FunctionalityGates control unwanted noise and unwanted signal pickup by closing a microphone or audio channel when the signal level drops below a set threshold. The primary use of Gates in live sound applications is for gating drum microphones and instrument pickups. The most common use for gates in installed sound reinforcement is for gating microphones used for speech reinforcement. Gated auto-mixers represent the most common use. Employing an individual Gate on a single microphone is less common, but not unheard of. Halogen provides advanced gain-sharing auto-mixer blocks that are easier to set up and provide better overall performance than a gated auto-mixer. The Gate block provides a basic tool for system designers to solve problems related to an individual microphone or audio channel.
Like an Expander, the gain reduction in a Gate occurs below the threshold. Like a Limiter, a Gate must respond very quickly to changes in level, requiring the use of an accurate peak detector in the side-chain. Gates differ from Expanders in that they use a fixed amount of gain reduction determined by the depth control. Gates remove background noise by closing the audio channel when the signal level drops below the threshold. Most Gate applications require side-chain equalization on an external side-chain input, so side-chain low-cut and high-cut filters are included in the Gate. The Gate is able to open very quickly without clicking by using look-ahead delay in the main signal path and pre-ramped gain control in the side-chain. The Gate hold function prevents the Gate from closing during brief excursions below the threshold.
Gates must turn on quickly. Accurately capturing and reproducing transient signals requires accurate peak detection. Accordingly, the Gate uses over–sampled peak detection. Look-ahead and pre-ramping techniques ensure fast and artifact-free gating. A look-ahead detector works by delaying the main audio signal a short amount while not delaying the side-chain signal. This allows examining the signal in advance to determine the appropriate response before an event. This action allows the Gate to turn on before the leading edge of the signal. This in turn allows the turn-on to ramp up instead of step on, avoiding the annoying click resulting from deep gate depth, high threshold and fast attack settings. With digital signal processing, it is possible to delay the main signal path as long as desired. The limiting factor is the amount of delay that the application tolerates. A delay of 16 samples (0.33 ms) with a sample rate of 48 kHz, combined with exponential ramping has proven to provide smooth, artifact-free gating.
For additional information on Gate functionality, take a look at the following RaneNotes:
- RaneNote 155: Dynamics Processors -- Technology & Applications (see Chapter 5)
See Also