How does the audio sync properly for telecined video?

[90sTV]

If you're converting 24 FPS to 30 FPS, wouldn't this mean the picture initially gets ahead of the audio, only for the audio to catch up later when duplicate fields are displayed?

Like, if you're showing two fields that correspond to a complete frame, that picture is now displayed for a shorter amount of time than it was on film, since the frame rate is higher to accommodate the extra fields.

Or is it just such a small difference that it's imperceptible to the viewer?

[johnmeyer]

When you telecine film the audio stays in perfect sync. That is because you assign a playback speed to the video that plays the different number of frames (more if you are converting 24 --> 30, or fewer if you are doing IVTC to go 30 --> 24) in the exact same amount of time.

[Brad]

The two fields are shown over the course of ~1/30 sec.

The film frame is shown over 1/24 sec.

The video fields occupy a shorter time, but consequently, a shorter portion of the audio track is played during this time. So sync is maintained.

In Avisynth, you can confirm this using something like ChangeFPS(60) on a 24fps file and comparing the length of audio per-frame with VirtualDub's audio display or AudioGraph.

[Cornucopia]

It's a type of jitter, but one that is small enough that it is rarely/occasionally noticeable, much less objectionable. At most, going from 24p to 60i, it could theoretically (though not as much in practice) deviate 1/2 field. That is 1/120th sec or 8.333 msec.

Others, correct me if I'm wrong...

Scott

[johnmeyer]

It's a type of jitter, but one that is small enough that it is rarely/occasionally noticeable, much less objectionable. At most, going from 24p to 60i, it could theoretically (though not as much in practice) deviate 1/2 field. That is 1/120th sec or 8.333 msec.

Others, correct me if I'm wrong...

Scott

I re-read post #1 and I guess you are probably answering what he was really asking, namely that won't there be a slight sync loss during the duplicate field. The answer is yes, but there is no way anyone would notice.

[Cornucopia]

I was.