Sound Devices 664 - Manual (Page 21)

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 664 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 “at-
tack time” and 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 664 limiters have a 1 ms attack time and a 500 ms release time.
The limiters are globally activated when the Setup Menu item LIMITERS > Limiters is set to On. This
activates both the input limiters (Inputs 1 through 6) and all output limiters. Sound Devices recom-
mends 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 664 input limiters have a threshold of +16 dBu (4
dB below clipping), while the limiters for the L, R, X1, and X2 tracks are adjustable from the Setup
Menu item OUTPUTS > L,R Limiter Thresh and OUTPUTS > X1,X2 Limiter Thresh.
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.
If the Activity LED is regularly in the yellow, reduce the amount of gain applied to the channel by
turning down the Trim control. See Input Activity LED for additional information.
When Inputs are linked as a stereo pair, the Input Limiters are also linked and perform the same gain
reduction equally to both inputs.
The Output Limiters prevent the L,R and X1,X2 signal from exceeding the user-set limiter threshold.
Setup Menu options LIMITERS > L,R Limiter Thresh and LIMITERS > X1,X2 Limiter Thresh allow
limiter thresholds to be set in 1 dB increments from +4 dBu to +20 dBu.
664 User Guide and Technical Information
15
Limit
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