Sabine THE DIGITAL DELAY ADVANTAGE User Manual

THE DIGITAL DELAY ADVANTAGE

A guide to using Digital Delays

Synchronize loudspeakers

Eliminate comb filter distortion

Align acoustic image

Contents

THE DIGITAL DELAY ADVANTAGE .....................................................1

- Why Digital Delays? ........................................................................................................................ 2

Loudspeaker Synchronization ..................................................................................... 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 Precedence Effect: Aligning the Acoustic Image..................................................4

THREE APPLICATIONS ........................................................................5

- Application I: Under-The-Balcony Speakers .................................................................................... 5

- Application II: Center Cluster with Front Fills................................................................................... 6

- Application III: Synchronizing the signals of a far-throw and 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

Special thanks to Hans
Drobilitsch of Hans
Drobilitsch Audio
GmbH. (Wollersdorf,
Austria) for his invalu­able technical advice.

stands to reason that the most intelligible sound systems are the ones that come
closest to emulating face to face communication. If this is your goal, a digital delay is essential to your
sound system.

Until recently, a digital delay’s cost was prohibitive for the average user. Only high-end applications could
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, we discuss the basic acoustical concepts needed to get the most out of your digital
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
to arrive at a certain point in the audience. Most of these tools are very sophisticated and tend to 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
a sharp click through the system. They moved the speakers until they could only hear a single sharp click
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.

Not all manufacturers acknowledge processing delays in their specifications, but you must take them into
account when synchronizing your system. Make sure all digital equipment is on and not bypassed when
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

Center cluster speakers offer several advantages over systems that have speakers mounted on the sides.
The most obvious advantage is that the distance to the closest and most distant locations in the audience
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.

All those in the audience who are closer to the performer than the center cluster will hear the direct sound
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

Many who took high school science may remember ripple tank experiments where waves are generated from
two separate point sources. The waves from each source combine to form visible interference patterns. In
some places, the wave crests and troughs are in phase so they combine to make a larger wave. In other
places the crests are out of phase, so the crest of one wave source is canceled by the trough of the other.
Ripple tank experiments show the interference patterns are strongest when the amplitude of the waves from
each source is equal.

A similar interference occurs in sound systems when a signal is delayed and mixed back into the original
signal. These interference patterns are called COMB FILTERS because their frequency response plots look
like the teeth of a comb (see Figs. 1 & 2). There are a number of common situations that cause comb filters.
For example, when the program is played through two loudspeakers, the loudspeaker that is farther away
interferes with the closer loudspeaker. Comb filters are also created when a performer is picked up by two
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
same amplitude); max. filter gain is +6 dB, 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,
and max. depth is -3). Reducing the amplitude
of the delayed signal reduces the comb filters'
effect.

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