PPM608

29. COMPRESSOR

Channels 1 to 4 have an in-line compressor circuit with a variable threshold. This is very useful for compression of vocals, for example.

When the incoming signals exceed 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 part of your song and the loudest part. Using a compressor, you are able to squeeze 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, and it is very useful for live sound as well.

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 hard on the ears too.

The following graph shows the input signal level going into the compressor, versus the output level coming out of it. It is the typical graph to see when compressors are discussed, and is just the kind of thing our engineers like to talk about during the company Christmas party.

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 blue curve. If the input is

–5 dBu (that is, below this threshold), the output is –5.

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 reaches +10 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).

+20

+15

+10

+5

+0

-5

SL

OP

E 1

:1

SOF

T KN

EE

SLOPE 6:1

-10

-10 -5 +0 +5 +10

INPUT SIGNAL STRENGTH dBu

+15 +20

The other blue curves represent in-between positions of the compressor knob, with higher thresholds before compression begins.

Separate compressors often have controls such as compression ratio, threshold, soft knee/hard knee, attack time, and release time. These last two affect how quickly the compressor kicks in when the input exceeds the threshold, and how quickly it is released after it drops below the threshold. In this powered mixer compressor, these parameters are specially chosen to give you the best overall performance.

Adjust the threshold carefully, so your dynamic range is still lovely, without distortion or overload during the performance. Run through a few practice songs and adjust the compression as required.

30. HI-Z SWITCH

Channels 5 and 6 are the only place to plug in guitars and basses directly to the powered mixer. These switches must be pressed in first.

If you connect a line-level device, such as a keyboard or drum machine, make sure these switches are out.

(They only affect the 1/4" line inputs of channels 5 or 6, not the XLR mic inputs.)

To use guitars or other instruments on other channels, you will need to use an external DI box first.

Without the DI box, (or if these switches are not pressed in) guitars will sound dull and muddy.

Owner’s Manual

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