AAC (Advanced Audio Codec)

The audio coding standard defined by ISO as part of the MPEG-2 specification.  Declared an international standard in April 1997, MPEG-2 AAC builds upon and extends the popular ISO/MPEG-1 Audio Layer-3 (MP3) audio coding format.  Compared to MP3, AAC provides higher quality music with approximately 30% of the storage space or bandwidth.  AAC provides for up to 48 audio channels and sample rates up to 96 kHz.

AC-3

See Dolby Digital.

AES/EBU

Audio Engineering Society / European Broadcast Union is a digital audio standard commonly used in linking professional digital audio equipment.  The AES/EBU digital interface is usually implemented using 3-pin XLR connectors, the same type of connector used in a professional microphone.  One cable carries both right and left channels.  AES/EBU is an alternative to the S/PDIF standard.

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 Ohm's Law.

amplifier (amp)

A device that increases signal level.  Many types of amplifiers are used in audio systems.  A pre-amplifier is used to increase the signal level from an audio source.  A power amplifier increases the signal level and lowers the impedance sufficiently to drive a speaker.  Pre-amplifiers and power amplifiers can be purchased as separate units or can be packaged together with other audio functions.  A receiver is an audio unit that combines an AM/FM or FM-only radio tuner, pre-amplifier, control, input-switching, processing, and power-amplifier functions built into a single unit.

Audio power amplifiers are primarily classified by their output-stage design.  Classes of analog-output audio power amplifiers include:

• Class A
• Class B
• Class AB
• Class AB plus B
• Class D (digital)
• Class G
• Class H

analog

The method of representing and storing audio information with continuously-variable physical quantities.  Audio in an analog electrical signal is represented by the varying of a voltage's rate of change (frequency) and strength (amplitude) to correspond to how the audio changes in frequency and volume.  Examples include cassette tapes and 78 rpm and LP records.  Contrast with digital.

bit (b)

Originally short for binary digit.  In digital terms, the smallest unit of information.  A bit represents either an on or off value represented by a 0 or 1.  Abbreviated b (note the use of lower case).  Also see hexadecimal.

bit stream or bitstream

Refers to a stream of bits transmitted over a communications line between two devices, such as an Internet audio or video stream via SHOUTcast.

byte (B)

A unit of digital value that consists of 8 bits.  Computer memory or storage is usually measured in the number of bytes such as kilobytes (KB), megabytes (MB), and gigabytes (GB).
Note the upper-case B.  Also see hexadecimal.

CD

Compact Disc--an optical-disc storage media that is designed to store audio, video, or computer data in a digital format (they can be mixed on the same disc).  CDs have a capacity to store up to 650 MB to 700 MB (megabytes) of data, or 74 to 80 minutes of music.  The digital information in a standard audio CD is encoded in a PCM format at a sampling rate of 44.1 Khz.

CDDA

Compact Disc Digital Audio--the standard as set forth by Sony and Philips in the mid-1970s.  Audio tracks are sampled at 44.1 KHz and 16 bits.

codec

Short for encoder/decoder, MP3 for example.  The encoder is the process used to create the file, the decoder turns that encoded file back into a digital video and/or audio stream.  Playback tools like WinAmp only need the decoder part of the pair.

compression

Methods used to reduce a file in size.  Various methods of encoding digital information can be used to reduce the size of an original digital recording, either by eliminating redundant information or information that is viewed as unnecessary or not critical (lossy compression--MP3, for example) or by using other means to preserve the full data stream (lossless compression--FLAC, for example).  PCM is an uncompressed digital format.  Various common encoding methods and their compression ratios compared to PCM are Dolby Digital (12:1) and DTS (3:1).

DAC

Digital-to-analog converter--circuitry that converts a digital bitstream to an analog signal.

DAB

See Digital Audio Broadcasting.

dB

The abbreviation for decibel.

decibel (dB)

A scale to measure the relative loudness of sound or strength of a signal.  Decibels are a logarithmic scale of relative loudness.  A difference of 1 dB is usually the minimum perceptible change in volume, 3 dB (actually double the acoustic power) is a moderate change in volume, and 10 dB is an apparent doubling in volume.  In audio equipment decibels are used to indicate the range of volume from the quietest to the loudest signal that can be handled.  Also see dynamic range.

