Mackie SP-DSP1 Quick Start Manual - page 5
Adjustable Parameters
MG (Minimum Gain)
This control defines the lowest level to
which the system can attenuate the program
signal. It has a minimum setting of –40 dB
and a maximum setting of 0 dB.
GR (Gain Range)
This control defines the amount of gain
change allowed by the Automatic Level Control.
It has a minimum setting of 0 dB and a maxi-
mum setting of 40 dB. The Gain Range setting
can never be greater than the Minimum Gain
setting, because the system gain from the
input to the output of the SP-DSP can never be
greater than 0 dB. Because of this, the Gain
Range control “re-scales” itself if the Mini-
mum Gain control is increased toward 0 dB.
As an example of how these controls work
together, if the user wanted his music levels
to operate in a range
±
10 dB around –15 dB
down from the input level, he would set
MG = –25 dB and GR = 20 dB.
NT (Noise Threshold)
This control sets the point at which the
noise level “triggers” the Automatic Level
Control to begin working. It has a minimum
setting of –80 dB and a maximum setting of 0
dB. The lower the Noise Threshold setting, the
more sensitive the algorithm is to ambient
noise levels in the room.
NR (Noise Range)
This control adjusts the ratio of how much
the program level changes as a function of
how much the noise level changes. It has a
minimum setting of 1 dB and a maximum
setting of 60 dB.
For example, if you set the Noise Range to
20 dB and the Gain Range to 30 dB, then a 20
dB change in the noise level causes a 30 dB
change in the program output level (assuming
the noise level has crossed the Noise
Threshold). These settings represent a 2:3
noise level to program level change. For every
2 dB change in noise level, the output level
changes by 3 dB. However, if you accept the
default settings of NR = 40 dB and GR = 40
dB, the settings represent a 1:1 noise level to
program level change. For every 1 dB change
in noise level, the output level changes by 1 dB.
Attack Time and Release Time
The Attack parameter defines the time, in
seconds, it takes for the Program Gain to
increase by 40 dB. Likewise, the Release
parameter defines the time it takes for the
Program Gain to decrease by 40 dB. These
parameters have a maximum value of 300
seconds (5 minutes). A short Attack Time
allows the program level to respond to every
shift in noise level. Increasing the Attack Time
allows the algorithm to “average” the noise
level and provide a relatively constant
Program Gain over time. To keep impulsive
bursts of noise from affecting the program
level, a good setting might be Attack = 20
seconds and Release = 20 seconds.
Bar Graphs/Metering
PI (Program Input)
This indicates the input level into the DSP.
It is important that the Program Input signal
level is sufficient prior to running the Auto
Calibration procedure. The input level should
be between 0 and –10 dB during the loudest
portions of the program material. Adjust the
level with the front panel up/down buttons.
MI (Microphone Input)
This indicates the ambient microphone
input level into the DSP. Again, it is important
that the ambient microphone signal level is
sufficient prior to running the Auto Calibration
procedure. The input level should be between
0 and –10 dB for the loudest noise plus
program expected.
It may be necessary to adjust the gain of
the ambient microphone. A good way to set
the MI level is to click the “Bypass” box on the
main screen. You may need to turn down the
program level if there is a lot of attenuation
set with the Minimum Gain (MG) control, or the
music will become very loud in bypass mode.
There is a trim pot located through an un-
marked hole just above the AMBIENT MIC
connector on the rear panel of the SP2400.
Use a nonconductive alignment tool to adjust
the pot until the meter indication is between
0 and –10 dB for the loudest noise plus
program expected.
PG (Program Gain)
This indicates the amount of gain the algo-
rithm applies to the program input. This can
never exceed the Minimum Gain (MG) setting.
PO (Program Output)
This indicates the output level from the
SP-DSP1. The Program Output is equal to the
Program Input (PI) level plus the Program
Gain (PG).
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