How Final Cut Pro Calculates Broadcast Wave File Timecode
at a Video Rate of 29.97 fps
If you import the same BWF file 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 timecode when importing BWF files.
In Final Cut Pro, three factors affect the calculated timecode value of an imported
BWF file:
 The true frame rate of NTSC video
 Mapping audio samples to video frames
 Drop frame indicators in BWF files
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 its true frame rate is
not simply 30 fps, nor is it 29.97 fps. The actual frame rate is 30 fps x 1000/1001, which
equals 29.9700299700299700 fps (repeating). Timecode calculated using 29.97 fps
(instead of 30 fps x 1000/1001) can be inaccurate, especially as the duration of media
increases. Final Cut Pro uses the more accurate calculation (30 x 1000/1001).
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 48,000 Hz and the video rate is 25 fps, every 1920
audio samples is equivalent to one video frame.
Chapter 20
Importing Media Files into Your Project
327
IV
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 Picture 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 BWF file, Final Cut Pro calculates timecode based on two parameters:
 The editing timebase (frame rate) of the currently selected sequence preset
 The audio sample rate of the imported BWF file
When the sequence preset frame rate is 29.97 fps and the sample rate of the BWF file is
defined in the SMPTE specification, Final Cut Pro uses the repeating frame pattern
defined in the SMPTE specification.
Drop Frame Indicators in Broadcast Wave Files
There is no ratified standard to indicate or detect whether a BWF 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 does not support reading this information. Instead,
Final Cut Pro always calculates non-drop frame timecode for BWF files. You can enforce
drop frame timecode during import or by using the Modify Timecode command. For
more information, see “
Importing Broadcast Wave Files
” on page 325.