MAC - AVAssetReader for TRIM - reading samples OUT OF ORDER

Question

I am trying to read and write the samples from a video file at a specific start point and end point for trimming a video. I am using AVAssetReader and AVAssetWriter.

The logic used here is -

STEP A:

• Create asset reader instance with the specified asset.
• Set the time range to the reader as per the start and end points. ( Say for example - start point = 5, end point = 15, file length = 55 sec )
• Start reading the samples.
• Get the sample's exact time stamp with respect to start point that we have passed in.
• Store the time stamp of the sample that is accurate with the start point.( could be 5.13 or so ). Say ta = 5.13
• Release the reader.

STEP B:

• Create a new reader instance with the specified asset.
• Set the time range to the reader as per the start and end points. ( Say for example - start point = 5, end point = 15, file length = 55 sec )
• Start reading the samples.
• Create a new sample buffer with sample timing info altered as ( sample buffer's time stamp t1- ta fetched from STEP A ) - This starts to write from 0 ( sample buffer's time stamp t2- ta fetched from STEP A ) ( sample buffer's time stamp t3- ta fetched from STEP A ) and so on till end point.
• Release the reader.

The code sample for the same is:

STEP A:

while ([assetWriterInput isReadyForMoreMediaData] )
            {
                CMSampleBufferRef sampleBuffer = [assetReaderOutput copyNextSampleBuffer];
                if (sampleBuffer != NULL)
                {
                    CMTime originalTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
                    float time = CMTimeGetSeconds( originalTime );

                   // This is where we store the time stamp 
                    fTimeTange = time;
                    [HelpMethods setCorrectEditTime:fTimeTange]; // This is stored globally 

                    // This is to release the readers and writers and start a fresh call with the stored time stamp ta
                    [delegate resetTimeRange];
                    return;
                }
            }

STEP B:

while ([assetWriterInput isReadyForMoreMediaData] ) 
         {
            CMSampleBufferRef sampleBuffer = [assetReaderOutput copyNextSampleBuffer];
            if (sampleBuffer != NULL)
            {
                CMSampleBufferRef finalBuffer = sampleBuffer;
                CMSampleBufferRef newSampleBuffer;
                CMSampleTimingInfo sampleTimingInfo;

                CMTime cmm1 = CMSampleBufferGetOutputDuration(sampleBuffer);

                CMTime originalTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
                float time = CMTimeGetSeconds( originalTime );

                // This is a helper method to get the stored ta at STEP A
                fTimeTange = [HelpMethods getCorrectEditTime];

                sampleTimingInfo.duration = cmm1;
                float milliseconds = (time - fTimeTange) * 600;

                NSLog( @"Timestamp in  milliseconds = %f", milliseconds );

                sampleTimingInfo.presentationTimeStamp = CMTimeMake(milliseconds, 600);
                sampleTimingInfo.decodeTimeStamp = kCMTimeInvalid;

                CMSampleBufferCreateCopyWithNewTiming(kCFAllocatorDefault,
                                                      sampleBuffer,
                                                      1,
                                                      &sampleTimingInfo,
                                                      &newSampleBuffer);
                finalBuffer = newSampleBuffer;


                BOOL success = YES;
                success  = [assetWriterInput appendSampleBuffer:finalBuffer];


                CFRelease(sampleBuffer);
                sampleBuffer = NULL;
            }

Since the time stamps of the samples are read in out of order fashion, we end up getting an error saying

"(kFigFormatWriterError_InvalidTimestamp) (decode timestamp is less than previous sample's decode timestamp)"

and the values of time stamps are -

• Timestamp in milliseconds = 0.000000
• Timestamp in milliseconds = 79.999924
• Timestamp in milliseconds = 39.999962
• Timestamp in milliseconds = 119.999886
• Timestamp in milliseconds = 200.000092
• Timestamp in milliseconds = 160.000137
• Timestamp in milliseconds = 280.000031
• Timestamp in milliseconds = 240.000061
• Timestamp in milliseconds = 319.999969
• Timestamp in milliseconds = 399.999908
• Timestamp in milliseconds = 359.999939 and so on

Any manipulation done to the presentation stamps results in out of order reading of samples.

Looking out a way to overcome this out of order reading of timestamps. Please let know.

Thanks in advance, Champa