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We have compiled a list of audio, video and home theater terms that you
might find helpful as you research your next purchase.
From A-Z, this glossary should have all the answers.
Glossary of Terms:
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D
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F
G
H
I
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Q
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A
A-B Test
A test between two components. For example, a test between two different loud speakers.
A-Weighting
Measurement based roughly on the uneven frequency sensitivity of the human ear. The influences of low and high frequencies are reduced in comparison to midrange frequencies because people are most sensitive to midrange sounds.
AAD
An acronym found on CD cases to indicate that the music was recorded and mastered on in analog (AA) and then stored digitally (D).
(See also ADD) (See also DDD)
AC
Alternating Current. An electrical current that periodically changes in magnitude and direction.
AC3
Audio Codec 3. This was the original and more technical name for Dolby Digital. Replaced by marketing mavens when they realized that Dolby's name was not in the title. Some RF modulated, 5.1-encoded laser discs were labeled as AC3. Later versions were labeled as Dolby Digital. (See also Dolby Digital)
Academy Curve
An intentional roll-off in a theatrical system's playback response above ~2kHz (to -18dB at 8kHz) to minimize noise in mono optical tracks. Some (many) transfers to home video of mono movies have neglected to add the Academy filter during transfer, giving many old movies a screechy sound they were never intended to have. A few home processors have an Academy filter option, making them a must for old-movie buffs. Has been used since 1938.
Acoustic Suspension
A sealed or closed box speaker enclosure.
Acoustically Voice Matched
Ensuring that the many different loudspeakers used in a Home Theater system have a sufficiently similar timbre match, such that when a sound is panned between them there is no change in character of the sound.
Acoustics
The area of study which deals with the behaviour of sound. Also the effect a given environment has on sound.
Active
Powered. An active cross-over is electrically powered and divides the line-level signal prior to amplification. An active speaker includes an active crossover and built-in amplifier.
ADD
An acronym found on CD cases to indicate that the music was recorded in analog (A) and mastered or remastered digitally (D) and then stored digitally (D). (See also AAD) (See also DDD)
AES/EBU
AES/EBU (Audio Engineering Society/European Broadcasting Union) is the name of a digital audio transfer standard. The AES and EBU developed the specifications for the standard. The AES/EBU digital interface is usually implemented using 3-PIN XLR connector.
Alignment
A class of enclosure parameters that provides optimum performance for a woofer with a given value of Q.
Alpha
In sealed enclosure designs, the ratio of Vas to Vb, where Vb is the volume of the box you will build. See also: Sealed Enclosure
Ambience
The acoustic characteristics of a space with regard to reverberation. A room with a lot of reverb is referred to as being live; whereas a room with little or no reverb is considered to be dead.
Ampere (a)
The unit of measurement for electrical current in coulombs per second. There is one ampere in a circuit that has one ohm resistance when one volt is applied to the circuit. (See also Ohms Law)
Amplifier
A device which increases signal level (increases it's amplitude). Amplifiers typically increase voltage, current or both.
Amplifier Classes
Audio power amplifiers are classified primarily by the design of the output stage. Classification is based on the amount of time the output devices operate during each cycle of signal swing. Also defined in terms of output bias current, (the amount of current flowing in the output devices with no signal).
Class A operation is where both devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The key ingredient of class A operation is that both devices are always on. There is no condition where one or the other is turned off. Because of this, class A amplifiers are single-ended designs with only one type polarity output devices. Class A is the most inefficient of all power amplifier designs, averaging only around 20%. Because of this, class A amplifiers are large, heavy and run very hot. All this is due to the amplifier constantly operating at full power. The positive effect of all this is that class A designs are inherently the most linear, with the least amount of distortion.
Class B operation is the opposite of class A. Both output devices are never allowed to be on at the same time, or the bias is set so that current flow in a specific output device is zero when not stimulated with an input signal, i.e., the current in a specific output flows for one half cycle. Thus each output device is on for exactly one half of a complete sinusoidal signal cycle. Due to this operation, class B designs show high efficiency but poor linearity around the crossover region. This is due to the time it takes to turn one device off and the other device on, which translates into extreme crossover distortion. Thus restricting class B designs to power consumption critical applications, e.g., battery operated equipment, such as 2-way radio and other communications audio.
