Limiter

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A limiter or limiter is a dynamics processing effects device in a rig or a plug-in that regulates the output level ( amplitude of the voltage of the audio signal ) down to a certain value. This is determined by the “Limiter Threshold”. The limiter is an extreme form of the compressor with a ratio of infinity to 1; however, a ratio of 10: 1 and higher is often referred to as limitation. Important components are an envelope tracker and a VCA .

Other parameters

Response time (English "attack")
The response time is the time interval after which the input signal is regulated down to the same level after the threshold level has been exceeded.
Fall time (English "Release")
The fall time is the time interval in which the signal is regulated back to the normal ratio of 1: 1 after falling below the threshold value.
automobile
The "Auto" function scans the signal for level peaks and sets the control times accordingly. The response time (attack) becomes longer as the input level drops.
Soft knee
This parameter rounds the "corner" of the characteristic curve at the level of the threshold value. The result is a smooth transition between the unprocessed level and the limited level.

Different types

Reverse control

If the threshold is exceeded, the limiter regulates the signal back to the value of the threshold until the attack time has elapsed. This can lead to output levels that are above the threshold value.

Override (clipping)

The limiter limits the signal by saturating the amplifier. The override limit is slightly above the threshold value. Due to the rounded shape of the clipping characteristic, the signal is first compressed and then limited. The harmonic distortion creates overtones . Tube limiters usually produce more pleasant overtones for humans than devices with transistor construction.

Forward control

This circuit is also called "Feed Forward" or "Look Ahead". More precisely, we have to speak of forward control, since there is no closed control loop as with reverse control . The signal is delayed by a short time interval (at least by the response time (attack) of the control chain, for example 0.2 ms). In this way, the VCA can reduce the audio signal to the permissible level before the limit value is exceeded. Whether this works exactly depends on the precision of the control characteristic, since there is no control as in the control loop. The psychoacoustic phenomenon of the reverse masking ensures that the control process is barely noticeable.

Designations

Brick wall limiter / clipper

These terms stand for a certain form of level limitation. A brick wall limiter or "clipper" is guaranteed not to output levels above the threshold. By increasing the input level, a high level of loudness of the audio material can be achieved quickly . In contrast to the normal limiter, a brick wall limiter takes into account not only the envelope characteristic of the past, but also that of the future, so that transients (e.g. from a pool ) are also brought below the threshold value. This “clairvoyance” is possible in digital music production because all signals are already available and future samples can be accessed without any problems. Brick-wall limiters as real-time effects devices lead the signal to be modulated through a delay line (which nowadays usually also works digitally), while the generation of the modulating envelope characteristic occurs immediately, whereby the level is already attenuated before the loud signal is modulated . This has the disadvantage that there is an audible delay, which leads to problems when performing live, because musicians hear the notes played late and thus easily get out of rhythm.

Intersample limiting

The constant increase in the loudness of music productions led to the problem of "intersample peaks". During the digital-to-analog conversion between the sampled values, levels beyond 0  dBFS (dB full scale) can occur. Level meters with a higher resolution than the audio material can display these. Some manufacturers therefore offer "intersample limiters" that sample the audio material at a high sampling rate and limit this overload.

Areas of application

Limiters are required in radio, cinema and television as well as in music production. A common application in recording is the limitation of dynamic sound sources (such as speech / song) to a defined level. Here the limiter has the function of a protective limiter to avoid distortion in subsequent devices. The limiter is also a standard tool in mastering studios. The increase in the average level ( RMS ) of a production is mainly achieved by limiting the level.

Protection of the speakers

In the live area, limiters are used to protect audio devices. This primarily concerns the protection of loudspeakers. The limiter is located in the signal chain directly in front of the output stage. The limiter takes on two tasks, on the one hand to limit the level of the average level (RMS) in order to avoid thermal overloading of the loudspeakers, whereby the parameters for the threshold value setting result from the RMS load capacity of the loudspeaker and the amplification factor of the output stage, and on the other hand Limitation of brief level peaks, primarily to avoid mechanical overloading of the transducers. The parameters for this (peak) threshold value setting result from the peak load capacity of the loudspeaker and the amplification factor of the output stage. Therefore, in principle, two limiters are required in order to implement loudspeaker protection with regard to thermal and mechanical overload. With many limiters integrated in control units, only one threshold value parameter can be set per output; There is therefore often an additional setting parameter for the crest factor (level ratio between average level [RMS] and peak level [Peak]). The setting parameter of the crest factor is also called "overshoot" by some manufacturers.

