Dolby Surround Sound

1. Introduction 

Surround sound is potentially available from stereo VHS/UHV/IRD/cable, MTS
broadcast TV, stereo video tape, laser video disc and a few audio CDs.  The
topic is included in my LD (laserdisc) article series because surround is
frequently the next enhancement sought after obtaining an LD player.

There has been a fair amount of discussion over the years in the Usenet
Netnews groups, and rec.arts.movies on the subject of
surround sound.  Much of it has concentrated on theory; why recovery of 3 or
4 channels of sound from 2 channels of data is [im]possible, what artifacts
are introduced, etc.  The purpose of this article is to give you an idea of
how surround relates to the rest of your home entertainment components, and
what benefits it conveys.

Article Contents:
 2.  The Experience of Surround - what does it sound like?
 3.  Putting Surround in Perspective - how important is it?
 4a. How does matrix surround work? - how is it encoded?
  b. How does discrete surround work?
 5.  The Hierarchy of Home Surround - a travel guide to the buzzword maze.
 6.  Surround System Configuration - what hooks up where.
 7.  How to select a matrix decoder/processor.
 8.  Some surround titles - suggested demo titles.
 9.  References - tutorials and test reports.
Some History:

Surround sound is not new.  Disney's "Fantasia" (1940) had discrete
6-channel sound, as did the Cinerama series of movies in the 50s.  Surround
sound in the home is not new either.  15 years ago it was called
"quadraphonic".  Indeed, the "Dolby Surround" system of today is not much
different from the CBS "SQ" matrix system of a decade ago.

"Dolby Stereo" = "Dolby Surround" = "Dolby MP", for home purposes.

Many of the "stereo" soundtracks used for your laser video discs and VHS
tapes have been encoded for surround since the mid-70s.  This is because the
encoding for "Dolby Stereo" (Dolby Motion Picture matrix, or simply Dolby
MP) is the same as for home "Dolby Surround".  It is easier for video
producers to simply transfer the encoded signal from the theatrical audio
master (or release print) to the video master, than to decode to simple
stereo or go back to the pre-encoded audio elements and remix.

More recently, several discrete, or "5.1 channel" surround systems have
been fielded theatrically.  These use a compressed digital audio format,
either on the 35mm print or on separate CD-ROMs, to provide increased
channel separate.  One of these schemes (AC-3, aka Dolby Stereo Digital),
threatens to invade the home.

Unfortunately, most of the press coverage of surround has focused on
technical specifications, and none that I have seen attempts to give you any
idea of what surround actually *sounds* like.

2. The Experience of Surround ________________________________

  * It may be a very long time before we have convincing 3-D video, but
    effective 3-D audio is here today.  The audio portion of a surround
    presentation fills the entire listening space.

  * The difference between surround sound anround sound and conventional stereo can be as
    dramatic as the difference between stereo and mono.

  * To demonstrate this, during a sequence when the surround channel is
    active, try switching "effects OFF" on the decoder or processor.  The
    consequence is that the sound collapses to the front of the room.
    Switch it on, and sound floods the room.

  * Observation:  you can notice the *difference*, but on a tastefully
    mastered surround program, you often don't consciously notice the
    presence.  The program is simply more involving.  The psychological
    distance between you and the program is reduced.  The image may still be
    at arm's length, but the sound joins you in the room.

  * Of course, on a tastelessly mastered work, herds of objects noisily
    zooming out into the room can become an irritation, but films like that
    are apt to be infested with myriad artistic defects of other kinds.

The surround effect is almost subtle.  Unless you have been informed that
you are listening to a surround-encoded program on a surround system, you
are likely to simply have a more engrossing viewing experience, without
necessarily knowing why.  You quickly get accustomed to hearing "image
foreground" sounds from around you, after all, in real life, that's where
they come from.

However, the general public is not clamoring for surround, 70mm, wide aspect
ratios or 60 frames/sec. in the theatres, because they are not consciously
aware of the contributions made by these technologies.  None, including
surround, is quite as profound and easily identifiable as the difference
between, for example, silent-vs-sound or B&W-vs-color.

Surround may not be getting the public attention it deserves, but then, the
lack of technology awareness is desirable in art.  You don't want the medium
to overshadow the message.

3. Putting Surround in Perspective 

It is possible to create a home surround theatre whose sound exceeds that
of your local 35mm "Dolby Stereo" movie hall.  You will soo "Dolby Stereo" movie hall.  You will soon be able to
match the sound of any AC-3/DSD halls that might exist in your area.

However, surround sound is NOT the first step in a home theatre.  If you
are watching VHS tapes on a 13-inch department-store TV and listening to
the audio through the TV, or even the 5-inch speakers of a $100 discount
store "rack" system, then begin your upgrade elsewhere.  Don't get
surround until you have high quality in the following other areas...

 * Audio signal:  Tape: The absolute minimum for tape is VHS linear (analog)
                  stereo.  VHS linear mono is incapable of surround, and you
                  may not be satisfied with linear stereo.  VHS linear may
                  provide seriously inferior surround due to poor azimuth
                  alignment at the head, and lack of Dolby-B on some decks.
                  VHS HiFi is an improvement over VHS linear, but HiFi has
                  compression and 60Hz cyclic artifacts that may eventually
                  bug you.

                  Broadcast: MTS stereo can provide acceptable surround,
                  IF (big if), your TV has a "real" MTS decoder (with dBX)
                  and your broadcaster isn't trashing the signal.

                  Satellite stereo is generally of high quality.

                  LD: The finest video sound is from laserdisc.  Any LD
                  player will provide FM stereo, and virtually all post-1985
                  players also have digital stereo.

