Parameters of Time Code

[ LiB ]

SMPTE has only two parameters that affect the format. The first is the frame rate, which is how fast the time code is going. The second is the frame count, which determines which frame numbers are skipped in order to stay aligned with the real time clock.

Now that you're armed with a basic understanding of SMPTE and how it works, I will describe each SMPTE format, what it is used for, and any quirks or problems associated with it.

Frame Rates

The different frame rates used in SMPT time code correlate to the different speeds of visual media. Here's a list of all major SMTPE frame rates not including special versions used in transfers. Those will be covered later.

NTSC B&W Video 30fps. 30fps is used only in music or audio-only applications. It will not line up with any sort of video signal, as all current video standards use either 29.97fps for NTSC, or 25fps for PAL/EBU. It does relate to pulled-up sample rates, which will be discussed later in this chapter.
NTSC Color Video 29.97fps. This is and has been the standard frame rate for NTSC color video since its creation. Make sure you know whether you are using the drop-frame or non-drop-frame version of this frame rate.
PAL/EBU Video 25fps. This frame rate is used in Europe and countries in which the PAL/EBU television system is used. Be careful not to confuse it with the film rate of 24fps.
Film & 24p HD Video 24fps. Film is often shot at 24 frames per second, but may also be shot at 29.97fps or 30fps depending upon production needs. Film may be transferred to video for use in a product whose final format is video or only for the purposes of editing, in which case a film cutter would return to the actual film to create the final version. Film-to-video transfers use different SMPTE frame rates in different situations. We will talk more about this later in the chapter. New HDTV or hi-definition television cameras (HD for short) can also use 24fps as their frame rate. This is to help them look more like film than traditional video cameras .

Frame Count

Recall that drop-frame time code skips certain frame numbers in order to maintain accurate real time in its display. The actual frames are not skipped, just their numbers. The frame count of SMPTE is determined by if and how frame numbers are skipped. Drop-frame only applies to 29.97 SMPTE as all the other frame rates follow the clock exactly. It is common in software to see 30fps have a drop-frame and non-drop-frame counts. This is somewhat of a misusage of the term (see the Caution below).

Drop Frame

In order to lose those extra 18 frames in every ten minutes, drop-frame must skip two frames every minute for nine minutes and none in the tenth minute, for a total of 18 frames in ten minutes. This cycle repeats every ten minutes. As there are 1800 frames of video in ten minutes at 30 frames per second, the 18 frame numbers that are skipped (remember, no video frames are skipped, just their numbers) correlate to the .1% speed change to color video at 29.97 frames per second. Now, the 29.97fps drop-frame time code number we see at the end of one real time hour reads 01;00;00;00 on the time code display. (See Figure 3.3.)

Figure 3.3. A time code calculator displaying SMPTE in drop-frame format. Notice the semicolons.

NOTE

BEWARE OF 30 DROP-FRAME TIME CODE

Many manufacturers, including Digidesign, label one time code format as 30fps drop-frame. This is not a real format. The term is commonly misused as a name for 29.97 drop-frame. If you think about it, 30fps does not need to drop frame numbers in order to be in sync with the real-time clock. There would be no normal use for 30 drop-frame time code. In Pro Tools, the time code options include both 29.97 drop-frame and 30 drop-frame. The 30 drop-frame is a true 30fps with skipped frame numbers. In relation to the real-time clock, this format runs faster, and after one hour of real-time, reads: 01:00:03:18. This format is very misleading and should only be used when correcting errors in existing time code. Beware, and double-check your time code format!

NOTE

THE SEMICOLONS IN TIME CODE READOUTS

Whenever you see a SMPTE time code number that has a semicolon separating the seconds from the frames numbers, you are looking at drop-frame time code. Sometimes, all separators for drop-frame are semicolons. But bewarejust because there are no semi-colons in the display doesn't mean it is not drop-frame code.

Non-Drop Frame

Non-drop frame time code is simply time code that does not skip any frame numbers (see Figure 3.4). Some video post-production houses use 29.97 non-drop frame as the in-house time code format; usually these houses work more with short duration projects, such as commercials. With times less than 60 seconds, the difference between drop-frame and non-drop frame is not significant. Some video editors prefer to use non-drop-frame SMPTE all of the time, so that every frame has a consecutive number. This can help make time code calculations easier.

Figure 3.4. The same calculator after converting one hour to 30fps non-drop frame. No semicolons.

[ LiB ]