The volume level of common sounds in decibels (relative to the threshold of hearing) are:

• 0 dB is the nominal threshold of hearing, 130 dB is the threshold of pain
• Whisper: 15 to 25 dB
• Quiet background: about 35 dB
• Normal home or office background: 40 to 60 dB
• Normal speaking voice: 65 to 70 dB
• Orchestral climax: 105 dB
• Live rock music: 120+ dB
• Jet aircraft (near-field): 140 to 180 dB

digital

The method of representing and storing audio information with numbers.  Audio in a digital signal can be represented by many different schemes.  One common scheme is PCM.  Contrast with analog.

dithering

The method used in digital systems to smooth out the transitions from one bit to another at low levels.  Dithering makes a digital recording sound and behave more like analog.  The better a recording is dithered, the higher its apparent resolution.  Also known as anti-aliasing, a similar technique is used in computer graphics to make curved and diagonal lines appear smoother.

Digital Audio Broadcasting (DAB)

Also known as digital radio, Digital Audio Broadcasting is a digital method of transmitting audio and other information over the airways to radio receivers.  In the United States, a technology has been proposed called IBOC that uses the existing AM and FM frequency bands assigned to radio stations.

DivX

A compressed-video format closely related to MPEG-4, and the player software that plays it.  DivX files usually carry the file extension AVI or, less commonly, DVX.  Formerly, the name of a failed DVD-based technology.

Dolby® Digital

A 5.1 multi-channel surround format consisting of discrete front left, front center, front right, rear left, rear right, and a low-frequency-effects (LFE) subwoofer channel.  Dolby Digital is compressed approximately 12:1 compared to PCM.

Dolby Digital Surround EX

An enhancement to the Dolby Digital 5.1 format where a rear-center channel has been added creating a 6.1 format.  The Dolby Digital Surround EX format does not add an additional discrete recorded channel.  The new rear center or "back" channel is matrix-encoded in the Dolby Digital left and right surround signals.  The recorded Dolby Digital audio consists of five discrete main channels and a sixth, low-frequency-effects (LFE) subwoofer channel.  The two rear surround channels carry the signal for the rear center or "back" channel.  The Dolby Digital Surround EX format made is debut in the movie Star Wars: Episode I - The Phantom Menace.

DSD

Direct Stream Digital--a digital encoding format developed by Sony and Philips that is used to record SACD albums.  DSD samples an audio signal at a fixed rate (frequency) just as in the PCM method.  However, instead of recording the volume (amplitude) as an absolute number as in PCM, the DSD method measures and records how much the volume has changed since the last measurement.  If the signal is sampled fast enough, the amount of change since the previous sample would be very small.  As implemented on SACDs, DSD has a sampling rate that's over 2.8 MHz (2.8 million times per second)!  At this high speed the changes in signal strength can be represented with one bit.  DSD also compresses the data, resulting in a 2:1 reduction.  DSD is able to provide a frequency response from DC to 100 kHz with a dynamic range of 120 dB.

DSB

Direct-satellite broadcast--Sirius and XM Radio are the leading examples.

DTS

Digital Theater Systems--a multi-channel (5.1) digital audio encoding format created by Digital Theater Systems.  The DTS format was introduced in theaters in 1993 with the movie Jurassic Park.  In addition, the DTS format has been applied to home CDs, laser discs, and DVDs.  DTS audio, as used in DVD-Video and CDs, is compressed approximately 3:1 from PCM recorded with a sampling frequency of 96 kHz and a 20-bit sampling size.  DTS is capable of encoding PCM with a sampling frequency up to 192 kHz and 24-bit sampling size.

DTV

Digital Television--the general term for SDTV, EDTV, and HDTV.

DVD

Digital Video Disk or (now) Digital Versatile Disk.  An optical disk storage media that can store video, audio, and computer data; the form factor is the same as that for CDs.  There are multiple standards and formats for DVDs.  Typically, a DVD-Video is referred to as a DVD.  Various DVD formats are:

• DVD-Video
• DVD-Audio
• DVD-ROM
• DVD-RAM
• DVD-R
• DVD-RW
• DVD+R
• DVD+RW
  

DVD-Audio

An audio standard for high quality audio recorded on DVDs that exceed both the capacity and reproduction quality of CDs.  A DVD-Audio disk is capable of recording audio with frequencies up to 96 Khz (compared to CDs at 20 kHz) and with a dynamic range of up to 144 dB (compared to a CD's 96 dB).  DVD-Audio also supports multi-channel recordings.