                  CD: A few titles have been release that are specifically
                  mixed for Dolby decoding.  Ambisonic CDs are not likely
                  to sound better in Dolby Surround than in plain stereo.

                  DVD: Digital Video Disc plans to use AC-3 5.1 discrete.
                  I have no information yet on whether or not DVD will
                  offer 2-channel re-encoded matrix output.

 * Video signal:  If you principally watch VHS tapes today, upgrading to LD,
                  satellite or at least S-VHS with HiFi is apt to provide a
                  higher return on investment than adding surround.

 * Monitor:       I suggest at least a 23-inch display (whether direct-view,
                  front- or rear-projected) with at least 350 analog lines
                  (or 640 discrete pixels) of real horizontal resolution,
                  composite video input, and capable of correct setup for
                  geometry, size (overscan), black level, white level and
                  chroma.  View at distances of between 4 and 8 screen

 * Audio chain:   The main (front) channels need to have speakers with
                  fairly flat on- and off-axis response, with no resonances,
                  no breakup or distortion at moderately loud listening
                  levels, and backed by an amplifier of adequate power that
                  adds no problems of its own.  The treble response needs to
                  be flat to 15KHz or more, and the bass response needs to
                  reach down to at least 35Hz - the lower the better, since
                  film/video programming has much more deep bass than music.
                  Unless you are using a subwoofer, the main speakers need
                  to be able to cleanly handle a lot of low bass.

                  If, for example, you can't tell the difference between CD
                  and pre-recorded audio cassettes on your system, you
                  probably need a complete audio upgrade.

Note on MTS:  VHF/UHF broadcasts encoded for NTSC-MTS stereo can carry
surround, but the stereo signal is often ruined by the local broadcaster or
cable operator, leaving you with a mono, [re]simulated stereo, out of phase
or highly distorted stereo signal.  Don't get surround just for MTS programs
unless you are certain that you have reliable access to solid stereo
programming, and you have decent MTS decoder in your TV (many so-called MTS
decoders lack the required dBX decoding, and are junk, even in some "hi end"

Note on Mono:  If you play a surround-encoded signal on a mono VCR or TV, or
through a mono audio system, mixing the left & right together, any sound
intended for the "surround" channel will be cancelled out altogether and
will be inaudible.  For this reason, some stereo surround material is
labelled "non-mono compatible".  In fact, the surround channel component of
*all* surround program material is non-mono-compatible.

4a. How does matrix surround work?

Matrix surround encode________________

Matrix surround encodes 4 channels (left, right, center,surround) into two
normal stereo tracks.  [Over]simply stated, sound intended for the
"surround" channel is recorded in the normal left & right stereo channels,
but out of phase with respect to each other.  Anti-phase is "rear"
(surround).  In-phase sound, balanced in both left and right is "center"
or "dialog".  Other sounds are left, right or some blend.

Some hall-dependent delay may be added (by the playback decoder or
processor) to the signal sent to the surround channel so that listeners far
back in the theatre won't hear the surround signal (esp.  simulated echo)
prior to the original front channel sound.  This also ensures that any
"front" sound that "leaks" into the surround channel is heard after the real
front sound, improving clarity (the "Hass" effect).  In the Dolby system,
the surround channel also employs mild (5 dB) Dolby B noise reduction.

Depending on the decoder, other signal processing and channel amplitude
manipulation (steering) may be applied to attempt to cancel various signals
out of channels where they aren't "supposed" to be.  Normally the effect is
subtle and effective.  To hear the steering in action (and struggling), play
a dual-audio program, such as a laserdisc with a monophonic soundtrack on
analog channel 1/L and a commentary on 2/R.  With Pro Logic, the sound will
jump all over the place.  (Of course, a really smart unit may just give up
in the face of such mistreatment, and shut down decoding.)

If you listen to an un-decoded surround program on an ordinary stereo setup,
you may detect the out-of-phaseness of surround signals (particularly on
headphones - see postscript).  The soundstage may appear to be wider than
your speakers, or you may actually have a psycho-acoustic experience of
sound from behind you (I noticed this on the "LadyHawke" LD, prior to having
a surround decoder.)

Technically stated, the "Dolby Stereo" MP Matrix ENCODING looks like:

             Source Sounds                                         As Encoded
  Left ----> + -------------------------------------------------+---> Left
 Source      ^                                                  ^     Total
             |                                         5 dB     |
             |                              100-7K     Dolby  +90 deg
  Ctr ---> -3 dB     Surround ---> -3dB ---> band ---> B NR --->|
 Source      |       Source                  pass      Encode -90 deg
             |                                                  |
             v                                                  v      Rt
  Right ---> + -------------------------------------------------+---> Right
 Source                                                               Total

Pre-recorded "surround" programs have Dolby Lt and Rt composite signals in
the left and right stereo channels.  To extract left/center/right/surround,
you need a DECODER.

Since several people have asked:  I do not know how the +90/-90 degree phase
shift is handled.  For sounds not correlated to L&R front, +0/-180 probably
works as well.  For correlated sounds, +0/-180 is probably safe for playback
systems that have rear delay.  Optimally, a frequency dependent shift is
probably used.  A pair of simple passive filter circuits might suffice.

4b. How does discrete (5.1) surround work?

Theatrically, there are three competing systems of discrete 5.1 surround:
 - AC-3, aka Dolby Stereo Digital (DSD), 16 bit, 384 kbps,  Digital (DSD), 16 bit, 384 kbps, 16:1 compression
 - DTS, Digital Theatre Sound, 20 bit, 1.2 Mbps, 3:1 compression
 - SDDS, Sony Dynamic Digital Sound (?)

