Apple QuickTime User's Guide download pdf (Page 37)

Chapter 4 Advanced Concepts 37

Choosing Frame Rate Options for Video Compression

When you export a QuickTime movie, you can set video compression options such as

the frame rate.

Frame rate is the number of individual images shown every second. A standard (NTSC)

video has a frame rate of 29.97 frames per second (fps), and the standard for film is 24

fps. The European standard (PAL) is 25 fps. QuickTime movies are sometimes played at a

slower frame rate to reduce bandwidth and CPU requirements.

If you choose a frame rate that’s lower than the movie’s current frame rate,

compression will cause frames to be deleted. If you choose a number that’s higher than

the movie’s current frame rate, existing frames will be duplicated. The file size will

change accordingly.

When choosing a frame rate, use a simple division of your current frame rate, such as

1/2, 1/3, 1/4, 1/5, and so on. For example, if your current frame rate is 30, use 15, 10, 7.5,

or 6.

Choosing Sound Compression Options

When you export a QuickTime movie, you can set sound compression options such as

the sampling rate.

Digitized sound is made up of sound samples. The more samples per second, the

higher the sound quality. To maintain quality, music requires a higher sampling rate

than spoken voice because music uses a wider range of frequencies. (Audio CDs use a

sampling rate of 44.1 kHz.) With QuickTime 6.3 and later, the proper sampling rate is

chosen automatically.

When you export a QuickTime movie using some compressors, you can also set the

sound sample size (the number of bits per sample), which determines how accurately

the dynamic range of the sound is reproduced. Eight-bit sound provides for 256

possible values, whereas 16-bit sound provides for 65,536 possible values.

For music that has very soft and very loud passages, such as orchestral music, choose

16-bit sound. For spoken voice or music with a fairly stable volume level, you can

choose 8-bit and get good results.

Exporting QuickTime Movies

If you have QuickTime Pro, you can export your QuickTime movie to a number of

different file formats, such as MPEG-4, AVI, and DV. In addition, you can export an audio

track to several different audio formats or export the individual frames of a movie as

separate image files.

LL1158.Book Page 37 Thursday, September 25, 2003 1:26 PM

1 2 ... 32 33 34 35 36 37 38 39 40 41 42 ... 49 50

No comments

Apple QuickTime User's Guide Page 37