The Science Behind Audio Limiting - Audio Mastering
A limiter is a type of compressor designed for limiting the level of a signal to a certain threshold. It is used to reduce dynamic range and increase loudness by pushing up the signal level. They are also used to prevent signals from peaking.
One of the main differences between a limiter and a compressor is its ratio. Compressors can have variable ratios, usually adjustable between 1.5:1 and 12:1. Limiters usually have a fixed ratio which cannot be changed. Depending on the unit, the ratio can be anywhere around 20:1 and ∞ (infinity):1. Some limiters are equipped with variable attack and/or release settings. These settings can potentially change the dynamics of the song, so choosing the right limiter is crucial.
On a compressor, the threshold parameter controls the gain reduction of the audio signal. The amount of gain reduction also depends on the ratio. If the audio signal crosses the threshold level, the exceeding amount will be reduced based on the ratio of the compressor.
Limiters also have threshold control which results in gain reduction, but the main difference here is that the gain reduction in most modern limiters is not decided by a ratio control but by the threshold ceiling. In certain limiters, the signal cannot go above the threshold. Anything louder than the threshold will be reduced to match the ceiling level.
Currently, there are several hardware and software devices that do more than just limiting. These broadly go by the name “maximisers”. It is a marketing term for a device that does compression, harmonic distortion and brick-wall limiting. Each manufacturer has their own idea for a maximiser unit and come up with a device which may consist of several colouring units packed inside. If you feel the need to add color to the audio, using maximisers is one way to go. It is important to note that many maximisers do not offer much control, so there is a chance that it may just end up making the mix sound worse. Always keep listening and use your own judgment to see what sounds best.
A useful feature in many modern limiting units is the look ahead feature. In the earlier days, traditional hardware limiters had no way of looking ahead while limiting the audio. A big, fast transient peak in the audio would mean that the limiter would start acting only after a small delay, due to the limitations of how hardware limiters work. This creates a distortion in the sound or even a pumping effect, which is unavoidable.
Software limiters have a way of bypassing this effect. With the Look ahead function, they can actually “look ahead” into the waveform and predict the amount of gain reduction needed to meet the threshold level. This means that the resulting audio will not contain any distortion or other artefacts.
However, this doesn’t mean that you should always turn your look ahead knob to the maximum possible value. Shorter look ahead times are better at preserving transients and increasing the apparent loudness in the track but they might generate distortion. Longer look ahead times make the track sound less loud and might push the transients back into the mix, but they are generally safer. Very short look ahead times (less than 0.1 ms) may cause hard-clipping and generate digital distortion. To avoid this, use oversampling if possible.
The end goal of all limiting units is to essentially catch all the peaks and limit them to the threshold. However, the “peaks” the limiter is trying to catch are sample peaks, and not true peaks.
Basically, the audio interface’s A/D converter takes samples of an analog signal and interpolates these samples to represent audio digitally. The accuracy of this interpolation depends on the sample rate. Therefore, there is a difference between the actual audio signal and its digital interpolation. Due to employing this method, some peaks are not accurately interpolated (See Figure above). These are called inter-sample peaks or true peaks. Many commercial music streaming services have strict sample peak and true peak guidelines to ensure smooth playback for their audience.
True peaks can broadly be dealt with either using true peak limiters, or using standard limiters with oversampling. To monitor the true peaks, make sure you are using precise true peak meters. Also be wary of the fact that increasing the look ahead time greatly helps in controlling true peaks, but it may affect the overall loudness and punch in the track.
In the process of making your track louder, it is easy to push the limiter in the DAW to reach the desired loudness levels. Pushing the limiter very hard is not recommended though, as doing this in itself sometimes creates artifacts like distortion/pumping. Proper gain-staging is the key to avoid this. Smooth, transparent limiting is easiest with a reasonably loud mix (around -6 dBFS) without straining the limiter. In cases where a lot of gain is required, it is better to do it in stages with multiple limiters, with each doing a little bit. And while you are pushing the limiters, make sure that there is no more than 3-6 dB of gain reduction per limiter (this value also depends on the limiter unit).
Types of Limiters
In general, limiters are compressors with very aggressive settings. With ratios above 20:1, they allow only a fraction of the audio signal past the threshold level. These limiters may or may not be equipped with the look ahead feature, and this may create a slight distortion or coloring of sound when trying to limit loud transients. These limiters can be used on instrument buses such as drums and percussions to make them sound bigger.
Brickwall limiters have a fixed ratio of ∞:1. This means that absolutely nothing gets past the threshold (unless there is inter-sample clipping, for which there are peak limiters). Almost all brickwall limiters have the look ahead parameter. This is just to make sure that the gain reduction is precise to the millisecond. These limiters are usually used as the final step in most mastering chains.
Both regular and brickwall limiters work on the concept of gain reduction. It is essentially like turning the volume knob down and back up very quickly to stay under the threshold. This happens so quickly that we barely notice it most of the times. If the attack and release times are not set absolutely right, there might be some pumping artifacts between transients because of the gain reduction going on.
In the end, there is no fixed guideline for limiting. Trust your ear and experiment with different things until you find the right sound.