All have the following general features in common:
 - The full channels are: left front, center, right front
                          left surround and right surround
 - The ".1" channel is:   subwoofer (limited bandwidth)
 - Sound is stored digitally
 - The digital data is compressed
The AC-3 and DTS systems also:
 - Discard some audio data to acheive a higher level of compression

Of the three systems, only AC-3 appears to be near delivery on home media
(LD and DVD).  LD media with AC-3 coding is already for sale. As of 2/95,
no players with AC-3 decoders or RF outputs had been released.

Full details of the AC-3 system haven't been revealed, but how it
generally works is as follows:  The 6 source channels are digitized, at 20
bits/sample, and apparently at 44.1 KHz.   The subwoofer channel is
low-pass filtered to eliminate sound above 300 Hz.   The raw combined
signal stream would be about 6 Mbps.  Data compression is applied to
eliminate redundancy, and psycho-acoustic masking rules are used to remove
sounds which theoretically can't be heard anyway.   The net result is a
data stream of 384 Kbps/sec (about 1/4 of the standard audio CD rate).

This data stream is easily stored in various nooks and crannies of
different media.  On 35mm film it occupies the space between the sprocket
holes (at 320 kbps).  On laserdisc, it replaces analog channel 2/R.

Most pre-AC3 players will ignore the AC-3 signal (and finding no analog
audio on 2/R, will feed the 1/L audio to both stereo outputs when analog
audio is selected).

1995 and later AC3-compatible players will either:
 - decode the AC-3 signal internally, feeding six (6) audio ports,
   probably via a ganged connector.
 - provide an RF output for an external decoder, much like the "AFM"
   output available today on some industrial LD players.
In either case, once decoded, your audio system will need to be able
to accept up to 6 separate audio inputs.  Many low-end surround systems
expect and support only simple two-channel (matrix surround) inputs.

Another unresolved question about AC-3 (and any other future home discrete
surround) is, if you have less than 6 channels in your audio system, can
the decoder provide:
 - subwoofer blended into L+R fronts?
 - split surrounds mixed for a mono surround setup?
 - center mixed into L+R for "phantom center"
 - mix-down to simple stereo, or even mono, where required?

AC-3 decoders for LD, at least, are required to also provide Dolby
Pro-Logic for decoding the normal digital and analog stereo tracks.

DTS and SDDS have yet to provide credible evidence of their intentions in
the home theatre market.  DTS could theoretically replace the normal
"RedBook" digital stereo baseband AM signal on LD, but would require
double-stocking of LDs, and an external decoder would be required for
existing player (assuming the had a raw digital output, as most
digital-sound models do).  SDDS relies on 2 or more external CD-ROMs in
the theatre, and presumably could do so in the home, but this seems highly

Indeed, the successful acceptance of any discrete implementation on LD
remains to be seen.  I personally have no plans to replace my current LD
player simply to obtain AC-3 compatibility.  Traffic in the Netnews group suggests that long-term LD enthusiasts
are confused about discrete surround and AC-3 in particular, and many seem
to share my opinion about the value of an "upgrade".   The average LD
owner, and potential LD owners, are probably hopelessly lost, and will not
be drawn to invest in it.

5. The Hierarchy of Home Surround 

There are several implementation levels (and a large number of buzzwords andevels (and a large number of buzzwords and
trademarked terms) associated with home surround.  Here is a condensed
taxonomy of home surround:

 Stage 0: Two-Channel Surround Emulation.  At this level there are only two
          front speakers and two channels of amplification.  A pre- or post-
          processor is used to tinker with the sound to produce ambiance or
          "surround like" effect at certain listening positions.  Not all of
          these techniques are marketed as alternative "surround" methods,
          but some are, and you need to beware of them if what you want is
          movie surround.

          Examples include:  Carver Sonic Holography, Hughes SRS, Q-sound
          and the "surround" mode of some low-end stereo(only) receivers.
          In my opinion, none of these are satisfactory for presentation of
          Dolby surround-encoded material.

 Stage 1: Two-Channel Speaker Bridging, also known as the "Hafler
          connection".  Because the surround signal is largely L-R, you can
          simply wire one or more surround speakers and a potentiometer
          across the (+) terminals of the left and right front speakers.
          Don't attempt this without first checking that your amp allows
          bridged connections (some will self-destruct), and don't expect
          wonderful results, particularly since left and right signal can
          bleed into the opposing speaker. If you want to try speaker
          bridging, see the June 1991 issue of Audio magazine for tips.

 Stage 2: Passive Matrix.  A simple decoder passes Lt to left, Rt to right,
          then isolates and subtracts the Lt and Rt, sending the Lt-Rt
          result to the surround speakers.  This at least eliminates the L/R

 Stage 3: "Dolby Surround".  A branded "Dolby Surround" decoder:
          * sends Lt-Rt (surround) thru a delay line (typically 20 mSec),
          * then thru a 7 KHz low-pass filter (to keep natural and azimuth
            error caused left/right source phase noise from being heard as
            surround) and
          * thru a 5 db (vs 10 on audio cassette) Dolby B-type noise reduction
          * A master volume control and input balance controls are also
          * Lt and Rt are isolated, and may also be summed (Lt+Rt) and sent
            to the Center output, but this is not required for Dolby cert.
          In any case, a maximum of 3 dB of separation is achieved between
          each adjacent pair of:  left-center-right-surround-left.

