使用AVAssetReader和timeRange实时读取样本

Question

以前我使用CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer从完整的音频文件中读取音频样本。 现在我想使用范围做同样的事情（即我指定时间范围......按时间读取一小部分音频，然后再返回并再次阅读）。 我想使用时间范围的原因是b / c我想控制每次读取的大小（以适应具有最大大小的数据包）。

由于某种原因，每次阅读之间总会有一个颠簸。 在我的代码中，你会注意到我启动AVAssetReader并在每次设置时间范围时结束它，这是b / c我无法在阅读器启动后动态调整时间范围（有关详细信息，请参见此处 ）。

可能是因为开始和结束读者太昂贵而无法产生持续的实时体验？ 或者还有其他方法可以做到这一点，我不知道吗？

还要注意，这个抖动或延迟发生在我设置时间间隔的任何时候......这让我相信以我的方式开始和结束读者对于实时音频播放来说太昂贵了。

- (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

我知道会发生什么:-D我花了将近一整天的时间来弄明白。

实际上，AVAssetReader会淡化前1024个样本（可能会多一点）。这就是您听到抖动效果的原因。

我通过在我真正想要读取的位置之前读取1024个样本来修复它，然后跳过1024个样本。

我希望它对你也有用。