SABINE GRAPHI-Q2 Specifications download pdf (Page 43)

9.4. USING DIGITAL DELAY

This section goes beyond the typical operating guide that only explains the front and back panel

adjustments of a piece of equipment. Instead, we discuss the basic acoustical concepts needed to get

the most out of the use of digital delay in sound systems. If you are familiar with these principles, feel

free to skip ahead. Some principles may require additional delay channels and options available with

the Sabine SDA102.

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. 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.

There are three distinct applications for digital delays. The first and most important is synchronization

of the loudspeakers to control excess reverberation and echo. Second, 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 be coming from the performer rather than from the loudspeaker.

9.4.1. Loudspeaker Synchronization

Sound travels at about 1,130 feet per second in air, or about 1 foot per millisecond. On the other hand,

electronic signals travel almost one million times faster through your sound system to the loudspeakers.

The main task for digital delays is to synchronize multiple loudspeakers so the sound traveling different

distances arrives at the listener’s ears at about the same time. Synchronizing the loudspeakers reduces

reverberation and echoes for improved intelligibility.

9.4.1.1. 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.

Calculating Delay Time Using Distance

Calculating delay time in terms of dis-

tance 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:

These measurements presume stan-

dard 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.

9.4.1.2. Processing (or Group) Delays

Converting signals back and forth from the analog to digital domain always delays the signal a little.

These conversion delays are often called processing (or group) delays, and usually range between 0.9

and 5 milliseconds. You will notice that Sabine delays always display the processing delay as the

smallest possible delay value. For the Graphi-Q, the processing delay is 1.38 milliseconds. You can

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.

SECTION NINE: SUGGESTIONS FOR OPTIMAL USE OF THE GRAPHI-Q2

( )

D (distance in feet)

1130

( )

D (distance in meters)

344

Delay (milliseconds) = 1000

1 2 ... 38 39 40 41 42 43 44 45 46 47 48 ... 57 58

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SABINE GRAPHI-Q2 Specifications Page 43

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