 Stage 4a. "Dolby Pro-Logic".  A Pro Logic" decoder starts with Stage 3, and:
          * replaces the simple Lt-Rt (surround) and Lt+Rt (center)
            extractions with an active adaptive matrix decode step.  For
            signals intended for one output, this circuit attempts to cancel
            them in the others.
          * It also analyses the soundfield for signal dominance, and
            focuses the sound toward those outputs.  The net result is that
            30 dB of separation is possible between any two channels.  Dolby
            Labs publishes a "Principles of Operation" pamphlet that goes
            into more detail.
          * Input balance is required.
          * Lexicon Pro-Logic decoders also include an "auto-azimuth"
            feature to correct for group delay errors (time shifts) between
            the two source channels.  Shifts as high as 50 microseconds are
            reportedly common in film-sourced sound (45 degree phase shift
            at 5 KHz).

          Incidentally, don't bother looking for any "Pro Logic" recordings.
          Pro Logic is used only in the playback processing.  The encoding
          (recording) of Dolby Surround always uses the "Dolby Stereo" (aka
          "Dolby MP") matrix described above.

          Pro-Logic ICs are not generally available to the public, although
          at least one "kit" based on the Mitsubishi M69032P has been
          reported in Australia.

 Stage 4b. When AC-3 matures, it will probably go here.

 Stage 5. "LucasFilm THX".  THX affects the presentation of movies at
          several points.  For the home, a THX-certified processor starts
          with Dolby Pro-Logic and adds:
          * Surround channel decorrelation - a digital pitch shift is used
            to make the (mono) surround signal "different" in the left and
            right surround channels.
          * Re-equalization of the front channels, to make the movie mix
            seem less "bright" in the home.
          * "Timbre Matching" - an equalization applied to the surround
            channel to make effects sound consistent when panned between
            front and surround speakers.
          * THX is also becoming a LaserDisc certification program.  The
            first title with "THX quality" presentation parameters will
            be the director's cut of "The Abyss", to be released in 1993.

          THX specifies front speakers with a reduced vertical dispersion
          (to minimize ceiling reflections) and two side-mounted surround
          speakers configured for dipole radiation.

          THX also recommends equalization for the L-C-R channels.  A THX
          equalizer will have 1/3-octave bands from 80 to 800 Hz,
          implemented as "interpolating constant-Q" circuits, and parametric
          equalization above 1000Hz and for the subwoofer channel.
          Reportedly, a THX-certified equalizer will include in-home

          THX is also a certification process for video programming,
          beginning with LaserDiscs in early 1993.  As with Pro Logic,
          THX-branded audio material is not "THX encoded".  THX mixes are
          merely a specific way of performing Dolby-MP sound mastering.  A
          THX-certified LD presumably will have purist aspect ratio, source
          element/transfer quality, colorimetry/timing, etc.  The telecine
          monitor is calibrated by LucasFilm and a "THX Vertical Interval
          Test Signal(TM)" is inserted during NTSC encoding of the signal.
          Various full-field test patterns are included after the end of
          the program on LD.

Other moviesound and home surround terms:

"Ultra Stereo", "Chace Surround" and "matrix surround" are DolbyMP/Surround-
compatible anti-phase encoding schemes that do not bear the Dolby logo.  You
may encounter these terms on program material.  They will work on your
passive, Dolby, Pro Logic or THX decoder/processor.

Hughes "SRS", Carver "Sonic Holography" and "Q-sound" are not, as far as I
know, Dolby-compatible.  They are 2-channel schemes that process the signal
on playback (SRS, Carver) or prior to recording (Q) and attempt to simulate
wider or 3-D sound placement with only the normal two front stereo speakers.
The effect may be limited to a small "sweet spot", and I don't recommend
additional Dolby Surround processing.  I have SRS on my Sony XBR TV, and
with or without my external Pro Logic decoder switched in, SRS-on is
principally a "hum enhancer" and "listener phase torture device".  SRS is no
substitute for Dolby, as far as I'm concerned.  I have heard Carver several
times, and am left uninspired.  Q-sound I have not heard extensively.

Ambisonics is a matrix encode/decode algorithm apparently intended mainly
for music reproduction.  I have no technical details.  Reportedly some
surround receivers introduced in 1992 will have Ambisonics in addition to

Dolby A, B, C, S and SR are noise reduction processes that have nothing to
do with surround except that Dolby MP uses a modified Dolby-B on the
surround signal, and VHS linear stereo uses (additional) normal Dolby-B on
both channels.  If your deck lacks Dolby B on the linear stereo, you can
expect mistracking of the modified (additional) Dolby B used on the surround
signal.  Dolby HX-Pro is a variable-bias technique for analog tape recording
and has nothing to do with tape playback, much less optical media or
surround sound.

Dolby SR-D is an early name for Dolby Stereo Digital (DSD, aka AC-3).

Cinema Digital Sound (CDS) is another reduced bit rate (lossy compressed)
digital sound encoding format for 35 and 70mm filmstock.   CDS replaced
the existing optical tracks.  CDS failed in the market, and is probably
gone for good, although it appears in some on-screen credit rolls.  "AC-2"
is Dolby's term for their family of 192 Kbps reduced-bit-rate data stereo
coders.  "AC-3" is is their "5.1 channel" surround version (320 Kbps).
Dolby has been requesting that developers of next-generation media leave
at least 320 Kbps of bandwidth available for AC-3.

"THX" is a LucasFilm trademark for several things, three of which are directly
related to home surround:

 1. "THX Theatre" - THX is a certification process.  Theatres bearing the
    logo are periodically tested to ensure that they meet LucasFilm
    standards for audio equipment, environment and playback of surround-
    encoded film.

 2. "Re-recorded in a THX theatre" - THX logos on films and recordings
    indicate that the final DolbyMP-compatible mixdown was done with the
    recording console and engineer located in an actual THX-certified
    theatre.  This is intended to ensure that the film audio will playback
    in a consistent and predictable manner in all THX theatres (and in homes
    equipped with THX certified components).

