Tech Note for Nerds

From The Frame Grab Tutorial: http://tvcrit3.tvcrit.com/content/view/58/102/.


Codecs, as explained by Curricular Computing, Dartmouth College:

One second of uncompressed NTSC video requires approximately 27 MB of disk space. Storing video onto desktop computers clearly requires compression, and playing video on desktop computers requires decompression. Hence, video compression algorithms, or "codecs" (short for compressors/ decompressors), were devised to handle the compression of video, and the decompression when it is played.

Codecs compress movies by looking at consecutive frames and storing just the differences (temporal compression), and/or by generalizing an image and removing redundant data (spatial compression). Many codecs use both spatial and temporal compression techniques. The process of analyzing each frame and compressing a movie can take a long time, but decompression takes place in real time to allow smooth playback.

Most codecs use lossy compression algorithms, which means the process of compression removes data from the original movie. You should never compress a movie multiple times, as each pass will lower the quality of the movie.

From www.dartmouth.edu/~cc/didactic/video/contents.html (dead link).

Apple provides a guide to the video compressors included with QuickTime. Moved to:

http://www.apple.com/macos/resources/macos9/us/help/quicktime4.1/ap/pgs/apCmprss.htm

Video compressors included with QuickTime

      Compressor   Comments    
    Animation Works best on computer-generated animations with broad areas of flat color. Doesn't work well when the scene has lots of color changes. You can use this for image or animation or video with an alpha channel. Lossless at 100%. Poor compression ratio.

    Apple BMP Used for still images when exporting in the BMP format. Does minimal compression. Inappropriate for video-based movie playback.

    Apple Video Very fast compressing and decompressing of video. Good compression ratios. Use for real-time capture of video, particularly when hard disk space is at a premium, or for testing clips. OK for hard disk playback. Image quality is poor when compressing enough for CD-ROM playback.

    Cinepak Commonly used for video movies that require CD-ROM playback. Very slow compressing, but decompresses fast on low-end processors. Good choice for streaming.

    Component Video A high-quality compressor. Good for capture on AV computers and as intermediate storage format. Low compression ratios. Larger files. Not good as a delivery format.

    DV - NTSC Used with Digital Video cameras manufactured in the U.S. and Japan.

    DV - PAL Used with Digital Video cameras manufactured in Europe.

    Graphics Good for 8-bit graphics files. Usually better than the Animation compressor in 8-bit. Slower to decompress than Animation.

    H.261 Designed originally for video conferencing. Very high compression ratios. Good for low data rates and for streaming live data.

    H.263 Very high compression ratios. Works optimally at window sizes of 352 x 288, 176 x 144, or 128 x 96. (Movie windows will be resized to one of these sizes.) Good for low data rates. Good for streaming live data.

    Intel Indeo Video 3.2 Cross-platform compressor, primarily designed for Windows. Use to compress video on Windows that will be decompressed on a Macintosh.
    Intel Indeo Video 4.4 Cross-platform, very high image quality.

    Microsoft Video Used to decompress video captured on Windows.

    Microsoft RLE Used to decompress video captured on Windows.

    Motion JPEG A

    Motion JPEG B

    Used to decompress files made with certain hardware Motion-JPEG cards when the card isn't available, or to compress in a format that can be played by certain hardware Motion-JPEG cards.

    None Good for capture only. Does almost no compression. Results in very large files. You can use this for image or animation or video with an alpha channel.

    Photo JPEG Ideal for high-quality compressed still images. Also useful as intermediate storage format for movies and for QuickTime VR panoramas. Too slow decompressing to use for video-based movie playback. No interframe compression.

    Planar RGB (sometimes called Photoshop) For importing photographic graphics with an alpha channel (for use when you have an image that you want to composite with video).

    Sorenson Ideal for streaming. Use if you plan to stream movies from a QuickTime Streaming Server.
    Sorenson 3 Even better for streaming than "Sorenson," but will not work with QuickTime versions before version 6 (released in 2002).

Sound compressors included with QuickTime

    QuickTime comes with a number of built-in audio compressors. You can install additional software for other audio compressors.

      Compressor   Comments    
    24-bit Integer

    32-bit Integer

    Increases the sample size to 24 or 32 bits. (The sample will be converted to 8 or 16 bits to play on current hardware.)

    32-bit Floating Point

    64-bit Floating Point

    Increases the sample size to 32 or 64 bits. This allows for more accurate conversion to other sample sizes, and the application of effects.

    ALaw 2:1 Internet standard for compressed audio in Europe and everywhere else except the United States and Japan. Low compression ratio and low quality. Generally not recommended.

    DVI 4:1 Used for reception of MBone broadcasts.

    IMA 4:1 Very good for music and other audio content. Must be 16-bit. Relatively low compression ratio.

    MACE 3:1

    MACE 6:1

    Older Macintosh compressors. Low quality. Not generally recommended.

    MPEG-4 Audio Good for playback and streaming.

    MS ADPCM Used to listen to audio compressed on Windows.

    QDesign Music 2 Excellent for music content. Extremely high compression ratio. Good choice for streaming.

    QUALCOMM PureVoice Excellent for voice content. Extremely high compression ratio. Good choice for streaming.

    uLaw 2:1 Internet standard for compressed audio in the U.S. and Japan. Low compression ratio and low quality. You can use this for streaming. Generally not recommended.

Compressors for streamed data

    These are the compressors you should use if you plan to stream movies from a QuickTime Streaming Server.

    Video compressors:

    Sorenson 3
    H.261
    H.263
    Motion JPEG

    Audio compressors:

    DVI 4:1
    QDesign Music 2
    QUALCOMM PureVoice
    uLaw 2:1

 

From www.apple.com/quicktime/resources/qt/us/help/ap/pgs/apCmprss.htm .


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