H.264 – (2003) A codec standard, also called MPEG-4 AVC (Advanced Video Coding). Performs compression of video signals for Blu-ray, and for most Internet streaming video.
H.265 – Also called High-Efficiency Video Coding (HEVC). The 2013 successor to H.264, co-developed with the ISO. It’s supposed to double the H.264 compression ratio without loss of quality, and support up to 8K video.
H.323 – For voice and video on a TCP/IP network, using associated signaling protocols. It creates a virtual telephone line using roughly 10 kb/s.
Someone who manipulates other people’s systems over the Internet, commonly for malicious or criminal reasons (see l337, malware). In public discourse, this has buried the first, original meaning, although some old-school hackers are resisting it.
Named for American physicist Edwin Herbert Hall (1855-1938), who demonstrated and explained it in 1879. The electrical polarization of a conducting surface by a magnetic field when current is flowing or when the conductor is moving through the field. For this to occur, the field must have a component normal (perpendicular) to the current or direction of movement. For example, the wings of an aircraft in flight exhibit the Hall effect due to movement through Earth’s magnetic field.
A class of simple block code proposed in 1950 by American mathematician Richard W. Hamming (1915-1998) that can detect up to two errors per block and correct one. Given a data block length k = [2m – 1 – m], where m > 1, the Hamming codeword length n = [2m – 1].
An integer measure of the difference between two binary sequences, calculated by modulo-2 addition. For example, the Hamming distance between 10110 and 11011 is 3. This is distinct from free Euclidean distance, which refers to the difference between symbol waveforms.
Heat-Assisted Magnetic Recording. Seagate’s emerging (2018) hard disk drive data-storage technology to enable writing very densely packed drives, which use magnetically hardened materials to prevent the spontaneous, thermally driven bit-flips that could otherwise occur with such tiny data cells. The write head can’t create a magnetic field both precise enough and strong enough to reliably write this material unaided, so HAMR heats the disk with focused laser light just in advance of the write head, briefly improving the mobility of the disk’s magnetic grains. Compare MAMR.
A hash is a fixed-length number created from some variable-length source known as the message, most often a file or password. A hash algorithm uses the message as input to generate the hash, also called the message’s digest or fingerprint. If the algorithm is well-designed, each hash is functionally unique – that is, the probability of any other message producing the same hash is so low as to be effectively zero. Even a one-bit change in the message should result in a completely different hash value.
In benign environments, hashing is used to check the integrity of data, making sure a file or message has not been corrupted or unexpectedly changed. Cryptographic hashing is meant to ensure that data has not been intentionally altered, or that a password is valid. It’s far more secure than a checksum or CRC, which are not meant for security purposes. Because the server performing the check needs to store just the hash, it avoids the risks that would come with storing the actual data.
Unlike data encryption, the hashing process can’t be reversed. Even if the hash value and hash algorithm are known, they can’t be used to recreate the message, because most of its information content has been discarded.
It is sometimes possible to guess short messages with a cumbersome tool called a rainbow table, a huge list of allowable messages and the hashes they produce. This brute-force approach can work only if the algorithm and the constraints on the message are known, and security measures such as reiterative hashing and salting the message with random additions make its job much harder. As a result, rainbow tables have gone out of use in favor of other cracking methods.
The name of a topic on social networking services such as Twitter. It’s a string of characters beginning with #, the hash symbol. It allows users to form instant discussion groups: sending a message containing a hashtag not currently in use creates a new virtual thread. Other users can view and add to the thread of messages with that hashtag.
Hashtags require no setup or registration, and have no owner – users are free to create new ones at will, or use any existing ones. Messages can contain multiple hashtags.
High-Bandwidth Memory. A memory technology released in 2015 for GPUSs. It’s primarily a layout change: instead of a set of memory chips side by side on a small circuit card, stack a set of thin memory modules atop one another, linked by TSVs. The much shorter connections mean much higher speed (initially 512 Gb/s) and reduced power consumption. It also requires fewer control logic circuits – one per stack rather than one per chip. It’s similar to the unsuccessful HMC, which was aimed at PC main memory. It beats GDDR5 on bandwidth and power consumption, but is more expensive. HBM2 compares similarly to GDDR6.
