SABINE THE DIGITAL DELAY ADVANTAGE User manual

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THE DIGITAL DELAY ADVANTAGE
A guide to using Digital Delays
Synchronize loudspeakers
Eliminatecombfilterdistortion
Alignacousticimage
Contents
THE DIGITAL DELAY ADVANTAGE .....................................................1
- Why Digital Delays? ........................................................................................................................ 2
LoudspeakerSynchronization .....................................................................................2
- How to Synchronize Your Signals .................................................................................................... 2
- Processing (or Group) Delays .......................................................................................................... 2
- Center Cluster Speakers.................................................................................................................. 2
Comb Filter Distortion .................................................................................................3
- Calculating Comb Filter Frequencies................................................................................................ 4
- Comb Filter Amplitude ..................................................................................................................... 4
- Correcting Comb Filters ................................................................................................................... 4
The PrecedenceEffect: Aligning theAcousticImage ..................................................4
THREE APPLICATIONS........................................................................5
- Application I: Under-The-Balcony Speakers .................................................................................... 5
- Application II: Center Cluster with Front Fills................................................................................... 6
-Application III: Synchronizingthesignals ofa far-throwand short-throw loudspeaker....................... 7
CALCULATING DELAY TIME USING DISTANCE................................7
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THE DIGITAL DELAY ADVANTAGE
Why Digital Delays?
The most intelligible sound occurs when two people speak face to face. The
sound is loud and dry and the direction of the sound aligns with the speaker. It
standstoreasonthatthemostintelligiblesoundsystemsaretheonesthatcome
closest to emulating face to face communication. If this is your goal, a digital delay is essential to your
sound system.
Untilrecently,adigitaldelay’scostwasprohibitivefortheaverageuser. Onlyhigh-endapplicationscould
justify the cost. But recent drops in component prices now put the benefits of digital delays within
affordable reach of every user.
There are three distinct applications for digital delays. The first and most important is synchronization
of the loudspeakers to control excess reverberation and echo. Secondly, digital delays help control
comb filter distortion, and finally, digital delays are useful for aligning the acoustic image so the
direction of the sound seems to come from the performer rather than the loudspeaker.
This guide goes beyond the typical operating manual that explains only the front and back panel
adjustments. Instead,wediscussthebasicacousticalconceptsneededtogetthemostoutofyourdigital
delay and present examples of several practical applications.
Loudspeaker Synchronization
Sound travels at about 1,130 feet per second in air, or about 1 millisecond per foot. On the other hand,
electronic signals travel almost one million times faster through your sound system to the loudspeakers
— effectively instantaneous. The main task of digital delays is to synchronize multiple loudspeakers so
the sound traveling different distances through air arrives at the listener’s ears at about the same time.
Synchronizing the loudspeakers reduces reverberation and echoes for improved intelligibility.
How to Synchronize Your Signals
There are several powerful tools available for precisely measuring the time a loudspeaker signal takes
toarriveatacertain point inthe audience. Mostof these toolsare verysophisticated and tendto be quite
expensive. Fortunately, simpler tools are sufficient for most applications.
In the 1930s, engineers synchronized the low and high frequency speakers in movie theaters by feeding
asharpclickthroughthesystem. Theymovedthespeakersuntiltheycouldonlyhearasinglesharpclick
coming from both speakers. You can use this same method with a common child’s toy called a clicker.
Pressing the thin metal strip makes a loud sharp click. A clicker is especially useful when synchronizing
the direct sound from the performer with the sound from the loudspeakers.
Alternatively, you can use a phase checker especially for synchronizing the signals of two loudspeakers
(either LF and HF or two full range systems), since most of the phase checkers include a click generator
and receiver. Phase checkers are quite affordable and have other uses besides synchronization.
Processing (or Group) Delays
Converting signals back and forth from the analog to digital domain will slightly delay the signal. These
conversion delays are often called processing (or group) delays, and usually range between 0.9 to 5
milliseconds. You will notice that Sabine delays display the processing delay as the smallest possible
delay value. You can simply bypass the unit for 0 seconds delay.
