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A Brief Surround Glossary
AC-3 - A data compression method, otherwise known as Dolby Digital, that uses psychoacoustic principles to reduce the number of bits required to represent the signal. Bit rates for 5.1 channels range from 320 kbps for sound on film to 384 kbps for digital television and up to 448 kbps for audio use on DVD. AC-3 is also what's known as a "Lossy" compressor (see Lossy Compression) that relies on psychoacoustic modeling of frequency and temporal masking effects to reduce bits by eliminating those parts of the signal thought to be inaudible. The bit rate reduction achieved at a nominal 384 kbps is about 10:1.
Bass Management - A circuit which utilizes the subwoofer in a 5.1 system to provide bass extension for the 5 main speakers. The Bass Manager steers all frequencies below 80Hz into the subwoofer along with the LFE (see LFE) source signal.
Bass Redirection - Another term for Bass Management.
Direct Radiator - A loudspeaker where the principal output is directed at the listening area. Universally used for the front channels in a multichannel sound system and widely used for the surround channels, direct radiators are often found to be better for localization and poorer for diffuse-field reproduction such as for reverberation and ambience than dipole radiator.
Dipole Radiator - A loudspeaker having a Figure 8 directional pattern and often used for reproducing the surround channels of a multichannel audio system by placing the listening area in the null of the Figure 8 pattern. Dipoles are often found to be better at reproducing enveloping sounds such as reverberation and ambience and poorer at localizing than a direct radiator. Also, dipoles simulate an array of loudspeakers in theaters when used in the home.
DLT - Digital Linear Tape. The current standard media used as a DVD production master for delivery to the pressing plant, it is a tape format similar to exabyte but with a much faster transfer rate and greater storage capacity.
Dolby Digital - The official Dolby name for AC-3 encoding. (see AC-3)
Dolby Prologic - An active matrix decoder that extracts four signals from 2 channel Dolby Surround encoded material. The four channels are left, center, and right front channels, and a single bandwidth limited mono surround channel. The amplitude-phase matrix decoder uses level difference between the two source channels, called LT and RT, to steer across left-center-right, and the phase difference to steer from front to surround.
Dolby Surround - A digital encoding system that combines four channels (Left, Center, Right and a limited bandwidth Surround channel) into two channels. These two channels can be summed together for mono playback, or played back as normal stereo. When the two channels are fed into the active Dolby Pro Logic decoder, the matrix is unfolded back into four channels again. The limited bandwidth Surround channel is reproduced through the Left Surround and Right Surround speakers. If the matrix is fed into a passive decoder, then only the stereo signal plus the surround channel is unfolded.
Downmix - To automatically extract a stereo or mono mix from an encoded surround mix.
DSD - Direct Stream Digital. DSD measures at a very high sampling rate whether an analog waveform is rising or falling. This yields a single bit; either a 1 for a rising wave or a 0 for one that's falling. This single bit is then recorded directly, which avoids many of the unwanted side effects occurring when using the LPCM process.
DTS - A data compression method developed by Digital Theater Systems using waveform coding techniques that takes 6 channels of audio (5.1) and folds them into a single digital bitstream. This differs from Dolby Digital in that the data rate is a somewhat higher 1.4Mbs, which represents a compression ratio of about 4 to 1. DTS is also what's known as a "Lossy" compression (see Lossy Compression).
DVD - Digital Versatile Disc. A new optical disc format similar to the CD but featuring greatly increased storage capacity and advanced interactivity and user features. This new format comes in 4.7, 8.5, 9.4, and 17Gig capacities and is separated into five specification "books". These are DVD-ROM, DVD-Video, DVD-Audio (soon to be released), DVD-R (write-once), and DVD-RAM (erasable).
DVD-A - The soon to be released audio format of DVD features a variety of sample rates (44.1, 48, 88.2, 96, 176.4, and 192kHz) and word lengths (16, 20, 24) for six channel surround sound audio. DVD-A also features many value-added capabilities such as text, graphics and video.
DVD-V - The video format of the Digital Versatile Disc. The DVD-V format allows for audio-only usage using either LPCM, Dolby Digital or dts encoding.
Encoder/decoder - An encoder is a device which takes multiple digital data streams (as in 6 channel surround sound) and compresses them into a single data stream for more efficient storage and transmission. The decoder will perform the opposite function, taking a single encoded bitstream and breaking it into multiple data streams.
