Rane C4 Operator's Manual - page 5
Manual-5
Output
Limit Threshold
Input
Output
Limit Threshold
Peak Limiting
A limiter is just a compressor with a really high ratio, right?
Well…not exactly. Although the two devices use similar termi-
nology (Threshold, Attack, Release, and so on), they serve two
completely different purposes and therefore operate in different
manners. Fundamentally, a compressor uses an rms detector to
keep the average level of a signal under control, while a limiter
uses a peak detector to act on the instantaneous level of the sig-
nal. Compare Figure 2 to Figure 7.
Primary uses of limiters: 1) preventing clipping and distor-
tion in power amplifiers, 2) protection of loudspeakers from
damage resulting from destructive transients [like the proverbial
dropped microphone] 3) preventing overs [clipping] during
digital recording 4) preventing overmodulation of the transmit-
ted signal in broadcast.
Figure 8 shows the effect of limiting a signal. The top line is
the original, full volume input signal we need to limit – perhaps
the unfortunate blast of noise when the vocalist inadvertently
unplugs the phantom-powered mic. The bottom line shows the
limited output. At no point does the output signal ever exceed
the threshold, demonstrating the limiter’s brick wall capability.
Contrast this with Figure 9, which shows a compressor set to a
high ratio (∞:1), and a fast attack time. Due to the compressor’s
averaging effect, the output easily overshoots the threshold, and
transient peaks are missed altogether. And it’s these peaks which
can potentially ruin your speakers or the perfect recording take,
and possibly your reputation.
Overshoot
Input
Figure 8. Limiter performance showing brick wall characteristic.
Figure 9. Compressor used as a limiter. Signal peaks can still exceed
threshold, even at high ratio settings.
INPUT
OUTPUT
SIDE-CHAIN
BROADBAND
GAIN CONTROL
PEAK
DETECTOR
GAIN COMPUTER
FILTER
THRESHOLD
RELEASE
Figure 7. Peak Limiter Block Diagram
-40
+20
-35
-30
-25
-20
-15
-10
-5
0
+5
+10
+15
Input dBu
-40
+20
-35
-30
-25
-20
-15
-10
-5
0
+5
+10
+15
O
u
t
p
u
t
d
B
u
Threshold
Figure 0. Peak Limiter Response Graph