Notallmanufacturersacknowledgeprocessingdelaysintheirspecifications,butyoumusttaketheminto
accountwhen synchronizingyour system. Makesure alldigital equipmentis on andnot bypassedwhen
synchronizing. Also, be careful to make an appropriate adjustment in your delay lines if you later add
any type of digital equipment to the system.
Center Cluster Speakers
Centerclusterspeakersofferseveraladvantagesoversystemsthathavespeakersmountedonthesides.
Themostobviousadvantageisthatthedistancetotheclosestandmostdistantlocationsintheaudience
is almost equal, so most listeners hear similar levels of amplified sound. Center clusters also offer two
other advantages regarding the visual imaging.
Studies have shown that people can detect even small horizontal changes in the direction of a sound
source, but vertical shifts are much less noticeable. This suggests that the sound from center-cluster
speakers is more likely to be visually aligned with the performer than loudspeakers placed on each side
of the stage.
Special thanks to Hans
Drobilitsch of Hans
Drobilitsch Audio
GmbH. (Wollersdorf,
Austria) for his invalu-
able technical advice.

!
Allthoseintheaudiencewhoare closer to theperformerthan the center clusterwill hear the directsound
from the performer before they hear the sound from the loudspeakers. This makes the sound seem to
come from the performer, not the loudspeakers. (See the Precedence Effect below.)
Comb Filter Distortion
Manywhotookhighschoolsciencemayrememberrippletankexperimentswherewavesaregeneratedfrom
twoseparatepointsources. Thewavesfromeachsourcecombinetoformvisibleinterferencepatterns. In
someplaces, thewave crestsandtroughs arein phaseso they combineto makea larger wave. Inother
placesthecrestsareout of phase,so the crestof one wavesource iscanceled by thetrough of theother.
Rippletankexperimentsshowtheinterferencepatternsarestrongestwhentheamplitudeofthewavesfrom
each source is equal.
A similar interference occurs in sound systems when a signal is delayed and mixed back into the original
signal. TheseinterferencepatternsarecalledCOMBFILTERSbecausetheirfrequencyresponseplotslook
liketheteethofacomb(seeFigs.1&2). Thereareanumberofcommonsituationsthatcausecombfilters.
Forexample,whentheprogramisplayedthroughtwoloudspeakers,theloudspeakerthatisfartheraway
interfereswiththecloserloudspeaker. Combfiltersarealsocreatedwhenaperformerispickedupbytwo
microphones, one closer than the other. You even introduce comb filters by mixing digital effects back
into the “dry”signal at the mixer’s effects loop.
Fig. 1:
COMB FILTERS. Input signal mixed with a 2
msec. delayed signal (both signals have the
sameamplitude); max.filtergainis+6dB,and
max. depth is - dB).
Fig. 2:
COMB FILTERS. Input signal mixed with a 2
msec. delayed signal (delayed signal has 10
dB less amplitude; max. filter gain is +2.5 dB,
andmax.depthis -3). Reducingtheamplitude
ofthedelayedsignalreducesthecombfilters'
effect.
"

"
Calculating Comb Filter Frequencies
The reinforcement and cancellation frequencies depend on the delay time (the time difference between
thearrivaltimeoftheoriginalsignalandthedelayedsignal).Thefrequencyofthefirstcancellationoccurs
at 1/(2 x t) Hz, where t = the delay time in seconds. The cancellations are separated by (1/ t) Hz. Fig.
3 shows how the comb filters change with the delay time.
Comb Filter Amplitude
If the original signal and the delayed signal are the same amplitude, the reinforced frequencies increase
in amplitude by 6 dB, while the out-of-phase frequencies cancel completely to - dB.
Comb filters cause a lot of problems. The frequencies that are reinforced are prone to excite feedback,
while the out-of-phase cancellations make the program sound thin and over equalized.
Try this simple experiment to hear what comb filters do to your sound.