Peak limiter in power amplifiers

If a limiter is used to limit the short-term peak level, it is also referred to as a “peak limiter”. Many manufacturers install peak limiters in power amplifiers. The fixed threshold value of this limiter, however, only relates to the peak load capacity of the output stage, so there is no protection with regard to the peak load capacity of the loudspeaker.

If an output stage without a peak limiter is approached with an excessive level, overmodulation occurs. All signal components above the permissible level are cut off (clipping) , which results in both DC voltage components (through asymmetrical limitation of the positive and negative half-wave) and additional overtones (clipping distortion) in the output signal. The direct current components can lead to a thermal overload of the woofers, while the clipping distortions, the energy of which in the high frequency range can be many times the energy of the distortion-free overtone structure, more likely to endanger the tweeters and mid-range drivers. Therefore one can speak of indirect loudspeaker protection at this point. It also explains the paradoxical fact that undersized and thus overdriven power amplifiers without peak limiters are more likely to cause damage to loudspeakers than oversized power amplifiers, because the latter may deliver excessive levels for the loudspeaker, but neither DC components nor clipping distortion.

noise protection

One application for specially equipped limiters is the area of ​​noise protection. These devices are used in discos and at events and are usually required by the authorities as a requirement to limit the sound pressure level.

These special limiters (sound level limiters) can be sealed so that the level cannot be manipulated by operating elements accessible from the outside. They may only be leveled by approved experts or by measuring points in accordance with §§ 26, 28 BImSchG .

broadcast

Transmission limiters are used in the transmission output of broadcast studios. Your task is to limit the frequency modulation deviation so that neighboring transmitters do not interfere with each other. One of the best known of these limiters is the Optimod . These devices are not only used as limiters, they often also intervene in the sound image.

Premastering

Today's digital workstations process audio material with a large word length . The last step is to reduce it to the 16 bits of the audio CD . Since a level increase is possible after limiting the level peaks, information from the lower bits of the high-resolution source signal can reach the usable dynamic range of the CD. For this reason, it makes sense to summarize level limitation and word width reduction as the last step in premastering.

Devices and plug-ins

Analog classics like the UREI 1176 LN or the Universal Audio LA2A have now achieved cult status. Today they are mainly used in recording studios because of their characteristic sound, because from a purely technical point of view, the new digital devices are better.

Jünger Audio limiters are widespread in the broadcasting sector , while the TC Finalizer caused a sensation in music production when it was launched. This works with a chain of effects, the last stage of which is a softclipper, which is controversial among experts. Nevertheless, the device is still widespread today, and its radio counterpart with the name DB Max is considered the most cost-effective form of broadcast processing .

In the early 1990s, the Israeli company Waves introduced the new secret weapon in the Loudness War with a plug-in called L1 . Its successor L2 was so successful that it was also available as hardware. Today there is a large selection of plug-in limiters for the TDM , AU and VST interfaces . In addition to the native versions, the market also offers those with a higher demand for computing power, which then require DSP-supported additional hardware from TC (Powercore) and Universal Audio (UAD). Plug-ins are also available for these platforms that can emulate legendary analog devices by reproducing both the desired and the undesired properties of the technology of the time and thus come very close to the tonal character of the original.

The best-known sound level limiters are SPL2 and SPL2 TS from DATEQ . They have a tamper-proof housing and do not change the dynamics of the audio signal, only the maximum level.

See also

literature

  • Thomas Sandmann: Effects and Dynamics. Professional know-how for mixing and mastering . 7th edition. PPV-Verlag, Bergkirchen 2008, ISBN 978-3-932275-57-9 (+ 1 CD-ROM).
  • Michael Dickreiter: Handbook of the recording studio technology. Volume 1, 6th edition. KG Saur Verlag, Munich 2008, ISBN 978-3-598-11765-7 .