Technical Q&A QA1600

QTKit Capture - Extracting SMPTE Timecode information from a QTSampleBuffer

Q: How can I extract SMPTE timecode information during device playback using the QTCaptureFileOutput object?

A: If the device you are capturing from is providing SMPTE timecode, the sample buffer (QTSampleBuffer) objects presented at the output will be tagged with timecode meta-data that may be accessed using the QTSampleBufferSMPTETimeAttribute key.

For example, both the QTCaptureFileOutput and QTCaptureDecompressedVideoOutput objects have a delegate method called captureOutput:didOutputSampleBuffer:fromConnection: which is invoked every time the output receives a new sample buffer. By using the QTSampleBuffer attributeForKey: method, the SMPTE timecode can be pulled out for each sample buffer recieved as shown in Listing 1.

Listing 1:

- (void)captureOutput:(QTCaptureFileOutput *)captureOutput
                      didOutputSampleBuffer:(QTSampleBuffer *)sampleBuffer
                             fromConnection:(QTCaptureConnection *)connection
{
    NSValue *SMPTETime = [sampleBuffer attributeForKey:QTSampleBufferSMPTETimeAttribute];
    if (SMPTETime) {
        NSLog(@"SMPTE Time: %@", QTStringFromSMPTETime([SMPTETime SMPTETimeValue]));
    }
}

A QTSampleBuffer may have other attributes as well. The following attributes are declared in QTSampleBuffer.h.

QTSampleBufferDateRecordedAttribute

Returns the date on which the media in the buffer was originally recorded.

The value is an NSDate.

The @"dateRecorded" string value can be used in key paths for key-value coding, key-value observing, and bindings.

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QTSampleBufferHostTimeAttribute

If the buffer is from a real time source, this attribute returns the buffer's host time.

The value returned by this attribute can be compared with the return value of CVGetCurrentHostTime or AudioGetCurrentHostTime to determine whether or not it is too late for the buffer to be processed in real time.

The value is an NSNumber interpreted as a UInt64.

The @"hostTime" string value can be used in key paths for key-value coding, key-value observing, and bindings.

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QTSampleBufferSMPTETimeAttribute

Returns the SMPTE timecode for the sample buffer, if it has one.

The value is an NSValue interpreted as a SMPTETime (defined in CoreAudio/CoreAudioTypes.h).

The @"SMPTETime" string value can be used in key paths for key-value coding, key-value observing, and bindings.

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QTSampleBufferSceneChangeTypeAttribute

If the buffer marks a scene change in the input content, returns a Scene Change constant. The returned constant specifies the type of scene change.

The @"sceneChangeType" string value can be used in key paths for key-value coding, key-value observing, and bindings.

QTSampleBufferExplicitSceneChange

Indicates that a scene change was explicitly marked in the sample buffers metadata.

This constant is returned by QTSampleBufferSceneChangeTypeAttribute specifying what kind of scene change, if any, is marked by a sample buffer.

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QTSampleBufferTimeStampDiscontinuitySceneChange

Indicates that the scene changed due to a discontinuity in time stamps between the current sample buffer and the previous sample buffer.

This constant is returned by QTSampleBufferSceneChangeTypeAttribute specifying what kind of scene chnage, if any, is marked by a sample buffer.

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References

• Introduction to QTKit Capture Programming Guide

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Document Revision History

Posted: 2008-03-17