Brickwall limiters
Limiting is generally defined as compression at a ratio higher than about 10:1, but there’s a special type of limiter called a ‘brickwall’ limiter, which guarantees no signal level increase at all when the input exceeds the specified ceiling.
This is achieved using a ratio all the way up at infinity:1, and an instantaneous attack time – a time-bending feat that can only be performed by means of a short lookahead delay, enabling the limiter to start reacting just before the actual transient occurs. The payback for this is a bit of inevitable latency that might need to be compensated for by the host DAW.
Modern brickwall limiters typically have highly program-dependent release stages, and allow you to push the levels much further than a clipper before they distort audibly. However, the smoothing of the gain reduction might make limiting short transients more audible than clipping, as the rest of the mix will be ducked momentarily as well.
If a limiter is set to zero lookahead, it will actually be clipping initial transients rather than limiting them; and the faster the release, the more distortion will creep in. That’s not necessarily a bad thing, but fast limiter settings are more likely to require oversampling to reduce aliasing.
Oversampling has another advantage, in that it tends to reduce inter-sample peaks. Oversampling literally generates samples between the existing ones, so the limiter can respond to inter-sample peaks it would otherwise be unaware of. Some limiters provide a separate True Peak limiting option, though, which will control inter-sample peaks without oversampling the signal path (probably by oversampling just the sidechain signal).
These days, it’s good practice to use True Peak limiting, with the ceiling set no higher than -1dBFS, giving some headroom to accommodate the higher peak levels that can result from lossy encoding such as MP3 or AAC.
Mastering limiters often include dither, as they’re designed to go last in the master chain. This makes sense if you’re mastering for CD, and you’re already at the correct sample rate of 44.1KHz, but if you’re creating a master intended for multiple formats at different sample rates, you might be better leaving this off. Instead, render a 24-bit file, and dither the CD versions to 16-bit later, after converting the sample rate.