Tech Tips:
Odd - Odd, Even - Even: Dual System and 24P
By Tom TalbotImportant concepts to keep picture and sound in sync from start to finish when shooting 24P.
Dual System is a term that in its simplest form means that audio is recorded separately from picture but can be locked together in some form or fashion. Typically on a dual system film shoot, picture runs at 24 FPS and sound (really sound Timecode) runs at a true 30 FPS. (Notice both 24 and 30 are EVEN numbers ). The same method could be applied to HD 24P shoots if you wanted to minimize change for an experienced film crew.
Because the Sony HDW-F900 provides several shooting speeds: 23.98, 24, 25, 29.97, or 30 frames per second many people ask us which one is the right speed to shoot with. Which frame rate (and therefore timecode rate) that is chosen is dependent on several things including shooting style, production look, etc. but also on whether or not audio will be recorded and whether the end product is intended for video or theatrical film release.
In today’s world of digital video, we often take for granted the fact that audio seems to automatically lock up to picture without issue. What makes this possible is the fact that audio is recorded either directly into a video camera at the same time that video is being recorded or that the timebase of the video camera and a field DAT recorder are set the same at 29.97. (Note that picture and sound timecode both use 29.97 or ODD numbers) Once recorded, it never has to change frame rate throughout the post production process. By this I mean that video productions start and end at 29.97 frames per second (FPS).
The same could be said for HD or for film if the editing and presentation process stayed in its native form from beginning to end as well. Shoot, edit, and present 24 FPS. The reason why this does not usually happen is due to simple economics. It is often too expensive (or too time-consuming and therefore costly) to edit in its original form. So an alternate Offline edit has to occur to allow for cost effective editing.
That said, to save money CONVERSIONS have to happen to get from film or HD into the less expensive “video” editing environments. To pass content into these offline editing systems we use standard definition 29.97 video as the medium to get content out of production into the editing environment (even if the edit is to be done at 24 FPS). Confused? The term Telecine is the common way to describe this conversion. Without getting into the specifics of the 2:3 telecine process, the most important thing to be aware of is the telecine two step – Slow down 24 by .1% in order to convert to 29.97 (Note the conversion from EVEN to ODD)
If picture is originally shot at 24 FPS, the HD playback deck (or the film telecine machine) has to do the following:
- Play back picture .1% slower (23.98 FPS)
- CONVERT to 29.97 FPS
- Resolution downconvert
- Record to standard definition video.
If audio was recorded separately, sound must also be slowed down .1% (sometimes referred to as 0.99
which is an ODD number) just prior to merging in order to marry it to the slowed down picture and maintain sync.. Most often a studio DAT machine such as the Soony PCM-7040 is used with the Non Linear Editing system and audio is digitized separately.When shooting HD “24p” your process will be much easier if you can record production quality audio inside the camera directly but actually record at 23.98 instead of 24. Because 23.98 converts cleanly up to 29.97 (no slowdown needed for picture or sound) originating at 23.98 is more elegant and less prone to sync errors.
Attention Producers: You will also save $$$ and time if your audio is already locked in camera because telecine dubs from HD to SD do not need to be manually synced by an Assistant Editor during post production.
So, If you have gotten this far I will now make your life much simpler with this simple memory aid:
Odd, odd – Even even.
What this means is that you should match an ODD picture rate (timecode rate) to ODD sound timecode or match an EVEN picture rate (timecode rate) to EVEN sound timecode.
Examples:
|
Picture |
Sound (TC) |
|
24 |
30 |
|
23.98 |
29.97 |
|
23.98 |
23.98 |
|
29.97 |
29.97 |
Some of you may notice that 23.98 actully could be called even. For this discussion 23 is what is being referred to when it is described as ODD.
