Because video isn't a perfect 30 frames per second, but rather 29.97*, syncing audio with a film transfer gets a little more complicated. Now you've got a video version of the film which is ALMOST real-time, but not quite. The difference isn't noticable by watching the picture only, but it presents a problem when trying to syncronize the sound track to the picture. If you don't slow down the sound accordingly, you will notice that the sound seems to drift away from the picture after 30 seconds or so. The difference in the rates is exactly the same as the difference between DROP FRAME and NON DROP FRAME video time code (0.03 frames per second). The drift would add-up to 18 frames every ten minutes.
*The true rate is of NTSC color video is 29.97002617 Hz which results in a discrepancy of 2.261 frames over 24 hours.
Put yet ANOTHER way, if you were to simultaneously play (1) the original film footage, (2) the video version of the film, and (3) the field sound , all at the same time, the film and field sound would stay in sync, but the video version would be a tad slower, noticably so after 30 to 60 seconds had passed.
So, because the film runs a little slower than 24 fps during the tape transfer, the sound must be slowed down slightly when placed in sync on a video copy of a film.**
** NOTE: If the film was shot at 30 fps, then the audio must have a x.999 speed adjustment because the film will be running at 29.97 fps when transferred to video.
** NOTE: If the film was shot at 24 fps, then the audio must have a x.999 speed adjustment because the film will be running at 23.976 fps when transferred to video.
In any event, unless the film was not shot at 23.976, or 29.97 fps, a x.999 speed alteration in the audio must occur in order for teh sound to sync up to the video transfer.
So what to do?
*If the field audio carries a 60Hz pilot tone (which is what most Nagras use to keep the motor going at a constant speed) you must lock the player to an external 59.94 Hz crystal to slow it down.
*If the field audio uses time code for sync (which is how many DATs work) and the time code on the tape is 30 fps, resolve it to 29.97 to slow it down.
NOTE: Digital field audio can also be speed-altered by changing the playback rate. (i.e. audio sampled at 44.1 KHz can be played at 44.056 KHz, or audio sampled at 48 KHz can be played at 47.952 KHz )
Fixing the sound on an Avid, or w/ProTools.
(ProTools is a non-linear SOUND design program which is often used in conjunction with the Avid. Other non-linear sound design software packages include AudioVision, Sonic Solutions and Deck II.)
If the audio was placed in sync on the video by a transfer facility, and the picture and track are digitized to the Avid together from that tape, set the ProTools hardware pulldown at x.99. This actually speeds the audio back up to its original rate because the Avid then plays the film at its original 24 fps. (by sampling at 44.056 KHz or 47.952 KHz)
For a film project where analog audio is transferred directly to the Avid for autosyncing to picture, no speed correction is required because the Avid then plays the film at its original 24 fps. (set the ProTools hardware pulldown at x1.0)
For a film project where digital audio is transferred directly to the Avid for autosyncing to picture, no speed correction is required because the Avid then plays the film at its original 24 fps; therefore, the audio playing at its original rate will sync to the picture. (the ProTools hardware pulldown is of no matter because the clock will detect and lock to the sample rate of the incoming digital audio; use Pro Tools Setup to arrange digital input).
Questions about sync?
Back to the Pulldown Explained page.