Hydrogen bomb. Also fusion bomb, thermonuclear bomb. A fission bomb (referred to as the primary) is used as a trigger to compress and heat a surrounding capsule of hydrogen-isotope fuel (the secondary) inside a thick jacket, often itself of a fissile material such as U-238. The fuel’s deuterium (hydrogen-2) and tritium (hydrogen-3) atoms fuse to produce helium-4, an extra neutron, and a lot of energy. This is a two-stage bomb, as conceived by Edward Teller and Stanislaw Ulam.
If a few hundred kilotons isn’t enough for you, the secondary can in turn be used to fuse an even larger third stage. The most powerful bomb ever tested (over Novaya Zemlya, USSR, 30 October 1961) was apparently a three-stage device with a yield equivalent to about 57 megatons of TNT.
Deuterium is stable and occurs naturally. Tritium must be manufactured and is radioactive, hence unstable. In addition, these hydrogen isotopes are gases, with all the storage problems that entails. Therefore, many hydrogen bombs instead use solid lithium deuteride, made from deuterium and lithium-6, as the fuel. The neutron radiation from the primary splits Li-6 atoms into He-4 and tritium, which then fuses with the deuterium.
High Definition. Referring to computer monitors, this means a width-to-height aspect ratio of 16:9 or, less commonly, 16:10, which can display HDTV or widescreen movies. The low end for HD resolution is 1280 × 720 pixels. See graphics.
High Density ##-pin. A high-density version of one of the D-sub cable connectors. The most common is the HD-15, the analog video signal connector for VGA and SVGA monitors (see graphics). It uses the DE shell with 15 pins (male) or sockets (female) in three rows, so it’s also called a DE-15 or HDE-15. There are HD-26, HD-44, HD-62, and 4-row HD-78 connectors.
High Definition Audio. Intel’s 2004 sound coder/decoder specification to replace AC‘97. It can support up to eight 96-kHz, 32-bit sampled audio channels, though most hardware implementations of it are less robust. The motherboard header in a PC with HD Audio is physically, but not electrically, the same 10-pin header used for AC‘97.
High-bandwidth Digital Content Protection. A copy-protection protocol created by Intel for systems that use HDMI (see graphics) and some other digital video connections. An HDCP-compatible media player requires that any connected display use a HDCP key to authenticate itself. Display manufactures pay a licensing fee for the keys, and agree to abide by anti-copying restrictions in the design of their equipment. On 14 September 2010, someone posted the HDCP master key online. It’s a 40×40 array of 56-bit values. The algorithm uses a 40-bit value containing 20 ones and 20 zeros to generate two private keys, one for the data source and one for the data sink (the reader/display node), so there are 240 = 1,099,511,627,776 possible key pairs.
Hard Disk Drive. Standard for PC data storage since the early 1980s, using a rapidly spinning disk and magnetic read/write heads. It’s a direct descendant of the Winchester drive. Early PCs used a 5¼" wide hard drive. In 1983, the now-dominant 3½" drive was introduced. The 2½" drive (1988) is for portable PCs, although even smaller drives are common. (2015) The SSD is gradually replacing the HDD in personal computers.
Unlike memory (see DRAM), hard drive capacity in bytes uses a decimal standard. For example, a 500 GB hard drive has 500 × 109 bytes of capacity, not 500 × 230. Also, formatting the drive for use consumes some small percentage of its capacity.
Each ring of a hard disk is a track. Under the old approach, each track had a fixed number of sectors. This wastes space. Servo information is the magnetic patterns that the drive uses to position the read/write heads; newer drives embed the servo information into the data, and use constant-size sectors so that outer tracks hold more.
The first HDDs used longitudinal recording, with the magnetic elements laid out horizontally. The industry later switched to perpendicular magnetic recording (PMR), standing the magnetic elements on edge so that they could be packed more tightly. This required materials with higher magnetic coercivity, a magnetically permeable substrate, and a smaller write head. Still more recently, there’s shingled magnetic recording, which overlays the tracks on the disk like rows of roofing shingles. It achieves higher data density, but makes writing more difficult, hence slower.
