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Operating Manual -4048A digital signal processor
7.3g Delay
In large installations or outdoor venues there are often many speakers in vari-
ous locations to get the best coverage possible. Since sound travels relatively slow
through air (1130 ft/s at 20ºC), multiple loudspeaker locations can create a situation
where the original audio signal, simultaneously leaving all loudspeakers, arrives at a
single point in the venue at several different times. Needless to say this causes prob-
lems, and what may be crystal clear sound directly in front of any one loudspeaker
can be unintelligible at the farther reaches of the venue with direct line-of-sound to
multiple loudspeaker sources.
The solution is to delay the audio signal to the loudspeakers located further
away from the primary source, so that sound comes out of the distant loudspeakers at
the exact time that sound from the main loudspeakers arrives. Within the 4048A, up
to 682 milliseconds of time delay are available on each output channel, allowing
secondary loudspeakers to be time aligned with the primary speakers up to 771 feet
(235m) away.
A second application for delay is to time
align discrete drivers within a cabinet or cluster. For
example, a typical three way speaker cluster would
have low end, midrange, and high frequency drivers
all located near one another. The different drivers
for each frequency band are not necessarily the same physical depth with respect to
the front of the loudspeaker cluster, so there exists the problem of the same signals (at
the crossover points) arriving at the cluster "wavefront" at different times, creating
undesirable wave interaction such as frequency peaks or cancellation. The solution
in this case, rather than fixing the frequency anomalies with EQ, is to slightly delay
the signal to the drivers closest to the cluster front.
Using the location of the driver diaphragm farthest back as a reference point,
measure the distance to other drivers in the cluster, and set the output delay for each
accordingly, with the driver diaphragm closest to the front getting the longest delay
and the driver at the very back getting no delay at all. The minimum adjustment is
0.02 milliseconds, or about ¼ inch.. When appropriate, always time align the loud-
speakers before applying EQ to the outputs of the 4048A.
7.3h Limiter
A full function compressor/limiter is included on each
output channel. A limiter is commonly used to prevent transient audio signal spikes from
damaging loudspeakers, manage analog and digital recording levels, optimize broadcast
levels, or "thicken" the sound of an audio source (compression). The adjustable parameters
include Limiter In/Out, Threshold, Ratio, Attack Time, Release Time, and Limiter Link.
The limiter threshold range is from -20dBu to +20dBu. This setting determines the
signal level above which gain reduction begins, and is indicated by the amber LED (S/L) in
the output meter section. Increases in audio level above the threshold will be reduced ac-
cordingly by the ratio settings.
The ratio control determines the amount of gain reduction above limiter threshold.
Ratio ranges from a gentle 1.2:1 to a brick-wall INF:1. To illustrate how the ratio control
works, imagine a commonly used loudspeaker protection ratio of 10:1, which means that for
every input signal increase of 10 dB above threshold, the output level will only increase by
1dB. The higher the ratio, the more pronounced the audio effect, so use the lowest ratio
possible to sufficiently address the problem.
Same sound arrives at
two different times.
Fix b
y
dela
y
in
g
secondar
y
speakers 177mS.
Speaker on
Main Sta
g
e
Secondar
y
Speaker
200 ft from Main Sta
g
e
Long Time Dela
y
For Remote Speakers
High - No Delay
Midrange Delay
12 Inches = 0.9mS
Low Delay
8 Inches = 0.6mS
Example: 12 Inches
Example: 8 Inches
Short Time Dela
y
For Driver Ali
g
nment