Mackie PPM1012 Owner's Manual - Page 17

Owner's Manual 25. COMPRESSOR (Channels 1-6 only) Channels 1 to 6 have an in-line compressor circuit +20 with a variable threshold. This is very useful for com- pression of vocals, and snare drums for example, so you +15 might consider connecting your microphones and drum mics to channels 1 to 6, rather than other channels. +10 OUTPUT SIGNAL STRENGTH dBu When the incoming signals exceed the threshold level set by this knob, the signal level is automatically com- +5 pressed. This reduces the dynamic range, and reduces the chance of distortion due to overloading the input +0 signals. SOFT KNEE SLOPE 6:1 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 harder on the ears too. The graph on this page 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. -5 SLOPE 1:1 -10 -10 -5 +0 +5 +10 INPUT SIGNAL STRENGTH dBu +15 +20 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. 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). The other blue curves represent in-between positions of the compressor knob, with higher thresholds before compression begins. Outboard 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 screams and high-notes, and adjust the compression as required. * Mr. Little, my math teacher thought that graphs might come in handy for me one day. Finally! Owner's Manual 17