Video refers to recording, manipulating, and displaying ‘television’ images. Digital video involves the recording or capturing, storing and manipulation of video as a playable sequence of bit-mapped digital images.
Until recently, most video was recording and stored in analog format on analog tape using standard video camcorders. Playback was accomplished using a simple analog VCR.
To convert these analog recordings to digital format, we use video capture technology. Video capture requires special hardware (e.g., a ‘capture card’) that converts analog TV signals into digital form for hard disk storage, usually with concurrent data compression (to save storage space). Usually, the compression hardware/software also serves to decompress the images when they are displayed. The abbreviated term for the system that performs both compression and decompression is codec.
Direct recording is accomplished using a digital video (DV) camcorder, a device that captures and stores video as discrete frames of digital images on digital tape. Typically, DV camcorders have built-in hardware codecs.
Digital video can be transferred directly to a computer’ mass storage devices either simultaneously with recording (called real-time acquisition) or stored on tape for later transfer.
There are dozens of formats used to record and playback digital video. However, only a few are commonly used or encountered in PC applications. These include Microsoft’s Video for Windows (*.AVI), Apple’s Quicktime video (*.MOV) and the Moving Pictures Expert Group’s formats (primarily MPEG-1 and MPEG-2).
AVI stands for Audio Video Interleaved and was introduced with Windows 3.0 as the standard Windows video format (also called Video for Windows). AVI files are limited to 320 x 240 resolution, and 30 frames per second. However, AVI files require no hardware for display, making this format the lowest common denominator for PC-based multimedia applications. Video for Windows supports several data codec techniques, including RLE, Indeo, and Cinepak. Unfortunately, even with compression, AVI files can be extremely large.
MOV stands for Movie. MOV files are the standard QuickTime video and animation format developed by Apple Computer. QuickTime is built into the Macintosh operating system and is used by most Mac applications that include video or animation. PCs can also run files in QuickTime format, but they require a separate (free) downloadable program (click here to download Quicktime). QuickTime supports most encoding formats, including Cinepak, JPEG, and MPEG.
With its high compression, QuickTime 4.0 was the first version to fully support streaming audio and video. The current version (QuickTime 5) also plays MP3s, MIDIs, and WAV files. Because it can handles multiple audio, video and graphic file formats, Quicktime is popular for multimedia development (see the separate section on multimedia).
MPEG stands for Moving Pictures Expert Group, the name of the ISO committee that develops digital video and audio compression standards, and -- by extension -- the name of the standard they have produced.
MPEG achieves high compression rate by storing only the changes from one frame to another, instead of each entire frame. Although MPEG is a lossy compression method, the loss of video data is generally imperceptible to the human eye. For this reason, MPEG generally produces better-quality video than competing formats, such as AVI and QuickTime. MPEG files can be decoded by special hardware or by software. While decompressing an MPEG-2 data stream requires only modest computing power, encoding video in MPEG-2 format requires significantly more processing power (and time).
There are two major MPEG standards: MPEG-1 and MPEG-2. MPEG-1 provide a video resolution of 352-by-240 at 30 frames per second (fps). This produces video quality slightly below the quality of conventional VCR videos. MPEG-1 requires transfer rates of no more than about 1.5 Mb/sec. Since this corresponds to the data retrieval speed from CD ROM and digital audio tape (DAT), MPEG-1 is the primary standard for CD-ROM video and audio applications.
MPEG-2, offers resolutions of 720x480 and 1280x720 at 60 fps, with full CD-quality audio. This provides full-screen, broadcast quality video in all the major TV formats, including NTSC and the newer HDTV. MPEG-2 is used commercially for DVD videos (on which a 2 hour video can be compressed into a few gigabytes) and for streaming video over digital cable or satellite TV.
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For a technical overview of digital video (with some emphasis on PAL), visit the PC TechGuide at:
http://www.pctechguide.com/24digvid.htm
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© Craig L. Scanlan, 2001. Version 2.0 - January 2002. Original version January 2001.