 3. THX crossover - LucasFilm lists recommended audio components for THX
    theatres.  They also make a crossover, bearing the THX brand, which is
    only used in actual motion picture theatres.

 4. Home THX - LucasFilm has a testing and certification process for home
    audio equipment.  Those models which are submitted by the maker, and
    pass the tests, may exhibit the branding.  THX branded equipment
    provides the promise of effective home theatre, but can still sound
    hideous if improperly set up and calibrated.  Some THX-branded equipment
    includes dealer installation and adjustment.  For amplifiers, THX
    merely provides an assurance of high power and quality.  The first
    THX-cert LD player (from Runco) was released in late 1994.

 5. THX certified surround decoders, equalizers, main speakers and surround
    speakers, on the other hand, must provide specific THX required
    functions (listed in Stage 5 above), as well as high general quality.

 6. THX certified LaserDisc media (described earlier).

The future.  Your humble prognosticator predicts:  AC-3 on LD will not be
a major factor in the market.  What may be a major factor is DVD (which
uses AC-3). DVD threatens to make major inroads into the VHS rental and
sell-thru markets, as well as capture some share of the LD market.   If
this happens, the makers of home theatre audio hardware will shift rapidly
to providing equipment with discrete inputs for 5.1 channels.

AC-3 has not been extensively A-B (much less A-B-X) compared to
high-quality matrix or less compressed discrete systems like DTS and
SDDS.  Until critical independent observers test and report, it is
difficult to predict the success of AC-3.  My personal opinion is that the
bit rate is too low for acceptable fidelity (or put another way, the 16:1
compression ratio is too high).  AC-3 may flop.

Regardless,  a Dolby Pro-Logic or THX matrix decoder will remain a viable
investment for the rest of this century.

6. Matrix Surround System Configuration

Here i_____________________________

Here is what a largish Dolby surround setup looks like in the home.  You can
easily get by with only four speakers/channels (I do).

                        ============== Screen
            _             ....._.....              _
           | |            :   | |   :             | |
          /___\           :  /___\  :            /___\
        Left Front        :  Center :          Right Front
    _____                      |                       _____
    \   /                      :                       \   /
     |_|                       |                        |_|
     | |  Surround             :              Surround  | |
    /___\                      |                       /___\
                  _____        |         _____
                  \   /        :         \   /
                   |_|         |          |_|
                 Surround      :       Surround

Room:  Any size room is acceptable if the decoder has adjustable rear delay.
    With fixed delay, you must get the surround speakers properly located
    with respect to the front speakers and the delay value.  See "surround"
    speakers below.

Main (Left&Right):  Uniform response from 35Hz (or lower) to 15Kz (or
    higher) is essential.  Reduced vertical dispersion is desired to
    minimize ceiling reflections.  This probably means that my Vanderstein
    2Ci's, which have the midrange radiator tilted up, are not ideal for

    Solid bass tolerance (if not response) is necessary in the mains.  Movie
    sound has a great deal of low bass.  Unless you are using a subwoofer,
    make sure that your intended speakers won't break up or be destroyed by
    challenging program material.  The LaserDisc of "The Dream is Alive"
    (Lumivision LVD9019) makes an excellent test.  Caution:  the shuttle
    launch sequences in this program have destroyed speakers.

Center:  In both the theatre and the home, only a small portion of the
    audience is sitting near the centerline.  Those near the sides might
    hear center channel sound (sounds equal in both left & right speakers)
    as coming from the front speaker nearest their side.  If the decoder has
    a "center" output, it attempts to isolate in-phase, balanced sound,
    usually dialog, which it thinks should be "front center".  It sends it
    to the center output and attempts to cancel it from the other outputs.
    That way, everyone hears "dialog" from the screen center speaker.

    Further, if you have no center speaker, and rely on the "center" sound
    to consist of an equal-and- in-phase signal emitted from both left/right
    front speakers, the sound waves will not arrive in-phase at all
    listening positions.  This is the so-called "comb filter effect".  For
    example, a 6-inch difference in distances to L/R speakers results in a
    180-degree group delay (and total cancellation of the direct wavefront)
    at 1000Hz (and many other frequencies).  LucasFilm reports that center-
    channel dialog is easier to understand if it comes from a single speaker.

    A THX-certified decoder, and many (but not all) Pro-Logic decoders, will
    roll off the center bass below 100Hz (-3dB @ 80Hz, -10dB @ 50Hz).  If
    the bass is rolled off, it will be delivered to the subwoofer channel,
    or merely left in the L and R main channels.  Although matching the
    L-C-R speakers is ideal, if you are certain that your decoder rolls off,
    you can use a smaller and less powerful speaker for "center".  I am not
    yet using a center speaker, and can't contribute further comment.

    Another case for a center speaker is horizontal sound pans.  If the
    speakers aren't identical, the timbre, and at some frequencies, even the
    phase of the sound will vary, providing a very distracting sonic image.

    If you don't plan a center speaker, the recommended positioning of the
    L/R mains is "close" to the screen.  If you must place the mains far
    from the screen, consider using the TV's own speaker(s) as the "center".
    Unfortunately, most TV speakers are junk, so you'll have to judge the
    efficacy of this by trial.  Be careful with "close" - the magnets in
    many speakers are powerful enough to skew the geometry and color of
    inadequately shielded TVs.

Subwoofer:  Although film sound has much more bass energy than music, due to
    sound effects, the case for a subwoofer in a surround setup little
    different than for a normal stereo setup.  If your front speakers have
    wimpy bass, and you don't want to upgrade them, and you have lots of
    money, get a subwoofer (and maybe an amp to drive it).  If your system
    can reproduce cleanly down to 40Hz, you are probably OK as is.  The
    subwoofer may generally be located anywhere in the room.

