioUrl选项：nil]；
n错误*错误=nil；
AssetTreader=[[AvAssetTreader alloc]initWithAsset:资产错误：&错误]；
如果（错误）
NSLog（@“创建读取器时出错：%@，[error debugDescription]）；
AVAssetTrack*songTrack=[asset.tracks对象索引：0]；
NSArray*trackDescriptions=songTrack.formatDescriptions；
numChannels=2；
for（无符号整数i=0；i<[trackDescriptions计数]；+i）
{
cAudioFormatDescriptionRef项=（cAudioFormatDescriptionRef）[trackDescriptions对象索引：i]；
const AudioStreamBasicDescription*bobTheDesc=cAudioFormatDescription GetStreamBasicDescription（项目）；
if（bobTheDesc&&bobTheDesc->mChannelsPerFrame==1）{
numChannels=1；
}
}   
NSDictionary*outputSettingsDict=[[[NSDictionary alloc]initWithObjectsAndKeys:
[NSNumber numberWithInt:kAudioFormatLinearPCM]，AVFormatIDKey，
[NSNumber numberWithInt:OUTPUTRATE]，AVSampleRateKey，
[NSNumber NUMBER WITHIT:16]，AVLINEARPCMBITDEPTHEY，
[NSNumber numberWithBool:NO]，AVLinearPCMIsBigEndianKey，
[NSNumber numberWithBool:NO]、AVLinearPCMIsFloatKey、，
[NSNumber numberWithBool:NO]，AVLINEARPCM为非交叉，
无]自动释放]；
AvassetraderTrackOutput*输出=[[AvassetraderTrackOutput alloc]initWithTrack:songTrack输出设置：输出设置ICT]自动释放]；
[AssetTreader添加输出：输出]；
[assetReader startReading]；

初始化FMOD和FMOD声音

//初始化FMOD
FMOD_结果结果=FMOD_正常；
无符号整数版本=0；
/*
创建系统对象并初始化
*/    
结果=FMOD:：系统\创建（和系统）；
错误检查（结果）；
结果=系统->获取版本（&V）；
错误检查（结果）；
if（版本设置软件格式（输出、FMOD声音格式、PCM16、1、0、FMOD、DSP、重采样、线性）；
错误检查（结果）；
结果=系统->初始化（32，FMOD_init_NORMAL | FMOD_init_ENABLE_PROFILE，NULL）；
错误检查（结果）；
//初始FMOD声流
CMTimeRange timeRange=[songTrack timeRange]；
浮动持续时间秒=timeRange.duration.value/timeRange.duration.timescale；
FMOD_CREATESOUNDEXINFO exinfo={0}；
memset（&exinfo，0，sizeof（FMOD_CREATESOUNDEXINFO））；
exinfo.cbsize=sizeof（FMOD_CREATESOUNDEXINFO）；/*必需的*/
exinfo.decodebuffersize=输出；/*样本中流更新的块大小。这将是传递给用户回调的数据量*/
exinfo.length=输出*numChannels*sizeof（有符号短）*持续时间秒；/*PCM数据的长度，以整首歌曲的字节为单位（对于Sound:：getLength）*/
exinfo.numchannels=numchannels；/*声音中的频道数*/
exinfo.defaultfrequency=输出；/*声音的默认播放速率*/
exinfo.format=FMOD\u SOUND\u format\u PCM16；/*声音的数据格式*/
exinfo.pcmreadcallback=pcmreadcallback；/*用于读取的用户回调*/
exinfo.pcmsetposcallback=pcmsetposcallback；/*用于查找的用户回调*/
结果=系统->创建流（NULL、FMOD\u OPENUSER、&exinfo、&sound）；
错误检查（结果）；
结果=系统->播放声音（FMOD_通道自由、声音、错误和通道）；
错误检查（结果）；