Stacktwo identicalfull-range loudspeakersas shown inFig. 4. Carefullyalign the HFhorns andwire the
speakersinmono. Standinfrontwhilelisteningtoyourfavoritefull-spectrumCD. Askafriendtomovethe
top speaker slowly away from you. The degradation in sound quality you hear is caused by comb filters.
The experiment is most dramatic when you use good quality speakers.
CorrectingCombFilters
Combfiltersareinevitable tosome degree inevery livesound system,and theycannot becorrected with
equalization. Fortunately,most combfilter problemscan be reducedto aminimum by synchronizingthe
signals and reducing the amplitude of the delayed signal. The examples below show several practical
applications.
The Precedence Effect: Aligning the Acoustic Image
HelmutHaaspublishedastudyin1951describingaseriesofexperimentsthatdemonstratedhowpeople
perceivedelayedsignalsandechoes. Inhisexperiments,alistenerwaspositionedbetweentwospeakers
placed3metersaway;onewasplaced45degreestotherightandtheotherwasplaced45degreestothe
left. Whenthesameprogramwasplayedthroughbothspeakerssimultaneously,thelistenerperceivedthe
acousticimage(thedirectionfromwhichthesoundseemedtobecoming)centeredbetweenthespeakers.
WhenHaasdelayedthesignalgoingtooneofthespeakersbysomewherebetween5to35milliseconds,
the listener perceived a shift in the acoustic image to the speaker heard first. While the delayed speaker
didnotcontributetotheapparentdirectionofthesound,itdidmaketheprogramseemlouderand“fuller.”
Fig. 3:
Comb filters get
closer as delay
time increases.
"
Fig. 4:
Comb filters noticeably
affect your sound.

#
Haas showed that you must increase the loudness of the delayed signal by about 8 to 10 dB (twice the
perceived loudness) in order for the acoustic image to move back to the original center position.
Increasing the loudness more than this, or increasing the delay somewhat more than 35 milliseconds,
makes the delayed signal sound like an echo.
ThephenomenondescribinghowtheacousticimagefollowsthesignalwehearfirstiscalledthePrecedence
Effect. The phenomenon that makes two distinct sounds heard less than 35 msec. apart seem like only
onesoundiscalltheHaasEffect. However,thetermsareoftenusedinterchangeablyinthesoundindustry.
THREE APPLICATIONS
APPLICATION I: Under-The-Balcony Speakers
Fig.5aboveshowsatypical situation where theperformer is amplifiedbya center clusterhanging above
thestage. Almosteverybodyintheaudiencewillenjoygoodsound,exceptthoseseatedintheshadowof
the balcony. So we add an under-balcony speaker to fill in the shadow.
Nowwehavesufficientvolumeunderthebalcony,butthesoundfromthetwospeakersarrivesatthelistener’s
earssome76to84millisecondsapart. Thetwosignals,alongwiththeirechoes,resultinanunintelligible
cacophony. Wemustdelaythesoundfromtheunder-balconyspeakertosynchronizethesignals. Dowe
set the digital delay to 76 or 84 milliseconds? Obviously, the geometry will not allow us to exactly
synchronizeeverylocation underthebalcony;wehavetocompromise.
First,youmustconsidertheprogramtype. Forspokenwordprograms,youwillproducethebestintelligibility
ifthesignalsfromtheunder-balconyspeakersarrivewithin10msec.ofthesignalsfromthecentercluster.
Thereforeweshouldsetthedelayto84-86msec. Youcanallowalittlemorereverberationforprogramsthat
are mostly music.
Next,wemusteliminatecombfilterdistortion. Findtheaxiswherethelevelsofthecenterclusterandunder-
balcony speaker are equal. Use the digital delay to precisely synchronize the speakers along this axis to
eliminate the most severe comb filters. Comb filters off the equal-level axis are much less of a problem
since a louder signal is not affected very much by a weaker signal.
Finally, you can experiment with adding 5 to 10 milliseconds delay to both sets of speakers to take
advantage of the Precedence Effect for the audience seated near the performer.
Inthefinalanalysis,everysettingisacompromise,andyourearhastobethefinaljudge. Checkthesound
in several different locations throughout the auditorium and correct the most severe irregularities.