DVD-RAM

A rewritable DVD format in which discs are (usually) encased in a carrier.  Treated by a PC like a Zip disk or a (really) big floppy.  Relatively rare.

DVD-ROM

A DVD that stores computer data and is read by a DVD-ROM drive attached to a computer or can be read by some players (especially JPEG picture files and MP3 audio files).  A DVD-ROM is read-only media.  DVD-ROM also includes recordable variations (DVD-R & DVD-RW, and DVD+R & DVD+RW).

DVD-Video

The format commonly referred to as "a DVD".  A DVD-Video stores video and audio information that is played by a DVD-Video player attached to a TV or video monitor.

dynamic range

The range between the loudest and the softest sounds that are in a piece of music or that can be reproduced by a piece of audio equipment without distortion.  This is a ratio expressed in decibels (dB).  In speech the range rarely exceeds 60 dB; in music the range can reach over 75 dB.

EDTV

See Enhanced Definition Television

Enhanced Definition Television (EDTV)

FireWire

The popular name for a high-speed digital standard for connecting peripherals such as digital video cameras, audio components and computer devices.  FireWire was originally developed by Apple Computer as a replacement for the SCSI bus.  IEEE 1394 is the formal name for the standard.  Vendors must obtain a license from Apple to use the term "FireWire".

FLAC

The Free Lossless Audio Codec.  Somewhat oversimplified, FLAC is similar to MP3, but lossless, meaning that audio is compressed in FLAC without any loss in quality.

frequency

The measurement of the number of cycles per second in an acoustic tone or an alternating current signal.  The technical term for cycles per second is hertz, abbreviated as Hz.  The range of human hearing is typically considered to range from 20 Hz to 20,000 Hz (also expressed as 20 Hz - 20 kHz).

Gigabyte (Gig or Gbyte)

One billion bytes. Written as 1 GB.  Gigabytes are used as a measure of digital storage space--for example, a standard DVD-R or DVD+R can hold about 4.7 GB.

HDTV

See High-Definition Television.

hertz

A measurement of frequency.  One hertz is equal to one complete cycle per second.  hertz is abbreviated as Hz.

Hexadecimal

00000000             00                    0

00000010             02                    2
00000011             03                    3
00000100             04                    4
00000101             05                    5
00000110             06                    6
00000111             07                    7
00001000             08                    8
00001001             09                    9
00001010             0A                  10
00001011             0B                  11
00001100             0C                  12
00001101             0D                  13
00001110             0E                   14
00001111             0F                   15
00010000             10                   16
00010001             11                   17
00010010             12                   18

------------------- through -----------------

High-Definition Television (HDTV)

HDTV refers to a complete product/system with the following minimum performance attributes:

• Receiver: Receives ATSC terrestrial digital transmissions and decodes all ATSC Table 3 video formats.
• Display Scanning Format: Has active vertical scanning lines of 720 progressive (720p), 1080 interlaced (1080i), or higher.
• Aspect Ratio: Capable of displaying a 16:9 image.
• Audio: Receives and reproduces and/or outputs Dolby Digital audio.

High-Definition Television (HDTV) Monitor refers to a monitor or display with the following minimum performance attributes:

• Display Scanning Format: Has active vertical scanning lines of 720 progressive (720p), 1080 interlaced (1080i), or higher.
• Aspect Ratio: Capable of displaying a 16:9 image.
• In specifications found on product literature and in owner's manuals, manufacturers are required to disclose the number of vertical scanning lines in the 16:9 viewable area, which must be 540p, 810i or higher to meet the definition of HDTV.

High Definition Television (HDTV) Tuner refers to a RF receiver with the following minimum performance attributes:

• Receiver: Receives ATSC terrestrial digital transmissions and decodes all ATSC Table 3 video formats.
• Outputs: Outputs the ATSC Table 3 720p and 1080i/p formats in the form of HD with minimum active vertical scanning lines of 720p, 1080i, or higher.  Additionally, it may output HD formats converted to other formats.  The lower resolution ATSC Table 3 formats can be output at lower resolution levels.  Alternatively, the output can be a digital bitstream with the full resolution of the broadcast signal.
• Audio: Receives and reproduces and/or outputs Dolby Digital audio.
  