从AvasseTraderTrack输出读取到环形缓冲区

AvassetraderTrackOutput*trackOutput=（AvassetraderTrackOutput*）[AssetTreader.outputs对象索引：0]；
CMSampleBufferRef sampleBufferRef=[trackOutput copyNextSampleBuffer]；
if（sampleBufferRef）
{
AudioBufferList AudioBufferList；
CMBlockBufferRef块缓冲区；
CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer（sampleBufferRef、NULL和audioBufferList、sizeof（audioBufferList）、NULL、NULL、0和blockBuffer）；
if（blockBuffer==NULL）
{
停止加载=是；
继续；
}
如果（&audioBufferList==NULL）
{
停止加载=是；
继续；
}
if（audioBufferList.mNumberBuffers！=1）
NSLog（@“numBuffers=%lu”，audioBufferList.mNumberBuffers）；
对于（int y=0；y我不熟悉FMOD，所以我不能在那里发表评论。Avassetrader不做任何“复制保护”的事情，所以这不成问题。（如果你能获得AVAssetURL，这首歌是无DRM的）

因为您使用的是非交错缓冲区，所以只有一个缓冲区，所以我猜您的最后一位代码可能是错误的

这是一些代码对我来说很好的例子。BTW，for for循环可能不会很有性能。你可以考虑使用<代码> MeMCPY <代码>……
如果您不受现有环形缓冲区的限制，
#define OUTPUTRATE   44100

// prepare AVAsset and AVAssetReaderOutput etc
MPMediaItem* mediaItem = ...;
NSURL* ipodAudioUrl = [mediaItem valueForProperty:MPMediaItemPropertyAssetURL];
AVURLAsset * asset = [[AVURLAsset alloc] initWithURL:ipodAudioUrl options:nil];

NSError * error = nil;
assetReader = [[AVAssetReader alloc] initWithAsset:asset error:&error];

if(error)
    NSLog(@"error creating reader: %@", [error debugDescription]);

AVAssetTrack* songTrack = [asset.tracks objectAtIndex:0];
NSArray* trackDescriptions = songTrack.formatDescriptions;

numChannels = 2;
for(unsigned int i = 0; i < [trackDescriptions count]; ++i) 
{
    CMAudioFormatDescriptionRef item = (CMAudioFormatDescriptionRef)[trackDescriptions objectAtIndex:i];
    const AudioStreamBasicDescription* bobTheDesc = CMAudioFormatDescriptionGetStreamBasicDescription (item);
    if(bobTheDesc && bobTheDesc->mChannelsPerFrame == 1) {
        numChannels = 1;
    }
}   

NSDictionary* outputSettingsDict = [[[NSDictionary alloc] initWithObjectsAndKeys:

                                    [NSNumber numberWithInt:kAudioFormatLinearPCM],AVFormatIDKey,
                                    [NSNumber numberWithInt:OUTPUTRATE],AVSampleRateKey,
                                    [NSNumber numberWithInt:16],AVLinearPCMBitDepthKey,
                                    [NSNumber numberWithBool:NO],AVLinearPCMIsBigEndianKey,
                                    [NSNumber numberWithBool:NO],AVLinearPCMIsFloatKey,
                                    [NSNumber numberWithBool:NO],AVLinearPCMIsNonInterleaved,
                                    nil] autorelease];

AVAssetReaderTrackOutput * output = [[[AVAssetReaderTrackOutput alloc] initWithTrack:songTrack outputSettings:outputSettingsDict] autorelease];
[assetReader addOutput:output];
[assetReader startReading];

// Init FMOD
FMOD_RESULT result = FMOD_OK;
unsigned int version = 0;

/*
 Create a System object and initialize
 */    
result = FMOD::System_Create(&system); 
ERRCHECK(result);

result = system->getVersion(&version);
ERRCHECK(result);

if (version < FMOD_VERSION)
{
    fprintf(stderr, "You are using an old version of FMOD %08x.  This program requires %08x\n", version, FMOD_VERSION);
    exit(-1);
}

result = system->setSoftwareFormat(OUTPUTRATE, FMOD_SOUND_FORMAT_PCM16, 1, 0, FMOD_DSP_RESAMPLER_LINEAR);
ERRCHECK(result);    

result = system->init(32, FMOD_INIT_NORMAL | FMOD_INIT_ENABLE_PROFILE, NULL);
ERRCHECK(result);