    Pay attention to where the low-pass crossover is.  Having a filtered
    subwoofer output in the surround process *plus* a filter or crossover in
    the subwoofer itself is not a "sound" idea.
Surround:  Notice that the side and rear speakers are *all* labelled
    "surround".  In a textbook Dolby theatre setup, there are a number of
    them (typically 8) spaced around the hall, they all emit the same signal
    and thhey all point toward the audience.  The point of having multiples
    is that each thus runs at a lower volume, the surround field is more
    uniform, and listeners near an individual surround speaker won't have
    their attention drawn to it.  Theatres also use multiple surround
    speakers to achieve coverage.

    The LucasFilm-recommended number of home surround speakers is TWO,
    located to the SIDE, and not behind the audience.  The rears are
    optional, but if present there must also be side speakers.

    Dolby recommends that the surround speakers be located 5 feet closer to
    the average listener than the front speakers, and that the "surround"
    signal be electronically delayed by 20 milliseconds (for a net arrival
    delay of 15 mSec compared to "front" sound).  The Dolby publications "a
    listener's guide" and "Pro Logic Principles of Operation" both include
    distance-time nomographs.

    Generally, T = Nd + Df - Ds, where
      T  is the delay setting,
      Nd is the net delay time in milliseconds (15),
      Df is the distance in feet from the listener
         to the nearest front speaker, and
      Ds is the distance in feet from the listener to
         the nearest surround speaker.
    The Shure recommendation for Nd is 20.  I suggest using the Dolby value
    (15), and if T works out to be a value that you can't set precisely
    because of fixed delay steps, chose the next higher delay setting.

    Unless you are using a THX processor, or a decoder that otherwise
    differentiates left and right "surround" signals (in movie or ambiance
    modes), a single channel of surround amplification will suffice.  One or
    multiple speakers may be used.  Whether to wire in series or parallel
    depends on the impedance of the speakers and amp.  If the two surrounds
    are at significantly different distances from the audience, separate
    amplification channels are needed to match levels.

Surround speakers:  You may be able to get by with modest surround speakers.
   In the Dolby mode of your decoder, the sound sent to these speakers is
   rolled off above 7 KHz, and although rolled off below 100 Hz during
   ENcode, it is NOT rolled off below 100 Hz during DEcode.  Any deep bass
   naturally out of phase in the original left and right sources will appear
   in in the surround channel (particularly if the decoder/processor has a
   subwoofer output).

   A case can be made for matching the speakers all around.  Several people
   have reported significant bass energy from their surround speakers, and
   some decoder/processors send full-range material to the surround speakers
   when in proprietary (non-Dolby) music surround, ambience or venue
   simulation modes.  There have also been press reports that some film
   producers are pre-emphasizing the bass in the surround mix, so that it
   will still be present after the encoding roll-off.

  The LucasFilm/THX recommendation is that the high frequency radiators of
   hoooorecommendation is that the high frequency radiators of
   home surround speakers NOT be pointed at the audience.  The original
   formerly THX-certified speaker from Cambridge SoundWorks ("The
   Surround", $400/pair), for example, has its bass cone pointed at the
   audience, but has two mid/high cones per unit, wired out of phase
   (dipole) and pointing sideways. The newer, less powerful "Surround II"
   ($250/pair, not THX) has dual sets of elements, all in dipole, and
   nothing pointing "forward".

   The Alantic Technology Surround speakers use a slightly diifferent
   scheme, wherein the drivers face each other at a 90 degree angle
   (instead of away at 180 degrees).  They are still wired anti-phase.
   and people who own them report being pleased with the performance.

   If you end up using ordinary forward-firing speakers, and find that you
   are too aware of them, try facing them straight up (and re-adjust channel
   level matching after rotating them).

   You typically don't need a 14-inch woofer or thermonuclear tweeter for
   the surround speakers, or much amp power for that matter.  Any decent
   bookshelf speakers and well-mated amp will do.  I'm using a retired 55
   W/ch receiver, driving a pair of CSW Surrounds.  The dipole configuration
   does diffuse the surround more than the traditional speakers (CSW
   Ensemble I's) I used to use.

   Further note:  If the speakers and amps are not all identical, it will
   not be trivial to ensure that they are all in phase and balanced.  For
   phasing, I suggest testing one pair (each non-identical) in a simple
   stereo setup (with a mono signal), and correlating the polarity markings
   on the binding posts.  Be sure to use the eventual amp channels for this,
   as some amps invert the signal.

7. How to select a matrix decoder or processor _____________

Step 7.1: Is your system ready for a decoder?

You may need (or want) a new main receiver or amplifier.  The surround
process requires exporting the raw stereo-matrix signal at the pre-amp
(line) level in the receiver/amp, then feeding th then feeding the decoded front signals
back in at that point.  The input stages of the receiver/amp handle the
matrix signal; the main output stages handle only the decoded "front"
                      Receiver or Amp
 __________          _________________
| Surround |---Lt---|  Pre  :: Power  |---L---|Spkr<
| Source   |        |       ::        |
| e.g. LD  |---Rt---|       ::        |---R---|Spkr<
| VCR,CD   |        |  Ext Proc Loop  |
`----------'        `--Out------In----'
                       |  |    ^  ^
                       v  v    |  |
                    .------------------.   .------.
                    |   In   Front-Out |   |      |
                    |                  |   |      |
                    |         Sur Left |-->| Rear |---|Spkr<
                    |                  |   |      |
                    | Surround Decoder |   |      |
                    |                  |   |      |
                    |         Sur Right|-->| Amp  |---|Spkr<
                    |                  |   |      |
                    | Center   SubWoof |   |      |
                    |  Out       Out   |   `------'
                        |         |
                        v         v
                     .-----.   .-----.
            >Spkr|---| Amp |   | Amp |---|Spkr<
                     `-----'   `-----'
                     Dialog    SubWoof
If you do not have "external processor" capability, but have a separate
"record in" selector switch, you'll need to:
  - route the Surround Source into the "LD" or "VCR" input as usual,
 - set "record source" to select that input,
 - route the record-out jacks to the decoder,
 - route decoder front-out back into "AUX" or "TAPE2" or a similar unused
   line input, and
 - select "AUX" or "TAPE2" on the receiver/amp main selector switches.

