Header
A block of data in a coded bitstream containing information about the data that follows.

Helical Scan
A DAT recording method whereby heads record data at an angle rather than in a straight line (linear), which uses the entire width of the tape.

Hertz (Hz)
The standard unit of frequency. One Hz equals one cycle (or vibration) per second. One kilohertz (KHz) equals 1,000 cycles per second, and one megahertz (MHz) equals 1,000,000 cycles per second. This standard unit is named after German physicist Heinrich Hertz (1857-1894).

Hi8 Video
The high-quality extension of the Video 8 (or 8mm) format, which features higher luminance resolution.

High-definition television (HDTV)
Any one of a variety of video formats offering greater visual accuracy (or resolution) than current NTSC, PAL, or SECAM broadcast standards. Current formats generally range in resolution from 655 to 2,125 scanning lines, having an aspect ration of 5:3 (or 1.67:1), and a video bandwidth of 30 to 50 MHz (5+ times greater than NTSC standard). Digital HDTV has a bandwidth of 300 MHz. HDTV is subjectively comparable to 35 mm film.

High Memory
In DOS -based systems, high memory refers to the memory area between the first 640K and 1 megabyte. It is also called the upper memory area (UMA).

High Memory Area
In DOS -based systems, the high memory area refers to the first 64K of extended memory.

High Sierra format
A standard format for placing files and directories on CD-ROMs proposed by an ad hoc committee of computer vendors, software developers, and CD-ROM system integrators. (Work on the format proposal began at the High Sierra Hotel at Lake Tahoe, Nevada.) A revised version of the format was adopted by the International Standards Organization as ISO 9660.

Hot Plugging
The ability to add and remove devices to a computer while the computer is running and have the operating system automatically recognize the change. Two new external bus standards -- Universal Serial Bus (USB) and IEEE 1394 - support hot plugging. This is also a feature of PCMCIA. Hot plugging is also called hot swapping. One of the oldest uses are hot swapping of Power Supplies.

Hybrid Coder
The basis for international standards for videotelephony and the prototype compression technique from which MPEG was designed. In the archetypal hybrid coder, an estimate of the next frame to be processed is formed from the current frame; the difference is then encoded by some purely intraframe mechanism. In recent years, the most attention has been paid to coders that include motion compensation, in which the estimate is formed by a two-dimensional warp of the previous frame, and the difference is encoded using a block transform (the Discrete Cosine Transform).

The key feature of the hybrid coder is the presence of a complete decoder within it. The decoder processes the difference between the current frame as represented at the receiver and the incoming frame. The receiver must therefore track the transmitter precisely, meaning that the decoders at the receiver and transmitter must match. The system is sensitive to channel errors and does not permit random access. However, it is on the order of three to four times as efficient as coders that use no prediction.

In practice, this coder is modified to suit specific applications. The standard telephony model uses a forced update of the decoded frame so that channel errors do not propagate. When a participant enters the conversation late or alternates between image sources, for example, residual errors die out and a clear image is obtained after a few frames. Similar techniques are used in versions of this coder developed for direct satellite television broadcasting.

Hybrid scalability
Hybrid scalability is the combination of two or more types of scalability.