Getting frame rate from AVCaptureVideoDataOutput

I'm using AVCaptureVideoDataOutput to grab frames, process them, and then write them to a MOV file using AVAssetWriter. I understand that to set the minimum frame rate for the data input, I need only write

myDataOutput.minFrameDuration = someCMTime;

How can I get the actual frame duration for a given cycle of

- (void) captureOutput:(AVCaptureOutput *)captureOutput didOutputS开发者_Go百科ampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection

I need the actual duration so that I can input an accurate duration time for the asset writer. I've been playing around with using CMSampleBufferGetDuration(sampleBuffer), but with limited success. Any idea how to get this value?

Here is my current captureOutput method implementation:

- (void) captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection 
{
    if([writerInput isReadyForMoreMediaData])
    {

        CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);

        if(imageBuffer){

            totalTime = CMTimeAdd(totalTime, CMSampleBufferGetDuration(sampleBuffer));


            if([adaptor appendPixelBuffer:imageBuffer withPresentationTime:totalTime]){
                NSLog(@"frame added");
                CVPixelBufferUnlockBaseAddress(imageBuffer,0);
            }else{
                NSLog(@"frame NOT added");
            }

        }else{
            NSLog(@"no buffer");
        }
    }else{
        NSLog(@"writerinput not ready");
    }
}

In the end, the problem was that I was using AVAssetWriterInputPixelBufferAdaptor, which requires the user to set the presentation time. Instead, I ended up just adding frames directly by calling appendBuffer on AVAssetWriterInput