FRONT OF HOUSE
LIMITING FUNCTIONS | Kent Morris
In transport , rails are what keep the train on the track . In audio , rails are what keep the signal within bounds . Live audio demands we raise levels enough throughout the signal chain to avoid the noise floor while simultaneously preventing those levels from reaching the clip point where distortion resides . In a contemporary sound system , once we perform the critical step of converting the incoming analog signal to a digital representation , termed ADC , we are dealing with mathematics and generally have an easier time of preserving clarity than years ago when we had to function continuously within the variability of the analog domain . Let ’ s walk through the process of getting and keeping proper levels in a digital mix system from the mic preamp through the power amp ’ s output to the loudspeakers .
Proper gain structure is the cornerstone of effective live sound reinforcement . Without it , the result is at best grating and at worst a sonic nightmare . Most digital mixers have a preamp gain range of about 60dB , which is broad enough to amplify inputs as diverse as a small diaphragm condenser mic requiring 50dB of gain to a keyboard needing only 10dB . The key is to dial in the preamp with the incoming signal at true performance level . On analog desks , the idea was to get the signal to 0dB on the meter , but in the digital world , the best practice is to arrive around the -12 mark on the meter for large format consoles and -18 for small format mixers . Analog has room above zero , but digital does not . At zero , digital distortion occurs and does so with a particularly nasty sonic signature .
The goal , then , is to achieve and maintain proper levels without resorting to help devices . Sometimes , though , that goal is not achievable , due primarily to performer-induced issues such as screaming , cupping mics and raising instrument levels far beyond what was presented in rehearsal . In that case , a good dynamic range reduction device , such as a compressor or limiter ( in more extreme cases ) is warranted and often necessary .
Compression essentially reduces signal level based on a threshold ( when ) by a ratio ( how much ) on a slope of attack and release ( how quickly in and out ). It is designed , in live applications , to control the extent of high levels relative to the overall level . It reduces , or compresses the top , and in doing so , raises the average . It does nothing to anything below the set threshold .
Limiting can be thought of as extreme compression in the sense a compressor set to infinity to one ratio is a limiter . A compressor is like a drop ceiling : it offers resistance to going above its position , but will allow some movement while a limiter is the steel roof above the drop ceiling . Nothing goes beyond it ; it is a brick wall . In order to preserve the signal as it flows through the audio system , any errant rises in level beyond the range of the circuitry must be brought within spec , either through
reduction at the preamp gain stage or by insertion of a compressor and / or limiter .
Limiting also has a significant role when using in-ear monitoring ( IEM ). If a signal gets out of hand on a floor wedge , the performer can physically move away from the speaker , but an IEM is secured in the user ’ s ear and presents an immediate danger . For that reason , limiting is vital when using IEMs . Most wireless IEM receivers have a limiter setting . However , wearers often bypass the limiter in order to bump the signal . This is a mistake , as continued exposure to levels beyond the limiter will result in permanent hearing loss . And , any momentary spike will , at the very least , cause a temporary hearing shift and autonomous compensation that will make it difficult to perform as intended . The limiters are there for a reason : use them . Engineers can help by employing limiters at the console away from the reach of the performer .
Compression and limiting should never be used as a “ babysitter ” on a sound system . They should only be employed when circumstances dictate their use . A compressor , properly set , can settle a vocal into the mix or make the drums stand out . Limiting can preserve a listener ’ s hearing in IEM usage and prevent destruction of vital system components when something goes wrong . They are both essential tools of the audio trade .
Kent Morris Kent is a 40-year veteran of the AVL arena driven by passion for excellence tempered by the knowledge digital is a temporary state .