For many years, hard drives used 512-byte sectors. Around 2010, manufacturers switched to 4096-byte sectors. See MBR, GPT.
High Definition DVD. Optical storage format intended to replace the DVD. It doesn’t use region codes like the DVD, but does implement AACS anti-copying technology. It officially surrendered to the similar Blu-ray standard in 2008.
Hardware Description Language. A programming language for defining electronic hardware, for purposes of circuit simulation and CAD/CAM. The code is compiled into a netlist, which can be used to program a PLD. The two most common HDLs are Verilog and VHDL.
Handheld Device Markup Language. A derivative of HTML, written to support Internet access from wireless devices such as handheld personal computers and smart phones.
High Dynamic Range. (2016) A digital TV technology that improves color range and contrast by combining separate but simultaneous videos of a scene taken with different light sensitivities. Viewing the enhancement requires a HDR-capable set.
The result of the read/write head physically contacting the surface of the spinning magnetic storage disk in a hard disk drive. This destroys data and damages the drive. Not the same as a disk crash, which is purely a software event.
Memory that the OS allocates dynamically to a running program for data and calculations. Unlike the fixed-length stack assigned to each individual program thread when code is compiled, the heap size is initialized at runtime, and the OS changes it while the program runs. This flexibility comes at the cost of a more complex structure, so heap calls are slower than stack calls.
A division of the ionosphere between 100 and 120 kilometers up, named for British scientist Oliver Heaviside (1850-1925), who proposed its existence in 1902 as the reason why radio waves at HF and below are able to propagate over the horizon. It exhibits one or more distinct maximums and sharp gradients of free electron density, and tends to absorb RF above ~10 MHz. It’s most pronounced in the daytime, and diminishes at night. Also called the E-layer.
The common form of hydrogen, protium, consists of one proton and one electron. Deuterium is a rare, stable, naturally occurring isotope that also contains one neutron. Heavy water is just H2O with a high proportion of deuterium atoms. This makes it denser than normal water, hence a more effective moderator for the fission chain reaction in a nuclear reactor.
Highly Elliptical (or Eccentric) Orbit. Refers to satellite orbits with a large difference between the apogee and perigee. Eccentricity can be as great as 0.75, which usually means passing through the Van Allen belts. The orbital inclination varies, though 63° is common – highly inclined orbits reduce the satellite’s exposure to belt radiation (see Molniya). The speed and period are likewise highly variable.
High Earth Orbit. Rather a vague term, and liable to be confused with (1). It usually means an orbital altitude of at least 20,000 km – above the second Van Allen radiation belt but below GEO. It can also mean orbits beyond GEO.
Named for German scientist Heinrich Hertz (1857-1894). The SI standard unit of frequency, indicating one occurrence per second. That is, if something repeats every T seconds, its frequency is [1/T] Hz. This is mainly for phenomena with T << 1: sound waves, electrical signals, electromagnetic (radio/light) waves.
The common higher-order units are kilohertz (kHz = 103 Hz), megahertz (MHz = 106 Hz), gigahertz (GHz = 109 Hz), and terahertz (THz = 1012 Hz). Units above THz and below Hz exist for academic thoroughness, but are seldom used. “Gigahertz” can be legitimately pronounced with either a hard or a soft G, but leading experts agree that, of the two, “jigahertz” sounds more ridiculous.
Wavelength (λ), the physical distance between wave peaks, is inversely proportional to frequency. In the case of EM waves, λ = [c / frequency], where c is the speed of light.
Nonlinearly mixing two signals of different frequencies, f1 and f2, to produce two beat frequencies, f3 = |f1 - f2| and f4 = f1 + f2. If f1 is a modulated carrier, f3 and f4 will receive its modulation, and a heterodyne receiver can tune to either of them (typically the lower f3, which is called the intermediate frequency, or IF) to demodulate it and reproduce the original baseband signal.