Encryption - An advanced form of digital copy protection.
Extensibility - A feature of DVD-A that gives it the capability of using any new digital encoding technology that may become popular in the future.
5.1 - A speaker system which uses three speakers across the front and two stereo speakers in the rear, along with a subwoofer.
In-Band Pink Noise - Filtered noise used for test purposes. This could be 20 to 80Hz for a subwoofer or 250 to 2.5kHz for the main system.
LFE - Low Frequency Effects channel. This is a special channel of 5 to 120Hz information primarily intended for special effects such as explosions in movies. The LFE has an additional 10dB of headroom in order to accommodate the required level.
Lossy Compression - A compression format that cannot recover all of its original data from the compressed version. Supposedly some of what is normally recorded before compression is imperceptible, with the louder sounds masking the softer ones. As a result, some data can be eliminated since it's not heard anyway. This selective approach, determined by extensive psycho-acoustic research, is the basis for "lossy" compression. It is debatable however, how much data can actually be thrown away (or compressed) without an audible sacrifice. Dolby AC-3 and DTS are lossy compression schemes.
Lossless Compression - A compression format that recovers all the original data from the compressed version. MLP is a lossless compression scheme.
LPCM - Linear Pulse Code Modulation. This is the most common method of digital encoding of audio used today and is the same digital encoding method used by current audio CD's. In LPCM, the analog waveform is measured at discrete points in time and converted into a digital representation.
MLP - Meridian Lossless Packing. This is a data compression technique designed specifically for high quality (96kHz/24bit) sonic data. MLP differs from other data compression techniques in that no significant data is thrown away, thereby claiming the "Lossless" moniker. MLP is also a standard for the 96kHz/24bit portion of the new DVD-audio disc, and will be licensed by Dolby Labs.
Premastering - The process of adding encoded audio and/or video, with extra program content such as different endings, out-takes, camera angles and bio material, and user interactivity such as menus and buttons to a DVD.
Red Book CD - The format standard for the audio CD that allows universal compatibility with any Compact Disc and any CD player. The format stipulates that the audio be recorded in stereo at 44.1kHz sample rate with a 16 bit word, among other things.
Reference Level - This is the Sound Pressure Level at which a sound system is aligned.
SACD - Super Audio CD. The SACD is a proposed format by Sony and Phillips containing the both the DSD process for very high quality multichannel audio and a Red Book CD layer which is compatible with any current CD player. This means that any current CD will play on a SACD player and any SACD will play on any current CD player.
Scalability - A feature of DVD-A that allows the producer to select from various sample rates (44.1, 48, 88.2, 96, 176.4, and 192kHz) and word lengths (16, 20, 24). It is also possible for the producer to assign different sample rates and word lengths to different channel families, such as 96/24 to the front speakers and 48/16 to the surrounds.
SMART Content - System Management Audio Resource Technique. This feature allows the producer to control the way the multichannel audio is played played back in stereo by saving one of 16 mixdown coefficients as control information to a data channel on the DVD-A.
Subwoofer - A very low frequency speaker with a frequency response anywhere from approximately 25Hz to as high as 120Hz. Depending on the application, either 40Hz or 80Hz is generally chosen as the crossover point.
Super Bit Mapping - A Sony process that down converts a 20 or 24 bit signal in such a way as to retain the maximum possible signal quality in only a 16 bit word.
Tripole - A surround speaker (trademarked by M&K) which combines both a direct radiator and a dipole in the same cabinet (see both Direct Radiator and Dipole).
THX - A set of technologies from Lucasfilm first developed for the cinema and subsequently for the home. In the theater, THX standardizes the sonic environment by stipulating not only the acoustics required but the playback equipment as well. In the home, both electronic and speaker strategies are employed in order to have the program material more closely match that of the dubbing stage.
UDF - Universal Disc Format. The file system used by DVD that eliminates much of the confusion that CD-ROM had due to the many different file formats used. All DVD formats use UDF and as a result have some level of compatibility with not only all DVD players, but with computers using DOS, OS/2, Windows, Mac, and Unix operating systems as well.
Watermarking - An embedded signal that applies a digital signature in the form of supposedly inaudible noise on a DVD-A. This digital watermark actually puts a faint image (either text or graphics) on the signal side of the disc which makes it extremely hard for pirates to duplicate. The watermark also identifies the artist, record company, catalog number and copyright holder.
