MULTI-CHANNEL ANALOG MIXERS WITH ENHANCED FX, USB RECORDING MODES, AND BLUETOOTH®
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Next to every gain knob – except the 1/8" stereo input gain knob – lies a level set LED. These LEDs are used with the gain control
to set the channel preamplifier gain just right for each source. If one or more channels are distorting, check the level set LEDs.
If they are on continuously, turn down the gain.
Compressor Knobs
The following channels of each mixer has an in-line compressor circuit with a variable threshold.
• P – On/Off compressor switches
• Pr – Channels 1–2
• Pr – Channels 1–4
This is very useful for compression of vocals, and snare drums, for example, so you might consider connecting
your vocal and drum mics to these channels, rather than one of the other channels.
When the incoming signal exceeds the threshold level set by this knob, the signal level is automatically compressed.
This reduces the dynamic range and reduces the chance of distortion due to overloading the input signals.
Dynamic range is the difference in level between the quietest and loudest parts of a song. A compressor
“squeezes” the dynamic range, resulting in an overall steadier, more constant volume level for the signal.
It helps sources, such as vocals, “sit” properly in the mix; it is very useful for live sound.
The compression ratio is fixed at around 6:1, with a soft knee response. The threshold can be adjusted clockwise
from off (no compression) to 0 dBu (max).
As an example, suppose the threshold is set to maximum.
An incoming signal reaches the threshold of 0 dBu.
As it increases beyond the threshold, it becomes
compressed at a ratio of 6:1. This means that even
if the input further increases by 6 dB, the actual output
only increases by 1 dB. This compresses the output
signal, so there is more protection to your system
from distortion and overload due to poor microphone
technique (say it ain’t so) and general pops, bangs
and heavy metal screaming. The soft knee means
that the compression slowly ramps up to 6:1 from the
threshold. It does not jump abruptly to 6:1, as this would
be hard knee compression, and harder on the ears too.
The graph to the left displays the input signal level going
into the compressor, versus the output level coming
out of it. It is the typical graph to view when compressors
are discussed, and is just the kind of thing our engineers
like to discuss during the company Christmas party
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.
If the compressor is off, then the input = output. For example, an input signal level of +5 dBu results in an output level of +5 dBu.
The diagonal line from lower left to upper right represents x = y, that is, input = output.
At the maximum compression, the threshold is set at 0 dBu, and the input to output relationship is represented by the lower
curve. If the input is –5 dBu (that is, below the threshold), the output is –5 dBu. As the input reaches 0 dBu, the output is a bit
less than 0 dBu. If the input is +5 dBu, the output is about +2 dBu. If the input r10 dBu, then the output is +3 dBu. Notice
the shapely curve of the soft knee between the diagonal slope of x = y and the compressor slope of 6:1 (the compression ratio).
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My High School math teacher, Mr. Marvin, thought that graphs might come in handy for me one day. Finally!
Pr
P
Pr
+20
-10
-5
+0
+5
+10
+15
+20
-10
-5
+0
SLOPE 6:1
SOF
T KN
EE
SLO
PE
1:1
+5
+10
+15
INPUT SIGNAL STRENGTH dBu
OUTPUT SIGNAL STRENGTH dBu