Another work-around is to:
 - feed the surround source (if you only have one) directly into the decoder
   Lt and Rt inputs,
 - feed the decoder "front" line outputs into the main (stereo) receiver or
   amp (AUX or other line-level in), and
 - feed the "center" "surround", "subwoofer" outputs directly to the
   secondary amps line-level inputs.

Step 7.2: Pick a Decoder or Processor (or Receiver with integrated decoder).

As far as brands and models, I cannot help you very much.  My only exposure
to surround has been via the Lexicon CP-1, which has both certified Dolby
Pro Logic and a variety of other modes (although not THX - for that you
need the CP-3).  I can't really say whether or not the lack of
auto-azimuth, use of ordinary Dolby or simple matrix decoding would be
disappointing by comparison to Lexicon's all-digital Pro Logic.

I would look for the following features (prioritized):
 * Pro Logic (adds less than $100 to new receivers nowadays).
 * Master volume control.
 * Auto-balancing on input (for programs recorded out of balance).
 * Adjustable rear-channel delay.
 * All calibrations from front panel and/or remote control.
 * Auto-test-tone program for calibration (more below).
 * Non-volatile storage of adjustable parameters (below).
 * Effects defeat (below).
 * Auto-azimuth to remove group delay of source program channels out of
   phase or independently time-delayed (as from SVA optical film tracks or
   when sharing a single DAC on LD).
 * THX certification.

When you demo, I suggest starting at the top so that you have a standard to
shoot for in a lower-priced decoder.  Have the salesperson run through the
setup procedure, and listen to a surround test disc (like Reference
Recording's LD-101, "A Video Standard").  This will show you how much
trouble the process is (or isn't) and more importantly, will ensure correct
store setup.  Far too often, I have heard simple stereo setups in stores
that are out of phase.  I estimate the chances of a correct surround demo at
about 5%, rising perhaps to 50/50 at a "high end" store or "video salon".

If you are considering getting an integrated receiver/decoder, I suggest
getting ONLY a receiver with the Pro Logic or THX brand, as it may otherwise
be difficult or uneconomical to upgrade a simple Dolby model later.

There are some things to watch for, lest you end up with missing or
duplicate components in the system and/or high "hassle coefficient":

  * Tuning decoder/processor input gain, balancing the outputs, setting
    surround delay, etc., are critical for acceptable performance.  Does the
    decoder/processor have easy step-by-step instructions?  Does it include
    a built-in pink noise generator for matching levels (or a separate
    tape/LP/CD with such a signal?); if not, consider getting a copy of the
    Reference Recordings LD-101 "A Video Standard" LD
  * Does the decoder/processor or receiver supply its own surround channel
    amp(s)?  If so, how many, and with what power?  Is   the power adequate
    for the speakers selected?  Is the impedance matched to the number of

  * If a subwoofer output is provided, is it producing flat response, or
    does it incorporate a low-pass filter?  What does the subwoofer itself
    require?  Does the subwoofer include its own amp?  How seamless is the
    response overlap between the subwoofer(s) and the bass response of the
    front and dialog (if any) speakers?

  * If your main speakers are driven by an integrated amp or receiver, does
    it have an "external processor loop" that allows separation of the
    pre-amp and power amp?  If not, you may encounter complications in
    signal source selection and front/rear volume balancing.

  * Is there a single master volume control for all channels, controlled by
    the remote?  Having that control duplicated on the front panel of the
    decoder/processor is a plus; a servo-driven ganged potentiometer is
    ideal.  (The Lexicon's have only volume buttons , no knobs.  The CP-1
    has only remote volume.  All have "mute".)

  * Are "effects" defeatable, allowing simple front-only stereo/mono?  Can
    you bypass the decoder/processor altogether for critical ordinary stereo

  * Does the decoder/processor have user-alterable presets for Dolby
    decoding, vendor-unique decoding, stereo ambiance enhancement and any
    other modes you will frequently use?  Are the settings non-volatile
    (preserved thru power-off)?

Finally, a feature to watch out for.  If the decoder does not bear the
double-D [)(] Dolby logo, find out why.  The missing logo indicates that the
vendor is unwilling to submit their design to Dolby for inspection, change
requests, re-submission, etc. leading to an official approval.  There are
reasons why this might be (which also apply to THX):

 A. They focused their design on low-cost and/or time-to-market, and were
    unwilling to pay the royalty for using the Dolby logo, and/or put up
    with the certification delay.

 B. They don't like the Dolby spec, and think they have a superior decoding
    scheme.  Of course, they could have done both their own and Dolby's.  If
    the decoder (alone) sells for more than about $500, this is probably the

 C. Their decoder is too primitive and/or low in quality to pass Dolby
    qualification.  It may also lack even simple processing, like surround
    channel delay.  If the decoder is built-in to a receiver, monitor or
    other component, listen carefully.  If possible, A/B-it against a
    quality stand-alone decoder.

