Frame Rates

Frame rate is the "speed" of a media stream, measured in frames per second. As with time values, the meaning of a frame rate depends on the context:

• Output frame rate: The frame rate of the final rendered project, defined by the group. When you render the project, each group becomes a separate media stream with its own frame rate.
• Source frame rate: The frame rate in which the source file was originally authored. The authored frame rate does not have to match the group's output frame rate. DES will automatically upsample or downsample the file as needed. For most media formats, DES can determine the frame rate by examining the format. A DIB sequence is an exception; you must specify the frame rate of a DIB sequence. (For more information, see Working with Sources.)

Playback rate: The apparent speed of a source clip when it appears in the project. For example, 10 seconds' of video can be fit into 5 seconds on the timeline. As a result, the speed of the clip increases by a factor of 2, as the following diagram illustrates.

(With an audio source, the pitch would shift as well.) The following formula determines a source clip's playback rate:

• Playback rate = (Media Stop – Media Start) / (Timeline Stop – Timeline Start)

Note that each of these three rates is independent of the others:

• You can speed up or slow down a clip by adjusting the media times; this does not affect the frame rate of the final output.
• You can increase or decrease the output frame rate without affecting how fast a file plays.
• You can mix, within the same group, source files that have different authored frame rates, and DES will upsample or downsample each clip to match the group's frame rate.

When you render a project, all times are rounded to the nearest frame boundary, as determined by the group frame rate. For example, suppose a video group has a frame rate of 30 fps. Each frame is roughly 33 milliseconds (ms). Suppose you add a 1.68-second source clip to the timeline, starting at time zero. The source does not end exactly on a frame boundary, so DES rounds the stop time to 1.6666 seconds (50 frames). If you seek to 1.68 seconds in the rendered project, you will actually seek past the end of the source, to the 51st frame.

However, DES does not overwrite the source's stop time. You might later change the group frame rate, or move the source to a new spot in the timeline where it rounds differently. Therefore, DES preserves the original stop time and rounds only when necessary. For more information, see IAMTimelineObj::FixTimes.