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On the left-hand side of the CD Scratch window are four gauge bars. These gauges show real-time audio characteristics of the Dynamics Processor. The first gauge is the Input level, second is AGC (Automatic Gain Control), third is the Compressor and Limiter, and the last gauge is the Volume Output level. There is also an oscilloscope underneath these bar gauges.
All number markings on the gauges are in the standard audio measurement scale of decibels (dB).
Input level: This gauge reflects the volume of the audio input into the Dynamics Processor.
AGC (Auto Gain Control): The yellow AGC gauge shows whether the AGC is currently boosting or cutting the volume of the audio. If it is a negative number, it is bringing the volume down. If it is a positive number, it is boosting the volume up. This is done first before the audio passes through to the compressor and limiter.
Compressor/Limiter: The Compressor and Limiter clamp down any audio which goes over a certain threshold of loudness. This protects speaker equipment, and keeps the audio level inside a tighter range.
Volume Output: This gauge shows the effect the AGC, Compressor and Limiter have had on the original audio volume represented by the Input Gauge. You will notice, in a preset like "Radio", the Output Gauge stays tightly within a set range, regardless of natural volume fluctuations of the input signal.
Note: There is an input level control in the Dynamics Processor dialog box. The input gauge shows the input level of the audio before it passes through this input control. Therefore there can be a difference between the input and output gauges even when no AGC or compression is currently taking place, simply as a result of the current setting of this input level control.
Oscilloscope: The oscilloscope underneath the bar gauges tracks the instantaneous overall alteration in audio level that is being applied as a result of the AGC/Compressor/Limiter. That's why, when you are in "Lounge" mode, the line is usually perfectly flat, however in "Radio" mode the oscilloscope shows a lot of activity.
In essence, the AGC, Compressor and Limiter all do the same thing: control/alter the volume of the audio. The difference is in the speed at which they exercise their control. For example, the AGC has a slower, more gradual control. It will allow very rapid increases in audio level to pass through unaffected. The Limiter is at the other extreme, within 1 millisecond it can clamp down on a passage of audio that is too loud.
The use of all three processors make for the best overall audio quality, in terms of faithfulness to the original signal, and yet desired control and effect.