Home Theater Terminology

This post explains the basic terminology of lexicons commonly used to describe Home Theater systems and components.

Amplification

Increase in signal level, amplitude or magnitude.

Amplifier

A device which increases the level of signal (by increasing the voltage or current). Some amplifiers are used to isolate or control a signal, and may not increase level – or may actually decrease the level.

Analog

An electrical signal whose frequency and level vary continuously in direct relationship to the original acoustical sound waves. “Analog” also may refer to a control or circuit, which continuously changes the level of a signal in a direct relationship to the control setting.

Anamorphically Squeezed

This process, which is used on few laserdiscs, a few DVDs and even fewer TV broadcasts, is used to achieve a widescreen image, where the image is considerably wider than standard NTSC fare, once it is ‘unsqueezed’. The wider image is squeezed into the skinnier aspect ratio, which is usually the NTSC standard of 4:3/1.33:1. Un-squeezing can be done with a ‘stretching circuits’ in the TV. The end result (if left un-squeezed) is a picture with really skinny objects. Another option which has less detail, but is more widely used is letterboxing the picture.

Aspect Ratio

This describes the ratio of the width of a picture to the height. The NTSC standard is 4:3. The current HDTV standard is 16:9. Modern movies range from 1.66:1 to 2.4:1. By far the most common ratios are 1.85:1 and 2.35:1.

Bipole Speakers

One type of surround speaker. In this instance two or more drivers are facing different directions, and their cones vibrate in phase. This causes an omni-directional sound.

Bypass

An alternate signal path that goes around a given circuit. A “hard wire” bypass uses a switch and a piece of wire to route the signal from the input to the output of a device. A “bypass” switch is sometimes called an “in-out” switch.

Crossover Frequency

In a 2-way loudspeaker system, the frequency below which the sound feeds the low frequency driver and above which the sound feeds the high frequency driver.

CRT Projector

One type of front projector. It consists of three tubes each putting out one color: red, green, and blue. They offer brightness and detail, but are difficult to setup, and convergence is required about two times a year.

Digital Light Processor (DLP)

Digital Light Processing (DLP) generates images by reflecting light off the surface of a digital micro-mirror device (DMD) containing hundreds of thousands of tiny mirrors, then through a color wheel and a lens and onto the screen. Higher resolution projectors have more mirrors in their DMD’s-reflecting a greater amount of light for brighter images.

Digital Micro-mirror Device (DMD)

A mirror that is very small that can be kept as is or tilted x amount of degrees in order to reflect light. As such, it is either on or off. It can be turned on and off at various rates per second to achieve different levels of brightness. Commonly used together to form micro-mirror “wafers” and are controlled by a Digital Light Processor (DLP).

Dipole Speakers

One type of surround speaker. In this instance two or more drivers are facing different directions (most commonly and by definition 180 degrees) and their cones are vibrating out-of-phase. This causes nulling out of the sound by the viewing area which forms a “figure-8” sound field.

Discrete

Audio information can be assigned and sent to exact speakers.

Dolby AC-3

The old name for the most popular 5.1-channel home theater sound system. Is now called Dolby Digital. Consists of front left/right speakers, a center speaker, left/right surrounds, and a Low Frequency Effects (LFE) channel, usually used with a subwoofer. See AC-3 vs. DTS.

Dolby Digital

1 – The new name for the most popular 5.1-channel home theater sound system. Used to be called Dolby AC-3. Consists of front left/right speakers, a center speaker, left/right surrounds, and a Low Frequency Effects (LFE) channel, usually used with a subwoofer.

2 – A 5.1-channel sound system. Used in some commercial movie theaters in which the sound is placed in between the sprockets on the film.

Dolby Pro-Logic

Most popular surround format. Almost any receiver nowadays has it. Uses matrixed surround in order to encode four channels of sound: left/right front channels, a center channel, and one surround channel. It is quite a common occurrence to see two speakers used for the one surround channel however, as well as a subwoofer to supplement the speakers.

Dolby Surround

Another surround format that came out before Dolby ProLogic. It consists of only three channels: left/right front channels, and one surround channel.

Driver

Another name for a loudspeaker; usually the term is used when the loudspeaker is coupled to a “horn” for acoustic coupling and controlled dispersion of sound.

Efficiency

In general sense, efficiency is the ration of energy output to the total energy input, expressed as a percentage. In speaker systems, efficiency refers to the ratio of total acoustic watts radiated to total electrical watts input. Home speaker systems of 1% to 3% efficiency are typical, while larger horn-loaded sound reinforcement speakers sometimes reach 10% efficiency or more. Efficiency should not be confused with sensitivity, which measures only the on-axis sound pressure level in relation to electrical input power.

Frequency

The rapidity of change in current of voltage in an electrical signal or of air pressure in an acoustical signal. Frequency is measured in cycles per second; 1 cycle per second (cps) is 1 Hertz (Hz). The higher a note on the musical scale, the higher its frequency.