Almost all modern RF receivers use super-heterodyning, invented by American engineer Edwin Armstrong (1890-1954): f2 is fLO, the signal from a tuned local oscillator (LO), so that f3 is constant. The subsequent filtering, amplification, and demodulation are then optimized for this IF, which is much more efficient than making all those functions tunable. 10.7 MHz is a very common IF for FM super-heterodyne receivers.
From “hexadecimal”. An Intel format for representing a microprocessor machine-language binary as an ASCII text file using hexadecimal numbers. The files typically have a .hex or maybe .h86 filename extension. A programmer device converts the Hex file into machine language that will actually run on the processor. Each line of the file is called a record, and consists of these six parts in order:
start code (a colon character)
two-digit count of the data bytes in this record
four-digit address offset of the data, relative to some base address
two-digit record type
data
two-digit checksum
Standard record types are 00 = data, 01 = end of file, 02 = extended segment address, 03 = start segment address, 04 = extended linear address, and 05 = start linear address. Record types 02 and 03 are only for 16-bit processors; types 04 and 05 are only for 32-bit processors. The checksum is the twos complement of the sum of all the other record bytes.
A number format with 16 digits. It’s the standard way to represent memory addresses and data values in microprocessors. It uses the digits 0-9 of the decimal number system, and represents decimal values 10 through 15 with the letters A through F (capitalization optional). For example, decimal 3995 is hexadecimal F9B. There are several conventions for attaching prefix or suffix characters to indicate that a value is hexadecimal, including 0xF9B, xF9B, hF9B, and F9Bh. Compare octal.
High Frequency. 3-30 MHz. Radio waves at this frequency are reflected down by the ionosphere and up by the ground, so they can travel far beyond line of sight. They’re also called ground waves. See RF.
Hybrid Fiber-Coax. The basis for cable TV: a network architecture with a fiber backbone, and coaxial cables for user access. The neighborhood box where the cable company’s optical fiber meets subscriber coaxial lines is called a fiber node.
The phone company has something equivalent, which it calls an ONU. The difference with cable is that each coaxial feeder coming from the fiber node supports multiple subscribers via a series of taps, meaning that each connected subscriber reduces available bandwidth for the rest. Each tap goes to a subscriber network interface unit (NIU) – the box on the side of the house. Upstream of all this is a secondary hub that supports a number of fiber nodes. Still farther upstream is a primary hub that’s tied into the trunk line and supports multiple secondary hubs.
If the system has been upgraded to support DOCSIS two-way data services, cable modem termination systems (CMTS) are located at either the secondary or primary hubs. These devices translate between the trunk’s packet-switched network and the MAC & physical layer communications protocols going out to the subscribers.
Human Interface (or Input) Device. Any device – keyboard, mouse, joystick, microphone, etc. – for human input to a system. The USB standard has a HID specification. Many HID drivers are written for it, enabling them to tap into a system’s USB driver to connect their hardware.
Hardware In the Loop. Also HWIL. An approach to testing of embedded systems in which all hardware components are actually present, rather than being simulated as in SIL.
The MS DOS XMS driver. It configures extended memory to XMS specs, and controls the high memory area. In the config.sys file, the command device=c:\dos\himem.sys is required before any device commands for programs or drivers that use extended memory.
Health Insurance Portability and Accountability Act. Federal legislation that regulates access to medical records kept by health-care providers. Caution: Health-care insurers routinely try to gain access to data that HIPAA blocks providers from giving them by encouraging patients to divulge it. This is legal.
High Performance Radio Local Area Network. A family of ETSI standards for wireless LANs. They’re similar to the 802.11 standards (same carriers and data rates), but support QoS, and ATM as well as Ethernet.
High-Performance Parallel Interface. An obsolete ANSI standard data bus that sends 32 or 64 bits in parallel at up to 800 Mb/s (mid-1990s). It was used for high-speed channels in I/O-intensive systems, e.g. supercomputers.
Hybrid Memory Cube. Commercially unsuccessful DRAM (computer main memory) technology developed by Intel and Micron and intended to replace DDR4 SDRAM. It used the same approach as HBM. See also Wide I/O.