// Init FMOD sound stream

CMTimeRange timeRange = [songTrack timeRange];
float durationInSeconds = timeRange.duration.value / timeRange.duration.timescale;

FMOD_CREATESOUNDEXINFO exinfo = {0};
memset(&exinfo, 0, sizeof(FMOD_CREATESOUNDEXINFO));

exinfo.cbsize            = sizeof(FMOD_CREATESOUNDEXINFO);              /* required. */
exinfo.decodebuffersize  = OUTPUTRATE;                                  /* Chunk size of stream update in samples.  This will be the amount of data passed to the user callback. */
exinfo.length            = OUTPUTRATE * numChannels * sizeof(signed short) * durationInSeconds; /* Length of PCM data in bytes of whole song (for Sound::getLength) */
exinfo.numchannels       = numChannels;                                 /* Number of channels in the sound. */
exinfo.defaultfrequency  = OUTPUTRATE;                                  /* Default playback rate of sound. */
exinfo.format            = FMOD_SOUND_FORMAT_PCM16;                     /* Data format of sound. */
exinfo.pcmreadcallback   = pcmreadcallback;                             /* User callback for reading. */
exinfo.pcmsetposcallback = pcmsetposcallback;                           /* User callback for seeking. */

result = system->createStream(NULL, FMOD_OPENUSER, &exinfo, &sound);
ERRCHECK(result);

result = system->playSound(FMOD_CHANNEL_FREE, sound, false, &channel);
ERRCHECK(result);

AVAssetReaderTrackOutput * trackOutput = (AVAssetReaderTrackOutput *)[assetReader.outputs objectAtIndex:0];
CMSampleBufferRef sampleBufferRef = [trackOutput copyNextSampleBuffer];

if (sampleBufferRef)
{
    AudioBufferList audioBufferList;
    CMBlockBufferRef blockBuffer;
    CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(sampleBufferRef, NULL, &audioBufferList, sizeof(audioBufferList), NULL, NULL, 0, &blockBuffer);

    if(blockBuffer == NULL)
    {
        stopLoading = YES;
        continue;
    }

    if(&audioBufferList == NULL)
    {
        stopLoading = YES;
        continue;
    }

    if(audioBufferList.mNumberBuffers != 1)
        NSLog(@"numBuffers = %lu", audioBufferList.mNumberBuffers);

    for( int y=0; y<audioBufferList.mNumberBuffers; y++ )
    {
        AudioBuffer audioBuffer = audioBufferList.mBuffers[y];
        SInt8 *frame = (SInt8*)audioBuffer.mData;

        for(int i=0; i<audioBufferList.mBuffers[y].mDataByteSize; i++)
        {
            ringBuffer->push_back(frame[i]);
        }
    }

    CMSampleBufferInvalidate(sampleBufferRef);
    CFRelease(sampleBufferRef);
}

CMSampleBufferRef nextBuffer = NULL;

if(_reader.status == AVAssetReaderStatusReading)
{
    nextBuffer = [_readerOutput copyNextSampleBuffer];
}                   

if (nextBuffer)
{
    AudioBufferList abl;
    CMBlockBufferRef blockBuffer;
    CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
        nextBuffer,
        NULL,
        &abl,
        sizeof(abl),
        NULL,
        NULL,
        kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
        &blockBuffer);

    // the correct way to get the number of bytes in the buffer
    size_t size = CMSampleBufferGetTotalSampleSize(nextBuffer);

    memcpy(ringBufferTail, abl.mBuffers[0].mData, size);

    CFRelease(nextBuffer);
    CFRelease(blockBuffer);
}

result = system->setSoftwareFormat(OUTPUTRATE, FMOD_SOUND_FORMAT_PCM16, 2, 0, FMOD_DSP_RESAMPLER_LINEAR);