How Final Cut Pro calculates Broadcast Wave File Time at a Video Rate of 29.97 frames per second

If you import the same Broadcast Wave file (BWF) into Final Cut Pro and other editing applications, certain timecode numbers may be calculated differently. The following section explains how Final Cut Pro accurately calculates time when importing Broadcast Wave files.

In Final Cut Pro, there are three factors that affect the calculated timecode value of an imported Broadcast Wave file:

1. The true frame rate of NTSC video:
   

   Timecode calculation is fairly straightforward, with the exception of NTSC-related frame rates. Calculating timecode for NTSC video is complicated because it's true frame rate is not simply 30, nor is it 29.97. The actual frame rate is 30 * 1000/1001, which equals 29.9700299700299700 (repeating). Timecode calculated using 29.97 (instead of 30 * 1000/1001) can be inaccurate, especially as the duration of media increases. Final Cut Pro uses the more accurate calculation (30 * 1000/1001).

   For more information, see SMPTE 170M-1999 "Composite Analog Video Signal - NTSC for Studio Applications."

2. Mapping audio samples to video frames:
   

   Timecode is calculated for digital audio files by mapping a certain number of audio samples to each video frame. When the sample rate is an integer multiple of the video frame rate, the timecode count can be accurately mapped to the sample count. For example, if the audio sample rate is 48000 Hz and the video rate is 25 fps, every 1920 audio samples is equivalent to one video frame.

   However, with NTSC-related video rates, there is no simple relationship between the number of audio samples per second and video frames per second. To avoid ambiguity, the Society of Motion Pictures and Television Engineers (SMPTE) specifies how many audio samples should be mapped to each video frame. The number of audio samples per frame is varied on a frame-by-frame basis to form a consistent pattern. Instead of an exact number of samples per frame, a consistent pattern of varying samples is spread over several frames.

   When you import a Broadcast Wave file, Final Cut Pro calculates timecode based on two parameters:

   • The time base (frame rate) of the currently selected sequence preset
   • The audio sample rate of the imported Broadcast Wave file

    

   When the sequence preset frame rate is 29.97 fps and the sample rate of the Broadcast Wave file is defined in the SMPTE specification, Final Cut Pro uses the repeating frame patterned defined in the SMPTE specification.

   For more information, see sections 3.15 and 14.3 of ANSI/SMPTE 272M-1994 "Formatting AES/EBU Audio and Auxiliary Data into Digital Video Ancillary Data Space."

    

3. Dropframe indicators in Broadcast Wave files:
   There is no ratified standard to indicate or detect whether a Broadcast Wave file was recorded with drop frame or non-drop frame timecode. Although there are several de facto ways this information is stored, Final Cut Pro 5.1.2 does not support reading this information. Instead, Final Cut Pro 5.1.2 always calculates non-drop frame timecode for Broadcast Wave files.

To modify the timecode of a Broadcast WAVE file to drop frame:

1. In the Browser, select the audio clip whose timecode you want to modify.
2. Choose Modify > Timecode, then choose Drop Frame from the Format pop-up menu.

Detailed SMPTE technical documents and specifications, including those referenced in this document, are available from the SMPTE website.