QSC PL218A User Manual page 14

FEATURES & SETUP-
FAST- Set both POWER LIMITER SPEED switches to FAST. This setting produces 20 milliseconds (0.020 seconds) of release time. The AVG mode will produce 8 milliseconds of
attack time, and PEAK mode will produce less than 1 millisecond of attack time. This setting will have low distortion down to about 1 kHz. It is suitable for high frequency protection,
but will cause a distinctly "muddy" effect when used on full range material.
MED- Set the upper POWER LIMITER SPEED switch to MED and the lower switch to FAST. This setting produces 200 milliseconds of release time. The AVG mode produces 80
milliseconds of attack time and the PEAK mode produces 10 milliseconds of attack time. This setting is the fastest possible setting for full range material and may still produce some
IM distortion for extremely low frequencies.
SLOW- Set the lower POWER LIMITER SPEED switch to SLOW and the upper switch to FAST. This setting provides 4 seconds of release time. The AVG mode produces 1.6
second of attack time and the PEAK mode produces 180 milliseconds of attack time. This setting will change the gain very gradually in the AVG mode, and sudden peaks may clip for
1 to 2 seconds. The PEAK mode will push overloads down quickly, but recovery time will still take many seconds.
POWER LIMITER TRACKING- Each channel's power limiter operates independently, with its own threshold and time constants.
Four tracking modes are available for your use:
1- No Tracking-- switch section 9 of both limiter adjustment DIPs should be set to OFF. This is the "usual" mode where neither channel's limiting effects the other.
2- CH 1 tracks the limiter activity of CH 2-- switch section 9 of channel 1's limiter adjustment DIPs should be set to TRACK CH 2 and switch section 9 of channel 2's limiter
adjustment DIPs should be set to OFF. This tracking mode is not often used, but is applied to paging environments with high ambient noise levels. Limiting on channel 2 will cause
limiting to occur on channel 1. Channel 2 is set up for a vocal-range horn and carries the voice page; its limiter is set for MED speed and PEAK mode with a power threshold and
gain setting that produces proper paging level. Channel 1 is set up for a full-range music speaker with its level set for comfortable listening against the ambient noise. When a
page drives channel 2 into limiting, channel 1 is driven "down" also, making the page more audible against the background noise. Some experimentation with channel 2's gain and
threshold will be required for best performance with a range of paging voices & levels.
3- CH 2 tracks the limiter activity of CH 1 ( bi-amp tracking ) -- switch section 9 of channel 2's limiter adjustment DIPs should be set to TRACK CH 1 and switch section 9 of
channel 1's limiter adjustment DIPs should be set to OFF. This tracking mode is useful for high frequency bi-amping , using fast protection on the horn without "ducking" the low
frequencies. When used carefully, this mode can protect each driver in a two-way system , while preserving musical dynamics and balance.
We offer the following suggestions as a starting point for bi-amp tracking:
TWO-WAY SPEAKER WITH A FAIRLY HIGH CROSSOVER FREQUENCY- Most of these systems have a large cone low frequency driver and a comparatively delicate high frequency
driver. The limiting is tailored for each driver; higher power threshold and slower limiter speed for the low frequency driver and a lower power threshold and faster limiter speed for
the high frequency driver.
The low frequency driver can handle short time overloads, so its limiter can be set to the slower responding AVG (average) mode with MED to SLOW overall limiting times. Short
peaks may clip, but the overall limiting will be smooth and gradual with no low frequency distortion and a minimum of "ducking". If there is too much clipping, try the CLIP
LIMITER or use the PEAK mode and MED speed.
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SIDE PANEL SWITCHES:
POWER LIMITER (continued from previous page)