Head-Mounted Display. A display built into a headset or goggles. It typically uses optics to create the illusion of a larger display farther away from the eye. The types used in virtual reality usually have dual LCDs with a distinct image for each eye, coupled with head tracking.
Hidden Markov Model. A computer speech recognition technique that represents how phonemes (basic sounds of a language) and allophones (phoneme variants) are pronounced, and how quickly they’re spoken.
Home Phone Network Alliance. A standard for running a LAN on telephone wiring, with network signals at a frequency safely above the 4 kHz range of telephone voice signals. HomePNA 3.1 (2007) supports coaxial cable as well, to integrate video equipment.
An obsolete set of wireless LAN technologies dating to the late 1990s, intended for home and later business use. They used a 2.4 GHz signal that carried about 10 Mb/s, but was not compatible with 802.11b.
In network lingo, a computer or network acting as bait for malware or hackers. It allows attacks to succeed in order to gather data on their methods, coding, and source.
A plaintext system file on many computers, including PCs, telling the system how to interpret specified Internet domain names. It takes precedence over whatever DNS server the system uses, so the file is a common way for users to block undesirable domains by mapping them to local IP addresses.
An up-to-date hosts file that blocks a long list of malicious or troublesome domains is available for download from Web sites such as http://someonewhocares.org/hosts. Just copy it over your existing hosts file (no filename extension), which is at /etc in Unix or Linux and at \Windows\System32\drivers\etc in all recent versions of MS Windows. Note that too big a file will slow down the computer.
Hewlett-Packard. Named for Bill Hewlett (1913-2001) and Dave Packard (1912-1996), who started it in 1939 in Dave’s garage in Palo Alto, CA – an event often considered to be the start of Silicon Valley. A major US manufacturer of electronics, printers, RF/electrical test equipment, and PCs.
Host Port Interface. A DSP port that connects DSP internal memory to host processor memory via the local bus. This allows the DSP and host to exchange interrupts.
High-Power Microwave. A type of weapon that creates high-power bursts of microwave radiation. It began as a military concept, with the intent to fry the electronics of enemy forces by inducing high currents. The ultra-wideband type, a.k.a. the E-bomb, does this with non-nuclear explosives and an extremely high-speed switch, generating an electromagnetic pulse (EMP) that diminishes rapidly with distance. The focused 1-10 GHz narrowband types (see HERF) can destroy even shielded electronics, and have a range of miles.
More recently, there are shorter-range civilian versions that can fit in a van or even a suitcase and use EMI to merely degrade the performance of digital systems, especially networks and microprocessors. With digital devices proliferating everywhere at higher frequencies and lower voltages, these less aggressive e-weapons suffice for most malicious purposes while requiring much less power and drawing less attention.
High Performance Routing. An IBM technology to send SNA traffic over frame- and cell-based networks. The HPR implementation of APPN provides a more efficient network than TCP/IP.
Heterogeneous System Architecture. (2014) An initiative at AMD to improve computer performance by making the integrated GPU more of an equal partner with the CPU. The key is compiling all system code into HSAIL (HSA Intermediate Language), which can map to either kind of processor.
High Speed Circuit-Switched Data. A protocol for the transition of 2nd generation (2G) cellular phone networks (GSM and IS-136) to 3G. It provides a maximum 57.6 kb/s per channel. “Circuit-switched” means that an RF channel is assigned for the duration of a call, as opposed to packet-switching. Because 3G systems are packet-switched, HSCSD is mostly for users who require dedicated connections (e.g., connecting to old dial-up systems), and hasn’t seen wide use. At the RF level, it’s essentially the same as GPRS.
High Speed Serial Data Connector. A connector standard for high-speed Gigabit Ethernet or Fibre Channel networks using 150Ω copper cable. The connector has 8 contacts and resembles an RJ45, but is shielded, and more rugged. It incorporates a small printed circuit to equalize the signals, extending the usable cable length to over 30m. TxD+ is on pin 1, TxD- on pin 3, RxD+ on pin 6, and RxD- on pin 8.
