Clapperboard used to synchronize sound with video.
Recorded video footage typically contains sound, or audio, along with video images. If you record your video footage on a consumer video camcorder, the audio and video are usually recorded together on the tape, disk or other media used to capture the video footage. The audio is “in sync” with the video, so that if you see someone speak in the video, the words match the movement of the person’s lips. Video-sync issues arise when the audio does not match the video. This can be caused by a number of factors.
Mismatched Frame Rate
When you record video footage, images are recorded in multiple consecutive frames. A frame is like a still photographic image; the camera records multiple frames every second. If your camcorder records using the NTSC (National Television System Committee) standard used in the United States, it records 29.97 frames per second. The PAL (Phase Alternate Line) system, used in Europe, records 25 frames per second. When you capture video footage to a computer for editing, you can separate the audio from the video and save the audio and the video as different files. You may want to do this if you are adding effects to the audio and video data. If you convert the video images from NTSC to PAL, you effectively shorten the running time of the whole video, as this process removes frames from each second of NTSC footage to create the PAL standard. If you then add the original audio track, which was recorded at 29.97 frames per second, the audio will get progressively out of sync with the video. Always convert audio and video together to keep them in sync.
Mismatched Sample Rate
When you record audio with video, the sound is recorded using a sample rate expressed in hertz (Hz). Camcorders and other audio-recording devices typically record at a sample rate of 44,100 or 48,000 Hz. When you create a new project in a video-editing program, you can set the audio sample rate used in the project. If you create a project with an audio sample rate of 44,100 Hz but your audio was actually recorded at 48,000 Hz, your sound will gradually get more and more out of sync with your video as you play it. The editing program takes the first 44,100 samples from the first second of your audio, applies them to the first second of the project, and the remaining 3,900 samples are rolled over into the next second. This happens for every second of footage, so the audio and video get progressively out of sync with each other. Create projects with the sample rate used when recording your sound.
In video-editing programs, the audio and video data are shown in a window known as a timeline. Imagine your video as a physical strip of film; the timeline is like having the film strip unrolled and stretched out horizontally in front of you. The editing program has separate timelines for video and audio, one on top of the other. If you have recorded your audio and video separately, you must match them up in the editing program timelines. This is why professional filmmakers use a slate, or clapperboard. When you use a slate, you can match the exact frame where the clapper hits the slate with the sound it makes in the audio recording. If the clapper audio and video are in sync, the whole video and audio in the timeline is in sync for that particular recording.