Hz

An abbreviation for hertz.

IBOC

In-Band On-Channel--a proposed technology for digital radio in the United States.  Digital radio is also being referred to as Digital Audio Broadcasting or DAB.  IBOC allows digital information to be transmitted simultaneously with existing analog broadcast in the same frequency bands that are currently assigned to AM and FM broadcast stations.

IEEE 1394

ISO

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kilobyte (K or kbytes)

1 kilobyte is 1,024 bytes.

(empty)

megabyte (Meg or Mbytes)

Precisely 1,048,576 bytes (1024 x 1024), but often rounded to "one million bytes".  Written as 1 MB.  Megabytes are used as a measure of digital storage space--for example, a CD can hold 650 to 700 MB.

MIDI

Musical Instrument Digital Interface--originally developed by Korg as a means of controlling multiple synthesizers from a single keyboard or computer, now a standard.  Also now often used to control stage lighting or on a computer to synthesize music directly on the PC.  The standard covers the connectors, signals, and MID file format.

MOD (file)

A type of computer-music file; think MIDI with the samples or synthesis instructions included.  Originally developed on the Amiga (in 1987), there are now multiple formats, including STM/S3M (ScreamTracker) and IT (ImpulseTracker)--a tracker is the program used to create MOD files in the first place--as well as the original MOD format.  There are literally tens of thousands of these files, many of them are excellent music, even the earliest ones--start here for more information.  WinAmp or (even better) MODPlug Player play these.

MP3 (MPEG-1 Audio Layer 3)

MPEG

MPEG-2

MPEG-4

A video codec that provides better video quality than the original MPEG-1 specification (comparable to the DVD/satellite-brodcast MPEG-2 standard) in about the same storage space.  These sometimes carry the file extension MP4, though some MOV and AVI files are actually MPEG-4.  Also see DivX.

Ogg Theora

Ogg Vorbis

A completely open, patent-free, professional audio encoding and streaming technology with all the benefits of Open Source.  Files usually have the OGG extension.  http://www.vorbis.com

ohm

A unit of electrical resistance (DC) or impedance (AC).  The symbol used is an omega (Ω).

Ohm's Law

PCM

Pulse Code Modulation--a method of encoding audio information in a digital format.  PCM samples analog audio information at a fixed sampling rate and measures the amplitude (volume) of the audio signal with a precision determined by the sampling size.  Audio is encoded on CDs in the PCM format with a sampling frequency of 44.1 kHz and a 16-bit sampling size.  A 16-bit sample size can contain 65,536 possible levels of sound volume which corresponds to a dynamic range of 96dB.  DVD-Audio can be encoded in PCM with a sampling frequency of up to 192 kHz and a 24-bit sample size.  A standard WAV file on a PC is usually encoded as PCM.

perceptual coding

A euphemism describing the lossy methods of audio compression that remove portions of the audio signal that cannot be easily perceived by the human ear.  Forms of perceptual coding in popular audio codecs include formats such as Dolby Digital, DTS, MP3, and AAC.  Contrast with FLAC.

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Ripper

A program that converts tracks of an audio CD from CDDA format to WAV format and saves them to storage media, usually a hard drive.

SACD

See Super Audio CD

SDDS

Sony Dynamic Digital Sound--a multi-channel (5.1 or 7.1) digital format developed by Sony for theatrical film.

SDTV

See Standard Definition Television

S/PDIF

Sony/Philips Digital Interface Format--a format for linking consumer digital audio equipment together based upon the AES/EBU standard.  S/PDIF typically uses unbalanced high impedance coaxial cables or fiber optical cables; however, high quality audio cables can be used for short lengths.  Good 75-ohm cable and good 75-ohm connectors can provide for lengths up to 25 feet.

Standard Definition Television (SDTV)

SDTV refers to a complete product/system with the following performance attributes:

Standard Definition Television (SDTV) Tuner refers to a RF receiver with the following minimum performance attributes:

Super Audio CD (SACD)

A technology for high resolution multi-channel digital audio recordings originally proposed by Sony and Philips.  SACD albums must contain one layer with the DSD digital encoding format which requires a SACD-capable player.  SACD albums can optionally contain a second layer encoded with the CDDA format (PCM  44.1 kHz  @ 16-bit) in stereo allowing compatibility with existing CD players.

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