Surround
Professional, October 1998, Volume 1, Issue 1, by K.K. Proffitt
Some
terms you need to know to start exploring the world of surround sound.
AC-3: A data
compression method developed by Dolby (also known as Dolby Digital) that
takes six channels of audio (5.1) and folds them into a single digital
bitstream. The norminal bit rate for six (5.1) channels is 384 Kbps, which
represents a sompression ratio of about 10 to 1, but is capable of a maximum
of 448 Kbps. AC-3 is also what's known as "Lossy" compression
(see Lossy Compression).
Bass Management:
A circuit which utilizes the subwoofer in a 5.1 system to provide bass
extension for the five main speakers. The Bass Manager steers all frequencies
below a selected frequency, such as 80 Hz into the subwoofer along with
the LFE (see LFE) source signal.
Bass Redirection:
Another term for Bass Management.
Direct Radiator:
A speaker where the sound shoots directly from the front of the cabinet,
as in the vast majority of stereo monitors.
Dipole: A
speaker where the sound emanates from the sides of the monitors, which
provides the advantage of additional coverage area. A Dipole works well
for ambience material but not as well for direct source material.
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 Pro Logic:
An active decoder which takes the two-channel Dolby Surround-signal and
unfolds it into Left, Center, Right, and a limited-bandwidth Surround
channel.
Dolby Surround:
A matrix encoding method 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 Dolgy 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.
DTS: A data
compression method developed by Digital Threater Systems that takes six
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
1.4 Mbs, 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 17 Gig 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).
Encoder/decoder: An
encoder is a device which takes multiple digital data streams (as in six
channel surround sound) and compressees them into a single data stream
for more efficient storage and transmission. The decoder will perform
the opposite funciton, taking a single encoded bitstream and breaking
it into multiple datastreams.
5.1: A speaker
system that 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 80 Hz 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 120 Hz of 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 phychoacoustic 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.
MLP: Meridian
Lossless Packing. This is a data compression technique designed specifically
for high quality (96 kHz/24 bit) 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 96 kHz/24 bit 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 biomaterial,
and user interactivity such as menus and buttons to a DVD.
Reference Level:
This is the Sound Pressure Level to which a sound system is adjusted when
pink noise at reference electrical level is played over one channel at
a time. It is generally measured with C weighting and the SLOW characteristic
of a sound level meter.
Real Time Analyzer:
Sometimes known as an RTA or Spectrum Analyzer, the Real Time Analyzer
divides the audio bandwidth into individual 1/3-octave bands and gives
a sound pressure level indication of each. Because of this, it is far
more precise than the more common Sound Pressure Level meters.
RTA: An abbreviation
for Real Time Analyzer or Spectrum Analyzer. See "Real Time Analyzer".
Spectrum Analyzer:
See "Real Time Analyzer".
Standing Waves:
A mutual reflection of sound waves back and forth between two surfaces
where the sound pressure varies greatly and where it varies very little.
This means that the level of a particular frequency (usually low) is very
loud or very soft depending on where you listen at in the room. Standing
waves usually occur in the bass frequencies and are worse in small rooms.
Subwoofer:
A very low frequency speaker with a frequency response anywhere from approximately
25 Hz to as high as 120 Hz. The crossover frequency to the main system
depends on the response of the main system, and could range from 40 Hz
to over 100 Hz in some consumer systems.
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.