HyperText Markup Language. The standard for creating Web pages by adding markup tags and other handling instructions to text files. It’s a narrow, fixed application of SGML, a much more complex markup system that predates the Web. HTML wasn’t intended to describe data formatting (although it can), so it was later supplemented by XML, a strict 1998 subset of SGML that describes content and offers more flexibility. This makes search engines more effective, among other benefits. However, even after the 1997 release of HTML 4, the language was not XML-compatible (unlike the equivalent XHTML). Modern HTML is paired with CSS, a method for separating the formatting of Web pages from their content and eliminating the repetitive work.
HTML 5, started in 2004, is an ambitious attempt to integrate XHTML and JavaScript into the language. While retaining backward compatibility with HTML 4 and older browsers, it also adds support for audio, video, and other Web technologies that used to require browser plug-ins (e.g. Adobe’s Flash or Microsoft’s Silverlight). The W3C and WHATWG are co-operating on compatible releases; the W3C approved theirs in October 2014.
Dynamic HTML (DHTML) allows manipulation of the data on a Web page without further calls to the server. It requires Java and some other stuff, so not all browsers support it.
High Temperature Superconductor. “High temperature” has traditionally meant warmer than liquid helium, which boils at 4.2 Kelvin (-268.93° C) under standard pressure, but colder than liquid nitrogen, which boils at 77.36 K (-195.8° C). Some newer HTS materials perform at still higher temperatures, above 100 K. As of 2020, there is at least one material that exhibits superconductivity at temperatures as high as 15° C – but only under enormous pressure. The search for a room-temperature superconductor (RTS) that doesn’t require such special treatment continues. See Cooper pair.
HyperText Transport Protocol. A member of the TCP/IP protocol suite at the Application layer, this is the client-server protocol that defines the World-Wide Web (WWW) by relaying HTML documents. It uses TCP/IP port 80 to open and manage Web connections. In 2015, the IETF released version HTTP/2. The previous version, HTTP 1.1, dates to 1999.
A Web server returns a response code for each new HTTP connection. Some of the best known codes:
200 – OK (no issues)
301 – redirect: URL moved permanently
302 – redirect: URL moved temporarily
401 – authentication required
404 – page not found
HTTPS (S for secure) is the abbreviation used in the URLs of Web sites that employ TLS or the older, less secure SSL to encrypt data sent to and from users. Encrypted data goes through TCP/IP port 443 instead of port 80.
Increasingly rare as network switches have become cheaper, a hub is just an extension attached to a switch port to distribute traffic to a distant point. It doesn’t make switching decisions. It simply broadcasts data from the switch or sending node to every other node, and nodes that aren’t the intended recipient ignore it.
Head-Up Display. Provides visual information to users without requiring them to take their eyes off their surroundings. Originally for cockpits of military aircraft, it has since found its way into civilian vehicles and mobile electronics, particularly AR.
Hird of Unix-Replacing Daemons. Hird, in turn, stands for Hurd of Interfaces Representing Depth. Again with the recursive acronyms. This is a kernel for the GNU OS, under development by members of the FSF since 1990 and still (2014) not ready for release.
High Voltage Direct Current. Most power lines carry AC. In a few long-distance or specialized cases, a line will be designed for HVDC instead. HVDC can connect large AC grids while keeping them decoupled so that they don’t have to be in phase; this is costly and inefficient, but simpler and safer to manage. Also, DC has lower transmission losses and infrastructure costs than AC, and this concern looms larger for some long-distance connections. See DC.
Intel trademark name for their early 2000s simultaneous multi-threading technology, introduced on the Pentium 4. Sometimes used carelessly to mean multi-threading in general.
Broadly, the dependence of a system’s change to a new state on the system’s current state as well as on input. This is generally desirable for control systems. A heater, for example, might maintain a temperature of 24° C by turning on below 22° and turning off above 26°. Allowing the current state (on or off) to determine response to input prevents a rapidly fluctuating on-off cycle.
Electrically speaking, hysteresis is the tendency of a material to retain magnetic flux after an applied magnetic field is removed, due to some of its atoms retaining the polar alignment imposed by the field. If an alternating field is applied, hysteresis acts to constantly oppose changes in current. This yields a hysteresis loop, where the flux within the system lags behind the applied field. See inductor.