Front Projector

One type of viewing device. This is a separate unit that projects the image onto a screen allowing screen sizes of over 300″.

High Definition TeleVision (HDTV)

New viewing standard with an aspect ratio of 16:9/1.78:1. It is slated to have over 1000 lines of resolution, as well as to have Dolby Digital be the official sound format. There is an analog system in Japan, and a digital system proposed by the Grand Alliance for the US. This system is supposed to co-exist with the current standard and eventually replace NTSC around the year 2006.

Impedance

The total opposition to the flow of alternating current in an electrical circuit. Impedance is measured in ohms.

LCD Projector

One type of front projector. Liquid Crystal Display transmits light through a tiny LCD screen and then projects it for a larger image. One major benefit is that convergence and adjustments are not required to perfect the picture. One drawback is that this technology results in pixellation.

Letterbox

This process, which is used on many laserdiscs and some TV broadcasts, is used to achieve a widescreen image, where the image is considerably wider than standard NTSC fare. The end result is a wider picture with black bands on the top and bottom of the screen, which reduces the overall resolution of the image. Another option with greater detail, but is less widely used is anamorphically squeezing the picture.

Light-Valve Projector

One type of front projector. It combines the technologies of LCD projectors and CRT projectors. They offer exceptional detail and brightness. Line Doubler/Tripler/Quadrupler doubles, triples or quadruples the number of lines that make up a picture, therefore increasing detail, and ridding the picture of scan lines. Usually used with front projectors.

Matrixed Surround

Term used to describe the process to make Dolby Pro-Logic compatible material. It fits four channels of sound into a space meant for two channels. The center channel is decoded by using material common to both left/right channels, and the surround channel is decoded by extracting the sounds with inverse waveforms. This process results in channel leakage.

Monopole Speakers

One type of speaker with all drivers facing one direction. Used for precise placement of sounds. Usually used in front and center speakers.

NTSC

The standard by which TV is broadcast in the US. It has a theoretical maximum resolution of 525 lines. Also has an aspect ratio of 4:3 or 1.33:1.

Ohm

The unit of measure of electrical resistance or impedance.

Omni directional

Equal sensitivity in all directions. Usually refers to non-directional microphones.

PAL

The standard by which TV is broadcast in Europe. It has a theoretical maximum resolution of 625 lines. Also has an aspect ratio of 4:3/1.33:1, and in some places 16:9/1.78:1.

Pan and Scan

A technique used in which the right and/or left edges of widescreen material is chopped off in order to fit the picture into a narrower aspect ratio, for example the NTSC standard of 4:3 or 1.33:1. Pepople who do this select the best part of the image to scan, and then if the whole image needs to be seen, scans across the rest of the frame.

Polysilicon

Polysilicon technology splits light into red, green and blue (RGB) components and directs each to its own liquid crystal display (LCD) panel. Each LCD creates an image for its respective color by blocking out portions of the light (similar to a film negative). The output or images of the three panels is then “assembled” by a prism and transmitted through a lens to project a fully saturated color image.

Rear Projector

A type of viewing device: A translucent pane of glass or acrylic with a customized coating and structure to optimally refract video and computer imagery projected onto the side farthest from the audience.

Receiver

Component used in home theater and stereo applications. A decoder, audio/video switcher, AM/FM tuner, and an amplifier built into one unit.

Refresh Rate

The rate at which the picture redraws itself in one second. Usually expressed in hertz (Hz).

Resolution

A term associated with the number of lines that make up the vertical portion of the picture. The higher the number, the more detailed the picture.

Screen

What the picture is projected onto. The screen is more important when it comes to front projectors, when the screen must be bought separately.

Subwoofer

This is a separate speaker used to handle the bass of movie soundtracks, and can be used with the Low Frequency Effects channel in the new digital sound formats. These speakers can sometimes handle frequencies as low as 15hz.

Surround Sound

The popular term used to describe an experience where the sound ‘surrounds’ you. This is best achieved using surround-encoded material, a receiver, and surround speakers.

Surround Speaker

Set of four, rear (surround), and center channel speakers. Used to create “life like” sound reproduction in movies.

THX

Rumored to stand for the Tomlinson Holman experience. Others say it was named after George Lucas’ first film, THX-1138. THX is a set of standards by which laserdiscs and video tapes are made, as well as by which home theater equipment is made. They are supposed to yield the highest quality in home theater.

Tweeter

A loudspeaker used in a 2-way or more complex speaker system to reproduce only the treble or high frequencies of the audible spectrum.

Watt

A unit of measure for electrical or acoustical power.

Widescreen

Term used to describe a picture in which the aspect ratio is wider than the NTSC standard of 4:3/1.33:1. Almost all movies made nowadays are shot in some widescreen format. To solve the problem of different aspect ratios, several different techniques can be used. Among them are anamorphic squeezing, letterboxing, and Pan and Scanning.

Woofer

A loudspeaker or driver in a 2-way or more complex speaker system that is used to reproduce only the bass or lower part of the audible spectrum.