Class AB operation allows both devices to be on at the same time (like in class A), but just barely. The output bias is set so that current flows in a specific output device appreciably more than a half cycle but less than the entire cycle. That is, only a small amount of current is allowed to flow through both devices, unlike the complete load current of class A designs, but enough to keep each device operating so they respond instantly to input voltage demands. Thus the inherent non-linearity of class B designs is eliminated, without the gross inefficiencies of the class A design. It is this combination of good efficiency (around 50%) with excellent linearity that makes class AB the most popular audio amplifier design.
Class AB plus B design involves two pairs of output devices: one pair operates class AB while the other (slave) pair operates class B.
Class D operation is switching, hence the term switching power amplifier. Here the output devices are rapidly switched on and off at least twice for each cycle. Since the output devices are either completely on or completely off they do not theoretically dissipate any power. Consequently class D operation is theoretically 100% efficient, but this requires zero on-impedance switches with infinitely fast switching times -- a product we're still waiting for; meanwhile designs do exist with true efficiencies approaching 90%.
Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. There have been several ways to do this. The simplest involves a single class AB output stage that is connected to two power supply rails by a diode, or a transistor switch. The design is such that for most musical program material, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks. Another approach uses two class AB output stages, each connected to a different power supply voltage, with the magnitude of the input signal determining the signal path. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight. Class G is becoming common for pro audio designs.
Class H operation takes the class G design one step further and actually modulates the higher power supply voltage by the input signal. This allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices. The efficiency of class H is comparable to class G designs.
Amplifier, Power
An amplifier without tone controls, usually with a higher power output than a line amplifier or pre-amp. Commonly used to drive loudspeakers.
Amplitude (amp)
The level (perceived as audible volume) of an electrical or acoustic signal. Shown as the value of the vertical axis on a typical graph of a sound wave.
Analog
An electrical signal analogous to the original sounds in non-digital form. For example, LPs and cassette tapes store audio in an analog form.
Any quantity which varies continuously without distinct steps. For sound waves in air, this refers to the continuous variation in air pressure; for an audio signal, this refers to the continuous variation in current or voltage.
Anamorphic
Process that horizontally condenses (squeezes) a 16:9 image into a 4:3 space, preserving 25 percent more vertical resolution than letterboxing into the 4:3 space. For the signal to appear with correct geometry, the display must either horizontally expand or vertically squish the image. Used on about two or three promotional laser discs and many DVDs. Also called Enhanced for Widescreen or Enhanced for 16:9.
Aperiodic
Refers to a type of bass-cabinet loading. An aperiodic enclosure type usually features a very restrictive, (damped), port. The purpose of this restrictive port is not to extend bass response, but lower the Q of the system and reduce the impedance peak at resonance. Most restrictive ports are heavily stuffed with fiberglass, dacron or foam.
Aperture Grille
In a color picture tube, a shadow mask consisting of a metal sheet with thin vertical openings instead of circular holes.
Aspect Ratio
The ratio of image width to image height. Common motion-picture ratios are 1.85:1 and 2.35:1. Television screens are usually 1.33:1 (also known as 4:3), which is similar to the Academy standard for films in the '50s. HDTV is 1.78:1, or 16:9. When widescreen movies (films with aspect ratios wider than 1.33:1) are displayed on 1.33:1 televisions, the image must be letterboxed, anamorphically squeezed, or panned-and-scanned to fit the screen.
ATSC
Advanced Television Systems Committee. Government-directed committee that developed our digital television transmission system.
Attenuate
To reduce the amplitude of an electrical signal usually by using a volume control, fader or pad. Also to reduce sound levels acoustically through the use of acoustic absorbers, resonators or structural materials.
Audio/Video (A/V)
Any system or component that involves both audio and video elements, like a receiver or preamplifier that switches video signals and processes multichannel audio singals (including surround sound decoding) for a home theater system.
Audiophile
An individual who draws great enjoyment from listening to music through high-quality audio reproduction systems.
Aux (auxiliary)
An input on a receiver, integrated amplifier or preamplifier that can be used to connect line-level source components. Also, any general-purpose A/V input. |
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