Why does Drop Frame (29.97fps) exist?

Question

is it because of sound sync issues?

Answer 1

Drop frame is a term that only applies to smpte time code. if you were to count television frames, every thirty frames would be a second (almost) so you could track time (almost) by counting frames. the drop frame is a convention for correcting frame count so that the minutes and hours match real time. the only person that needs be concerned about drop frame is the editor. when doing frame accuate edits based on time code, he may find that a certain TC in point or out point does not "exist" because that number happened to be skipped when time code was assigned because of the drop frame convention.

i am not sure what you mean by sound sync, but i am guessing that you are talking about digital tape recording. if both the sound and video is digital, then they share a common clock when recording. the clock for the audio is not precisely 48k because that would cause audio words to not be aligned with video frames, making a glitch in edited tape.  this has to do with NTSC video standards and nothing to do with drop frame. All NTSC video is 29.97 fps whether time code is associated with it or not. There are no missing frames. if you were to play back the digital audio track with a clock that ran at 48k exactly instead of a clock locked to the video, then this audio would eventually get noticably out of kilter with the associated video. a professional video editing program must take external sources of audio, that was recorded at true 48k and conform it to fit the video clock. this conforming process is usually not done for amateur grade work.

if you are asking why television is 29.97 fps instead of 30, that has to do with making color television backward compatible with older B/W television sets. the aural carrier is very precisely set at 4.5 MHz above the visual. in order to maintain a fixed integer relation allowing the color subcarrier to interleave with horizontal scan frequency, the frame rate was adjusted very slightly from the original B/W 30 fps.

Answer 2

What Lare is trying to say is that Drop Frame 29.97 exists as a problem solver to a specific issue.

Your camcorder probably records 30 exact frames per a second.  The NTSC TV signal most people watch is 29.97 fps.  Those numbers don't match up exactly, and this can cause a problem.  The video from a digital camcorder may not display correctly on a standard TV.  

This problem is not very common now.  It was sometimes seen several years ago when most people had an older TV with electronics that were developed and manufactured before the first digital camcorders (miniDV and Digital8) came out.  Connecting the first miniDV and Digital8 camcorders to some of these older TVs and trying to play the video resulted in an erratic picture.

The Drop Frame feature was inserted into editing programs to adjust the 30fps digital video to display slightly faster at 29.97fps.  Later, the feature was permanently programmed into the electronics of television sets in case someone connected a digital camcorder directly to a TV.  Most people back then did not take the time to edit their home videos on their Window 98, 2000, or iMac G3 computers.  They just connected the camcorder directly to the TV to watch the video tapes.

Once Windows XP and iMac G4 computers came out with free editing programs and the home user got into editing videos, the digital camcorders and televisions had already been improved to eliminate this problem.  The only reason Drop Frame remained in editing programs after this point was in case you need to supply an NTSC video to a television station whose system ran only on 29.97fps.