使用AVAssetReader和timeRange實時讀取樣本

Question

以前我使用CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer從完整的音頻文件中讀取音頻樣本。 現在我想使用范圍做同樣的事情（即我指定時間范圍......按時間讀取一小部分音頻，然后再返回並再次閱讀）。 我想使用時間范圍的原因是b / c我想控制每次讀取的大小（以適應具有最大大小的數據包）。

由於某種原因，每次閱讀之間總會有一個顛簸。 在我的代碼中，你會注意到我啟動AVAssetReader並在每次設置時間范圍時結束它，這是b / c我無法在閱讀器啟動后動態調整時間范圍（有關詳細信息，請參見此處 ）。

可能是因為開始和結束讀者太昂貴而無法產生持續的實時體驗？ 或者還有其他方法可以做到這一點，我不知道嗎？

還要注意，這個抖動或延遲發生在我設置時間間隔的任何時候......這讓我相信以我的方式開始和結束讀者對於實時音頻播放來說太昂貴了。

- (void) setupReader 
{
    NSURL *assetURL = [NSURL URLWithString:@"ipod-library://item/item.m4a?id=1053020204400037178"];   
    songAsset = [AVURLAsset URLAssetWithURL:assetURL options:nil];

    track = [songAsset.tracks objectAtIndex:0];     
    nativeTrackASBD = [self getTrackNativeSettings:track];

    // set CM time parameters
    assetCMTime = songAsset.duration;
    CMTimeReadDurationInSeconds = CMTimeMakeWithSeconds(1, assetCMTime.timescale);
    currentCMTime = CMTimeMake(0,assetCMTime.timescale); 
}

-(void)readVBRPackets
{
    // make sure assetCMTime is greater than currentCMTime
    while (CMTimeCompare(assetCMTime,currentCMTime) == 1 )
    {
        NSError * error = nil;
        reader = [[AVAssetReader alloc] initWithAsset:songAsset error:&error];
        readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track
                                                                  outputSettings:nil];

        [reader addOutput:readerOutput];
        reader.timeRange = CMTimeRangeMake(currentCMTime, CMTimeReadDurationInSeconds);

        [reader startReading];

        while ((sample = [readerOutput copyNextSampleBuffer])) {
            CMItemCount numSamples = CMSampleBufferGetNumSamples(sample);
            if (numSamples == 0) {
                continue;
            }

            NSLog(@"reading sample");               

            CMBlockBufferRef CMBuffer = CMSampleBufferGetDataBuffer( sample );                                                         
            AudioBufferList audioBufferList;  

            OSStatus err = CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
                                                                               sample,
                                                                               NULL,
                                                                               &audioBufferList,
                                                                               sizeof(audioBufferList),
                                                                               NULL,
                                                                               NULL,
                                                                               kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
                                                                               &CMBuffer
                                                                                   );



            const AudioStreamPacketDescription   * inPacketDescriptions;
            size_t                               packetDescriptionsSizeOut;
            size_t inNumberPackets;

            CheckError(CMSampleBufferGetAudioStreamPacketDescriptionsPtr(sample, 
                                                                         &inPacketDescriptions,
                                                                         &packetDescriptionsSizeOut),
                       "could not read sample packet descriptions");

            inNumberPackets = packetDescriptionsSizeOut/sizeof(AudioStreamPacketDescription);

            AudioBuffer audioBuffer = audioBufferList.mBuffers[0];


            for (int i = 0; i < inNumberPackets; ++i)
            {

                SInt64 dataOffset = inPacketDescriptions[i].mStartOffset;
                UInt32 packetSize   = inPacketDescriptions[i].mDataByteSize;            

                size_t packetSpaceRemaining;
                packetSpaceRemaining = bufferByteSize - bytesFilled;

                // if the space remaining in the buffer is not 
                // enough for the data contained in this packet
                // then just write it
                if (packetSpaceRemaining < packetSize)
                {
                    [self enqueueBuffer];           
                }

                // copy data to the audio queue buffer
                AudioQueueBufferRef fillBuf = audioQueueBuffers[fillBufferIndex];
                memcpy((char*)fillBuf->mAudioData + bytesFilled, 
                       (const char*)(audioBuffer.mData + dataOffset), packetSize);                                                                

                // fill out packet description
                packetDescs[packetsFilled] = inPacketDescriptions[i];
                packetDescs[packetsFilled].mStartOffset = bytesFilled;

                bytesFilled += packetSize;
                packetsFilled += 1;

                // if this is the last packet, then ship it
                size_t packetsDescsRemaining = kAQMaxPacketDescs - packetsFilled;
                if (packetsDescsRemaining == 0) {          
                    [self enqueueBuffer];              
                }                  
            }

            CFRelease(CMBuffer);
            CMSampleBufferInvalidate(sample);
            CFRelease(sample);
        }

        [reader cancelReading];
        reader = NULL;
        readerOutput = NULL;

        currentCMTime = CMTimeAdd(currentCMTime, CMTimeReadDurationInSeconds);
    }


}

Answer 1

我知道會發生什么:-D我花了將近一整天的時間來弄明白。

實際上，AVAssetReader會淡化前1024個樣本（可能會多一點）。這就是您聽到抖動效果的原因。

我通過在我真正想要讀取的位置之前讀取1024個樣本來修復它，然后跳過1024個樣本。

我希望它對你也有用。