If you go to the trouble to look at the source clip timeline with waveforms (sampleplot) turned on, you may discover the audible sync mark of the slate is about 1/2 frame before, or after, the leading boundary of your picture sync frame. This question would then arise: "Is something wrong here?". Odds are, the answer is "No."
For those of you familiar with editing film (real, physical, touchy-feely film), you know the experience of syncing dailies and finding the audible slate mark is in the center of a frame. There is also the dilemma that arises when the slate obviously closed between picture frames, but that is another worm in this can o' worms. When working with 35mm film you can slip the track by one to three perforations so you can get the slate to sound-off right at the frame line. With 16mm you are usually stuck deciding to go with the slate pop 1/2 frame early, or late. There is little difference when dealing with sync dailies on video or in a DNL picture editing system. Audio workstations like AudioVision and Pro Tools are capable of subframe, sample accurate edits.
Video picture and audio, as well as DAT audio are bounded by frame lines which are delineated by timecode addresses; it is possible for slate pops to occur in the middle of a frame on those media as well. When syncing audio to film in telecine the picture slate mark is located, and the audible slate is located on the sound source, to the nearest frame just as in film syncing. Then the two elements are edited together.
"But it looks in sync when I play the video tape!"
When looking at the video and audio playing together from the tape, they may look more perfectly in sync than when you later digitize that material in a 24 fps film project principally because 2-3 interlaced video at 30 fps hides 1/2 frame sync errors better than 24 fps progressive scan (aka 24p).. FYI: When Avid plays 24 fps material in a film project the edit monitor is displaying 24 progressive (with the monitor refreshing at what ever rate it is set to by the OS), and the 2-3 frame/field sequence is reestablished in the video signal sent to the client monitor, with the playback speed referenced to 30Hz.
A summary of the reasons why video might look more in sync:
1) The latency of the image on a video monitor, (it tends to decay, rather than disappear instantly).
2) Interlacing of fields. The 2-3 pulldown process adds fields/frames in addition to the fact that each field is temporally separated. (i.e., if a slate pop hits perfectly at the timecode boundary of a frame, the audio is already out of sync by 1/2 frame in field 2!!).
3) 30 fps is faster than 24 fps.
What you might see in the timeline...
