U. Abbreviation for Modular Unit. See rack space.

UDMA. Ultra Direct Memory Access. A protocol that offers twice the transfer rate of ATA by allowing devices direct access to RAM. UDMA/33 offers up to 33 MB/second performance, while UDMA/66 provides up to 66 MB/second data transfer.

UI. See user interface.

UL. Underwriters Laboratory. A public safety agency dating back to the 1890s. The UL is under-written, or supported, by insurance companies. Its mission is to test electrical products for safety, to establish standards, and to certify products.

Ultra ATA (a.k.a. ATA-4). A high-speed version of the ATA protocol that provides burst transfer rates up to 33 MB/second.

Ultra DMA. See UDMA.

Ultra SCSI. A family of parallel SCSI protocols, some of which use LVD (Low-Voltage Differential) technology to greatly increase performance, increase maximum cable lengths, and increase the maximum number of devices on a SCSI chain. The Ultra Wide SCSI versions double the data path from 8-bit to 16-bit, doubling performance. See Table U.1.

ultrasonic. Frequencies above the upper limit of human hearing, which is typically accepted as 20 kHz.

U-Matic. A 3/4-inch videocassette recording format developed by Sony. The Sony PCM-1600 adaptor allowed a U-Matic video machine to record stereo digital audio, and the machines quickly became the standard format for delivering CD masters for duplication. In fact, one story has it that the CD-standard sampling rate of 44.1 kHz is derived from the U-Matic’s horizontal-sync rate. The subsequent PCM-1610 and PCM-1630 decks also used U-Matic format videocassettes for storage and were popular for mastering applications. By the 1990s, Sony’s 1/2-inch Betacam format had made the U-Matic machines largely obsolete, though U-Matic machines are still in use today.

unbalanced. An audio connection that has a positive wire but that uses the cable shield to serve as both the negative signal conductor and the ground. Unbalanced cables do not offer noise cancellation (there is no common mode rejection) and are susceptible to noise and interference pickup. For this reason it is best to keep unbalanced cable runs as short as possible; generally, 30 feet is given as the upper maximum for unbalanced cable lengths.

uncolored (a.k.a. straight-wire). A characteristic of a device that introduces no coloration or tonal changes to a signal. Just as coloration is desirable for certain applications, lack of coloration is equally desirable for other applications. This term is typically applied to microphones and mic preamps, as well as to other types of gear.

undo. A command/function in most computer programs that allows the last operation to be reversed or undone, returning the data to where it was before the operation. Most modern programs offer many levels or even unlimited undo, allowing the user to revert the data back to an earlier point, before it was edited or other operations were carried out. See also redo.

unformatted capacity. The maximum data capacity of a disk drive before it has been formatted to work with a computer. Some portion of the disk space is required for formatting and “housekeeping” information, so the formatted capacity will always be smaller than the unformatted capacity. What really matters to the end user is the formatted capacity, but drives are usually advertised and labeled with their unformatted capacity.

unidirectional. Literally, “in one direction.” Unidirectional is generally used as a synonym for cardioid with reference to microphone polar patterns, though technically, the term could be applied to any directional microphone. See also cardioid.

uninstall. To remove a program from a computer. Usually this involves more than just deleting the program itself from the machine’s hard drive. Typically, a program installer puts a variety of support files, drivers, and other software bits into the computer’s system and other places. All of these must be removed to completely de-install a program. Most software packages that use an installer application will also uninstall their software, removing all traces from the computer.

unison. Two or more vocal or instrumental parts of the same pitch happening simultaneously.

unison mode. A mode available on many synthesizers that layers detuned oscillators for a fatter sound, usually at the expense of reduced polyphony.

unity. See unity gain.

unity gain. Unity gain is where the gain of a signal going into a device equals the gain coming out. Unity gain is helpful for maintaining gain staging, operating levels, and good signal-to-noise ratios.

Universal Binary. Macintosh-compatible software that contains executable code for both older PowerPC processors as well as the newer Intel processors. When the software is installed onto the computer’s hard drive, the operating system detects that the program is Universal Binary–compatible and automatically loads the code that is compatible with the processor type.

Universal Serial Bus. See USB.

UNIX. A trademarked name for the computer operating system developed at Bell Labs in 1969 that is popular for servers and high-end computer workstations at universities, government and research facilities, and major companies. By 1984 there were more than 100,000 UNIX installations in the world, with 750,000 in place by 1987. Over the years, many companies developed their own implementations of the OS. In 1993 Novell, who owned UNIX at the time, transferred the trademark to the X/Open Company (now part of The Open Group), which developed a single specification for APIs. UNIX pioneered, provided, or popularized many features that are now common in other operating systems, such as the hierarchical file system, the TCP/IP networking protocol (important for the development of the Internet), and many more. UNIX is a multitasking, multi-user OS with protected memory (which prevents a program from interfering with another program) that uses many small utilities along with the kernel, or master control program. Current implementations built upon the UNIX standard include Linux, Macintosh OS X, Solaris, and more.

unzip. To extract data-compressed files from a zip archive.

UPS. Uninterruptible Power Supply. A battery backup device that automatically takes over to provide AC power if the normal electric service is interrupted. A UPS typically isn’t intended to provide power long-term; rather, the UPS is intended to give the user time to save any critical data and shut down equipment before the power runs completely out.

upsampling. Increasing the sampling rate of digital audio. Typically, digital audio is upsampled so that filtering and other processes can be performed at very high frequencies, reducing artifacts that can be heard in the audible frequency range. Upsampling is sometimes confused with oversampling, though the two are different: Upsampling is a type of sample rate conversion in the digital domain, while oversampling is operating a converter at a very high sample rate. Upsampling is used, for example, by some plug-ins to increase the precision of DSP processing calculations.

USB. Universal Serial Bus. A standard for connecting peripheral devices to a computer. USB was jointly developed by a number of manufacturers called the USB-IF (USB Implementers Forum). USB is a hot-pluggable, plug-and-play protocol that supports simultaneously connecting up to 127 devices in series to a computer. There have been three versions: USB 1.0, USB 1.1 (which fixed a few problems), and USB 2.0, which offers higher speed performance. USB supports three data rates: Low (1.5 Mbps) for keyboards, mice, and other interface peripherals; Full (12 Mbps) for storage devices, audio interfaces, and more; and High (480 Mbps), which is supported in USB 2.0 for high-speed storage, audio interfaces, and other bandwidth-hungry devices. There is a Super Speed mode (4.8 Gbps) that is under development at this writing.

USB flash drive. See jump drive.

USB stick. See jump drive.

USB 1.0. See USB.

USB 1.1. See USB.

USB 2.0. See USB.

user-definable. A parameter or preference that can be set by the user.

user interface. The controls, display, and other items that allow a human to operate a piece of gear or software.

user preset. A program or preset created by the user of a piece of gear, as opposed to a factory preset, which is created by the manufacturer and included with the product.

utility. A piece of software designed to perform a specific function, usually a housekeeping task, such as disk maintenance, various types of setup, and a wide variety of other duties.

UV22HR (a.k.a. UV22). Apogee Digital’s patented technology for increasing dynamic range at lower resolutions, similar to dithering or bit-mapping, but using a 22-kHz “bias” tone to modulate the lowest bits in the 16-bit signal. UV-22 is said to provide resolution equal to that of 20 bits using 16-bit digital audio, without increasing background noise.