Fig. 5:
Overheadviewofunder-balconyapplication.

$
Application II: Center Cluster with Front Fills
Fig. 6 below describes a typical application that has a stage with a microphone, a center cluster above
the stage, and front fills in front of the stage. There must be thousands of installations throughout the
world like this that "get by" without digital delays. But with the Sabine digital delay, you can improve the
intelligibility and add a new quality without ringing up any significant costs. Use the digital delay in this
situation to align the visual image with the acoustic image. The program is much more enjoyable when
the amplified sound seems to be originating with the performer, not the loudspeakers.
Findacentralplaceintheaudiencewherethecenterclusteris6to8dBlouderthanthedirectsoundfrom
the performer. Delay them so that their sound arrives 5 to 8 milliseconds after the direct sound from the
performer. Experiment by bypassing the digital delay in and out to hear how the source of the sound
seems to move from the loudspeakers to the performer and back. Now your ears have the same
directional information as your eyes, so the performance will sound more natural and exciting. The best
seats in the house just got better.
What about the front fills? Their purpose is to add intelligibility and listening comfort to the first few rows
nearest the stage by filling in the areas missed by the center clusters. Simply add about 8 msec. to the
frontfillstotakeadvantageofthePrecedence Effect.
The8msec.settingpresumestheperformerisstandingonthefrontfewfeetofthestage. Butsomestages
arewellover30feetdeep. Whatifthereisasecondperformerstanding25feetbehindthefirst? Thedirect
soundfromhisorhervoicewillreachthefirstfewrowsabout25msec.afterthefirstperformer's. Theaudience
willperceivethefirstperformerdirectlyandthesecondperformerthroughtheloudspeakers.
We can add the advantage of the Precedence Effect to the second performer by placing an digital delay
in the mixer's channel insert point and adding a 25 msec. delay.
Certainly taking advantage of the Precedence Effect is not as obvious to the audience as eliminating
feedback, but it is nice to know you did all that is possible to make the performance enjoyable.
Fig. 6:
Synchronizingcenterclustersandfrontfills.

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Application III: Synchronizing the signals of a far-throw and short-throw loudspeaker.
In order to reach the proper coverage in larger venues, we often stack two full range speakers - a short-
throw center cluster for the audience below and a far-throw speaker for the back of the auditorium. It is
almostimpossibletoperfectlyalignthestackedspeakersmechanically,socombfilterdistortionbecomes
a problem in the area where the levels from both speakers are equal. The same thing happens with
speakers mounted on the right and left sides.
It is impossible to remove comb filters with equalization, but a Sabine digital delay eliminates them in
short order without affecting the spectral balance for the rest of the audience. Find the axis where the
levels from the two speakers are equal. This is where the comb filters are most severe. Carefully adjust
the digital delay so that the signal from both speakers arrives at precisely the same time.
Use the same procedure to align speakers within a cluster when necessary.
CALCULATING DELAY TIME USING DISTANCE
Calculating delay time in terms of distance is a common and accepted method. For a good start,
estimate the delay at 1 millisecond per foot between speakers. Use the following equation for more
precise estimates:
Delay (milliseconds) = 1000
OR
Delay (milliseconds) = 1000
These measurements presume standard temperature (68 degrees F, 20 degrees C) and pressure
(29.2 in. Hg., 760 mm Hg.). Sound travels slower in cooler or drier air and at higher pressures. For
example, the speed of sound decreases about 0.61 meters per second as the temperature drops
from 20 degrees C to 0 degrees C.
Fig. 7:
Aligningfar-andshort-throwspeakers
(thelevelfrom both speakers is equal).
Sabine, Inc.
13301 Highway 441 •Alachua, Florida 32615 USA •( 904) 418-2000 •[phone] •Fax: [phone]
www.SabineUSA.com Digital-Delay.p65
000216 - hto
© 2000 Sabine Inc.
( )
D (distance in feet)
1130
( )
D (distance in meters)
344
is louder
is louder A & B at
equal
levels