Sound Devices 633 - Manual (Page 25)

Limiters
Limiters prevent clipping by attenuating signals that surpass a set threshold. The amount of attenua-
tion is defined by the “ratio” of the limiter and expressed as two numbers. All 633 limiters use a 20:1
compression ratio. This means that signal that exceeds the threshold by 20 dB will exit the limiting
stage at only 1 dB above the threshold.
The time it takes for limiting to begin once signal has exceeded the threshold is referred to as the
“attack time”. The time it takes for limiting to cease once signal has fallen back below the threshold
is referred to as “release time”. Signals that exceed the threshold faster than the limiter’s attack time
can still cause clipping. The 633 limiters have a 1 ms attack time and a 500 ms release time.
The limiters are globally activated when the Setup Menu item LIMITERS > Limiter Enable is set
to Hard Knee or Soft Knee ( See "Limiter Knee", page 19 ). This activates both the input limiters
(Inputs 1 through 6) and all output limiters. Sound Devices recommends using the limiters at all
times. Limiters are present on both mic and line-level inputs as well as the Master L,R tracks and
the X1,X2 tracks. The default threshold of all limiters is 16 dBu and can be adjusted from 4 dBu to
18 dBu (in 1 dBu increments) from Setup Menu options LIMITERS > Input 1-6 PostFade Threshold,
LIMITERS > L,R Threshold, and LIMITERS > X1,X2 Threshold.
In normal operation, with a properly set gain structure, the threshold of the Input Limiter is rarely
reached. Without Input Limiters, high signal conditions can overload a channel and cause distor-
tion. The Input limiter is working when the respective input’s Input Activity LED illuminates yellow.
Limiting activity will also be displayed as a yellow square on the right side of the input’s meter on
the Main Screen. If the limiting activity is regularly indicating, reduce the amount of gain applied to
the channel by turning down the Trim control.
Limiter Knee
The “knee” of the limiter determines how the limiter operates in relation to the set threshold. A
“hard knee” leaves all program material below the threshold completely unaffected and attenuates
only those peaks above the threshold. A “soft knee” attenuates the program material slightly before
the threshold (about 6 dB) for a more gradual, tape-like sound.
The knee parameter for the 633’s limiters is set with Setup Menu option LIMITERS > Limiter Enable.
Linking Limiters
When two limiters are linked, both channels will be limited when signal reaches the threshold
in any channel of the pair. Linking inputs as a stereo pair (see Input Linking) also links those in-
puts’ limiters. Limiters for L,R and X1,X2 channel pairs can be linked with Setup Menu options
LIMITERS > L,R Linking and LIMITERS > X1,X2 Linking.
Limit
er
s
633 User Guide and Technical Information
19