The Limiter object can be used to create a hard-stop peak limit on an input signal. Because it operates on a feedback basis only, there is the possibility for the input signal to overshoot the limit set by the user for a short duration while the algorithm pulls down the signal gain.
The basic trade-off of any feedback limiter is strict regulation to the threshold versus distortion. The more aggressively the limiter is working to pull down (attack) the signal level to the threshold, the more distortion will be produced. Distortion, fluttering and/or pumping can also be produced when the signal level is released from regulation, if the resulting gain lift happens too fast.
Limiter Operation Description
- Attack Phase – When the Limiter detects that the Threshold value has been exceeded, it reduces the gain at a rate set by a combination of the Attack parameter and how much the input signal exceeds the Threshold value. The gain is reduced until the signal level is at or below the Threshold value. The faster the gain is reduced, the more distortion will be created. There is a minimum attack time time period during which the algorithm is determining signal level.
- Hold Phase – After the signal drops below the Threshold value, the Limiter will hold the gain steady for a time determined by the Hold parameter. It stops the gain reduction phase and begins to hold. Then at the point when the signal has fallen below the Hold threshold value, the Release phase begins.
- Release Phase – The gain is raised back to the starting gain, set by the Gain parameter, at a rate determined by the Release parameter. The faster the gain is raised, the more distortion will be created.
Limiter Tuning Parameters In-Depth Description
- Gain – this is a gain stage before the limiting portion of the algorithm, which sets the starting gain. This gain is applicable before limiting starts, after limiting is fully released and while no limiting is occurring. Gain changes during limiting will be added to this value.
- Threshold – this is the peak level under which the Limiter will attempt to keep the signal. Overshoot can happen during the Attack phase, both while the algorithm is computing signal level and while reducing gain to meet the Threshold value.
- Hold Threshold – this is the signal level, relative to the Threshold value, under which the Release phase will begin. This small offset keeps the algorithm from changing phases too quickly and provides an extra guard against exceeding the Threshold value, helping to reduce fluttering and distortion.
- Attack Time – this is how fast, and thus how aggressively, the Limiter will pull down the signal to meet the threshold. If strict adherence to the Threshold parameter is desired, then Attack should be set very low. This will produce more distortion than a more relaxed Attack value, but if the input signal is not anticipated to exceed the Threshold by very much, then the distortion will be relatively low. Worst case is when the Attack is set low and the input signal level is high compared to the Threshold value. There is a minimum attack time the user can set, which corresponds to the time period during which the algorithm is determining signal level.
- Hold Time – this sets the time during which the Limiter will hold the gain value steady. Having a non-zero value here can smooth the Limiter operation so that fluttering and pumping are less audible.
- Release Time – this sets the time during which the Limiter will begin raising the gain back to the original value. Extremely short release times cause very fast gain changes which can lead to fluttering, pumping and distortion.