Video time and time codes

6.30 AM Sunday morning.

I finally get the relationship between timebase and frame rates in camera and in editing and  a clearer understanding of time codes . Big learning moment for me as I have been very confused by dialogue boxes telling me there is a mismatch.  Just so I don’t forget. Paraphrased from Adobe Premiere 6.0 Classroom in  a Book.

Measuring video time

Hour, minutes and seconds are not an accurate enough for video editing because it requires precise synchronisation. One second may contain several events.

Timebase and frame rate interaction.

In a video project, time is measured by specifying the project timebase eg a timebase of 25 means that each second is divided into 25 units.

The exact time at which an edit occurs depends on the timebase specified, because it only occurs at a time division. Using a different timebase causes the divisions to fall in a different place.

The source time rate determines the time increments in a source clip eg shoot at 30 frames per second (fps) and the camera documents as 1/30 of a second (missing out what happened between exposures). Therefore the lower the fps the less information is stores and vice versa.

Editing software allows the user to specify the project frame rate. Therefore a project frame rate of 15 fps means that the  proramme will create 15 frames for each second. However, for smooth playback the source and project must be identical. If not frames must be added or dropped so it is preferable to ensure that the two match.

However, this is not always practical because multiple viewing platforms are required. Phase Alternating Line (PAL) for broadcasting uses 25fps, motion picture uses 24 (23.9) and  web 15.

To reduce jerky playback it is important to record in a frame rate that divides evenly into the timebase. eg so recording at 30fps outputs at 30, 15 or 10fps will play smoothly.

When time systems don’t match the most important value to set is the timbase which is set to the most critical final medium.

The mantra is the most predicatable results occur when the timebase and frame rate are multiples of one another. The best results occur when they are identical.

No more You tube undecipherable messages about frame rate mismatches.

Equally…

Timecodes

The time code is a frame number. However frames are counted differently acording to the output.

Premier defaults to the SMPTE (Society of Motion Picture and Television Engineers) where a duration of 00:01:15:10 indicates a clip of 1 minute 15 seconds and 10 frames. Feet and frames is another system.

Drop frame time codes are represented with semi-colons and occurs where the source is not excactly a match eg NTSC (National Television System Committee, the US standard) at 29.97fps.  “ Premier renumbers the first two frames of every minute  except for the tenth minute”  ie the frame following 59:29 is 1:00:02. Confusingly no frames are lost because it does not drop frames just frame numbers.

 

Update

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