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Measuring video time

In the natural world, we experience time as a continuous flow of events. However, working with video requires precise synchronization, so it's necessary to measure time using precise numbers. Familiar time increments—hours, minutes, and seconds—are not precise enough for video editing, because a single second might contain several events. This section describes how Premiere 6.0 and video professionals measure time, using standard methods that count fractions of a second in frames.

How the timebase and frame rates affect each other

You determine how time is measured in your project by specifying the project timebase. For example, a timebase of 30 means that each second is divided into 30 units. The exact time at which an edit occurs depends on the timebase you specify, because an edit can only occur at a time division; using a different timebase causes the time divisions to fall in different places.

The time increments in a source clip are determined by the source frame rate. For example, when you shoot source clips using a video camera with a frame rate of 30 frames per second, the camera documents the action by recording one frame every 1/30th of a second. Note that whatever was happening between those 1/30th of a second intervals is not recorded. Thus, a lower frame rate (such as 15 fps) records less information about continuous action, while a high frame rate (such as 30 fps) records more.

You determine how often Premiere generates frames from your project by specifying the project frame rate. A project frame rate of 30 frames per second means that Premiere will create 30 frames from each second of your project.

For smooth and consistent playback, the timebase, the source frame rate, and the project frame rate should be identical.

Editing Video Type Frames per second
Motion-picture film 24 fps
PAL and SECAM video 25 fps
NTSC video 29.97 fps
Web or CD-ROM 15 fps
Other video types, e.g., non-drop frame editing, E-D animation 30 fps


NTSC was originally designed for a black-and-white picture at 30 fps, but signal modifications made in the mid-20th century to accommodate color pictures altered the standard NTSC frame rate to 29.97 fps.

Sometimes the time systems don't match. For example, you might be asked to create a video intended for CD-ROM distribution that must combine motion-picture source clips captured at 24 fps with video source clips captured at 30 fps, using a timebase of 30 for a final CD-ROM frame rate of 15 fps. When any of these values don't match, it is mathematically necessary for some frames to be repeated or omitted; the effect may be distracting or imperceptible depending on the differences between the timebase and frame rates you used in your project.

A. 30 fps video clip (one-half second shown) B. Timebase of 30, for a video production When the source frame rate matches the timebase, all frames display as expected.

A. 24 fps motion-picture source clip (one-half second shown) B. Timebase of 30, for a video production. To play one second of 24 fps frames at a timebase of 30, source frames 1, 5, and 9 are repeated.

It is preferable to capture your clips at the same frame rate at which you plan to export your project. For example, if you know your source clips will be exported at 30 fps, capture the clips at 30 fps instead of 24 fps. If, this is not possible (for example, DV can only be captured at 29.97 fps), you'll want to output at a frame rate that evenly divides your timebase. So, if your capture frame rate and your timebase are set at 30 fps (actually 29.97), you should output at 30, 15, or 10 fps to avoid “jerky” playback.

When time systems don't match, the most important value to set is the timebase, which you should choose appropriately for the most critical final medium. If you are preparing a motion picture trailer that you also want to show on television, you might decide that film is the most important medium for the project, and specify a timebase of 24.

A. Timebase of 30 (one-half second shown) B. Final frame rate of 15, for a Web movie If the timebase is evenly divisible by the frame rate, timebase frames are included evenly.

A. Timebase of 24 for a motion-picture film (one-half second shown) B. Final frame rate of 15, for a Web movie. The time is not evenly divisible by the frame rate, so frames are included unevenly. A final frame rate of 12 fps would generate frames more evenly.

The important thing to remember is this: You'll get the most predictable results if your timebase and frame rate are even multiples of one another; you'll get the best results if they are identical.

For more information, see “Measuring time and frame size” in the Adobe Premiere 6.0 Technical Guides found in the Support area on the Adobe Web site (www.adobe.com/support/techdocs/topissuespre.htm).

Counting time with timecode

Timecode defines how frames are counted and affects the way you view and specify time throughout a project. Timecode never changes the timebase or frame rate of a clip or project—it only changes how frames are numbered.

You specify a timecode style based on the media most relevant to your project. When you are editing video for television, you count frames differently from counting frames when editing video for motion-picture film. By default, Premiere displays time using the SMPTE (Society of Motion Picture and Television Engineers) video timecode, where a duration of 00:06:51:15 indicates that a clip plays for 6 minutes, 51 seconds, and 15 frames. At any time, you can change to another system of time display, such as feet and frames of 16mm or 35mm film. Professional videotape decks and camcorders can read and write timecode directly onto the videotape, which lets you synchronize audio, video, and edits, or edit offline (see page 119).

When you use the NTSC-standard timebase of 29.97, the fractional difference between this timebase and 30 fps timecode causes a discrepancy between the displayed duration of the program and its actual duration. While tiny at first, this discrepancy grows as program duration increases, preventing you from accurately creating a program of a specific length. Drop-frame timecode is an SMPTE standard for 29.97 fps video that eliminates this error, preserving NTSC time accuracy. Premiere indicates drop-frame timecode by displaying semicolons between the numbers in time displays throughout the software, and displays non-drop-frame timecode by displaying colons between numbers in timecode displays.

Drop-frame timecode uses semicolons (left) and non-drop-frame timecode uses colons (right).

When you use drop-frame timecode, Premiere renumbers the first two frames of every minute except for every tenth minute. The frame after 59:29 is labeled 1:00:02. No frames are lost because drop-frame timecode doesn't actually drop frames, only frame numbers.

For more information, see “Timecode and time display options in the Adobe Premiere 6.0 Technical Guides found in the Support area on the Adobe Web site (www.adobe.com/support/techdocs/topissuespre.htm).

Interlaced and non-interlaced video

A picture on a television or computer monitor consists of horizontal lines. There is more than one way to display those lines. Most personal computers display using progressive scan (or non-interlaced) display, in which all lines in a frame are displayed in one pass from top to bottom before the next frame appears. Television standards such as NTSC, PAL, and SECAM standards are interlaced, where each frame is divided into two fields. Each field contains every other horizontal line in the frame. A TV displays the first field of alternating lines over the entire screen, and then displays the second field to fill in the alternating gaps left by the first field. One NTSC video frame, displayed approximately every 1/30th of a second, contains two interlaced fields, displayed approximately every 1/60th of a second each. PAL and SECAM video frames display at 1/25 of a second and contain two interlaced fields displayed every 1/50th of a second each. The field that contains the topmost scan line in the frame is called the upper field, and the other field is called the lower field. When playing back or exporting to interlaced video, make sure the field order you specify matches the receiving video system, otherwise motion may appear stuttered, and edges of objects in the frame may break up with a comb-like appearance.


For analog video, the field order needs to match the field order of the capture card (which should be specified in the preset). For DV, the field order is always lower field first. Be sure to select the correct preset first; doing so will correctly specify the field order.

Interlaced video describes a frame with two passes of alternating scan lines.

Progressive-scan video describes a frame with one pass of sequential scan lines.

If you plan to slow down or hold a frame in an interlaced video clip, you may want to prevent flickering or visual stuttering by de-interlacing its frames, which converts the interlaced fields into complete frames. If you're using progressive-scan source clips (such as motion-picture film or computer-generated animation) in a video intended for an interlaced display such as television, you can separate frames into fields using a process known as field rendering so that motion and effects are properly interlaced.

For more information, see “Processing interlaced video fields” in Chapter 3 of the Adobe Premiere 6.0 User Guide and “Interlaced and non-interlaced video” in the Adobe Premiere Technical Guides found in the Support area on the Adobe Web site (www.adobe.com/support/techdocs/topissuespre.htm).

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