Step 7.3: Install, calibrate and enjoy your surround system.

Unfortunately, many calibration programs jump from channel to channel, never
turning on pairs simultaneously.  It is very difficult to accurately set
levels this way, particularly if you must leave the central seating position
to make the adjustments.  I use a sound pressure level meter (Radio Shack
33-2050, about $40), parked head-height at the listening center.  Since only
comparative levels matter, you can also use a microphone feeding any metered
recorder.  Adjust the record level (in PAUSE) to about 0dB, and set levels
for all channels.  Incidentally, if using "phantom center channel mode" (no
center/dialog amp/speaker), it may not be possible to get the "center"
channel's level to exactly match "left" and "right".

And since someone asked...
The setup for "master volume" on an outboard processor (like a Lexicon) is:
 - set input gain on processor so that 0dB test tone from input media
   on main non-variable source (e.g. LD player) hits max on processor
 - set processor master volume to max
 - adjust main amp gain for "loudest level you'll ever listen at".  "The
   Abyss", "The Dream is Alive" and "T2" are good setup sources, as all
   have below normal digital track amplitudes (so that peaks don't clip).
 - reduce master vol to a nominal level and balance surround amp gain
 - stick "No Touch" labels on amp gains
 - use only processor master volume for system volume

Step 7.4: Use of a surround system for music.

If after reading all of the above, you suspect that there is an awful lot of
processing being done on the original stereo signal, you are correct.  Do
you want to have all that switched on when playing ordinary stereo music on
the same system?  I suggest "no", unless the music was specifically recorded
for surround (as a few CDs have been recently).

When playing music on my system, I switch from Pro Logic mode to "small hall
ambiance".  If your decoder doesn't have any alternate modes, it is doubly
important that it have an "effects off" mode.  You may not like what passive
matrix, much less Pro Logic or THX, do to non-surround stereo music,
particularly if you are critical audiophile.

8. Some matrix surround titles ________________________________________

If you get an opportunity to demo surround, make sure you are using source
material that is worthy of the system.  Be advised that:

  a. Many video sources with surround sound aren't so identified on the
     media jacket.  Sometimes your ears or t or the "Dolby Stereo in Selected
     Theatres" that appears in the trailing credits are your only clues.

  b. Conversely, the appearance of "Dolby" in the trailing credits is no
     guarantee that a Dolby-ized stereo master was used for the video
     release (although it is rare that this is not the case).

  c. Even if "Dolby" or "surround" appears on the jacket, the effect may
     be less than dramatic, and may be largely ambience and echo.

The following laser video disc (LD), from the IMAX movie, is what I use for
surround demos:

The Dream is Alive {CAV}              (Ferguson, 1985) Lumivision  LVD9019

The following two laser disc titles also have very effective surround
programs.  I cannot vouch for the non-Criterion pressings of
"Ghostbusters". I cannot tell you anything about tape editions.  "The
Abyss" is also the first THX-certified LD.  The earlier 1561-85 theatrical
edition also has an excellent soundtrack, but is not THX certified.

Empire of the Sun {CAV}              (Spielberg, 1988) WB         11844
Empire of the Sun {CLV}              (Spielberg, 1988) WB         W11573
  (P-51 attack scene)
Ghostbusters {CAV}                     (Reitman, 1984) Criterion  CC1181L
Ghostbusters {CLV}                     (Reitman, 1984) Criterion  CC1182L
  ("Slimer" scene in hotel)
The Abyss: Special Edition (CL/AV)  (Cameron, 1989/93) Fox        1988-85

There are several test discs available for calibrating systems (and
verifying that your dealer's demo system is correctly set up).  The most
easily available is:

A Video Standard                          (Kane, 1989) RR         LD-101

The following LDs also have surround programs.  Although they are somewhat
less dramatic than those above, they are more pronounced than several other
"surround" discs I have examined.

Ben-Hur {the current letterbox edition}  (Wyler, 1959) MGM        ML101525
Die Hard                             (McTiernan, 1988) CBS/Fox    1666-80
Dragonslayer {letterbox}               (Robbins, 1981) Bandai     LA098L14046
Dragonslayer {cropped}                 (Robbins, 1981) Paramount  LV1367
LadyHawke {current letterbox}           (Donner, 1985) WB         12370
LadyHawke {cropped}                     (Donner, 1985) WB         11464LV
Ruthless People                (Abrahams,Zucker, 1986) Touchstone 485AS
Star Trek III - The Search for Spock     (Nimoy, 1984) Paramount  LV1621
Star Trek III - The Search for Spock[WS] (Nimoy, 1984) Paramount  LV12954
Star Wars    {letterbox}                 (Lucas, 1977) CBS/Fox    1130-85*
The Witches of Eastwick                 (Miller, 1987) WB         11741A/B

One title to avoid:
Dolby Technologies: How They Work                      Pioneer    05458
Although a useful tutorial, it contains NO demo material.

Many of the articles referenced below also list demo titles.

* This is the new letterbox CLV edition with digital sound.  1130-80 (CLV)
  and 1130-84 (CAV) are cropped, although probably still surround. A THX
  certified edition is expected in 1993.

9. References ___________________________________

Available free from:  Dolby Laboratories
                      100 Potrero Avenue
                      San Francisco
                      CA   94103-4813
Write for: "Dolby Pro Logic Surround Decoder - Principles of Operation" - Principles of Operation"
           "Dolby Surround - a listener's guide"
           "Heard Any Good Movies Lately?" (a list of Dolby Stereo films)
           "Question about Dolby Surround"
           "What is Dolby Surround"