Limiter

A limiter or limiter is a dynamic effect machined device that down regulates the output level ( amplitude of the voltage of the audio signal ) to a specified value. This is determined by the " Limiter Threshold" ( threshold ). The limiter is an extreme form of the compressor with a ratio ( control ratio ) from infinity to 1, often, however, even at a ratio of 10:1 and above spoken of limitation. Important components are a envelope follower and a VCA.

• 3.1 " Brickwall Limiter " / " Clipper "
• 3.2 " Sample Inter limiting"

• 4.1 speaker protection
• 4.2 Peak limiters in amplifiers
• 4.3 Noise Protection
• 4.4 Broadcasting
• 4.5 Pre-mastering

Other parameters

Various types

Back control

When the threshold is exceeded, the limiter regulates the signal until the expiration of the response time (English " attack time " ) back to the value of the threshold. This can lead to output levels that are above the threshold.

Overload ( clipping)

The limiter limits the signal by the saturation of the amplifier. The clipping level lies slightly above the threshold. By a rounded shape of the Clippingkennline the signal is first compressed and then limited. This creates the harmonic distortion overtones. Limiter with tube technology produce for the people most pleasant overtones than devices in transistor design.

Feedforward

This circuit is also called " feedforward " or " Look Ahead". Precision must actually be spoken of feed-forward control, since no closed-loop feedback control as in the present. The signal is delayed by a short time interval ( at least the response time (attack ) of the timing chain, for example, 0.2 ms). Thus, the VCA reduce the audio signal from exceeding the limit on the permissible level. If this works exactly dependent on the precision of the control characteristic line, since no control is performed as in the control loop. The psychoacoustic phenomenon of backward masking ensures that the control process is hardly noticed.

Designations

"Brick Wall Limiter " / " Clipper "

These are the names for a certain form of level limiting. A "Brick Wall Limiter " or " Clipper " are guaranteed no level above the threshold of. By reinforcing the input level can be achieved as quickly a high loudness of the audio.

"Inter- sample limiting"

Due to the constant increase in the loudness of music productions have been encountered the problem of "inter sample peaks '. So it may come in the digital- to-analog conversion between samples to levels beyond 0 dBFS (dB Full Scale). Water level indicators with higher resolution than the audio material can represent. Some manufacturers offer therefore "Inter Sample limiter ", which sample the audio at a high sampling rate and limit this distortion.

Applications

Both radio, cinema and television as well as music production limiters are required. A common use when recording is to limit dynamic sound sources ( such as voice / vocals) to a defined level. Here, the limiter has the function as Schutzbegrenzer to avoid distortions in subsequent devices. Also in mastering studios, the limiter is a standard tool. The increase in the average level (RMS ) of a production is achieved mainly by the level limit.

Speaker protection

In the live range delimiters are used to protect audio equipment. This mainly concerns the protection of speakers. Here is the limiter in the signal chain right before the power amp. In this case, the limiter takes two tasks. Firstly, the level limit of the average level ( RMS) to avoid thermal overload of the loudspeaker. This results in the parameters for the threshold value of the RMS handling of the speaker and the gain of the amplifier. On the other hand, the level limit of momentary peak levels (peak ) in order to avoid mechanical overload of the transducer in the first place. The parameters for this (peak) threshold adjustment arising from the peak capacity of the loudspeaker and the gain of the amplifier. Therefore, one needs, in principle, two limiters to implement a speaker protection with regard to the thermal and mechanical stress. With many built- in control units limiters can only specify a threshold parameter for each output; often is, therefore, an additional adjustment parameters for the crest factor ( ratio level between average level [ RMS ] and peak [ peak ] ) exists. The setting parameters of the crest factor is referred to by certain manufacturers, also known as " overshoot ".

Peak limiters in amplifiers

If a limiter for limiting the short-term peak level ( Peak) are used, one also speaks of clippers (English "peak limiter" ). Many manufacturers who use clippers in power amplifiers. The fixed threshold limiter this only relates to the peak power of the output stage, a protection in terms of peak power handling of the loudspeaker is therefore not given.

If a stage without clippers approached with excessive level, it comes to overdrive. Here are all signal components cut above the permitted level ( clipping), whereby both DC components ( due to asymmetric boundary of the positive and negative half-wave) as well as additional harmonics ( Clippingverzerrungen ) arise in the output signal. The DC components can lead to thermal overload of the woofer, while the Clippingverzerrungen whose energy can be a multiple of the energy of distortion-free overtone in the treble range, endanger more treble and midrange. Therefore, one can speak of an indirect speaker protection at this point. It also explains the paradoxical at first glance fact that undersized and thus overdriven amplifiers with no clippers more damage to speakers cause as oversized output stages, as the latter, although, where appropriate, provide excessive level for the speakers, but no DC components still Clippingverzerrungen.

Noise

An application for specially equipped limiter is the noise protection. These devices come in nightclubs and at events for use and are usually required by the authorities as a support for limiting the sound pressure level.

These special limiter ( sound level limiters ) are sealable, so that the level can not be manipulated by externally accessible controls. You are only allowed by authorized experts or by measuring points according to § § 26, 28 BimSchG be leveled.

Broadcasting

In the transmit output of broadcast studios broadcast limiter can be used. Your task is to limit the stroke of frequency modulation, so do not interfere with adjacent stations are mutually exclusive. One of the best known of these limiters is the Optimod. These devices are not only used as a delimiter, often in addition also intervened in the sound.

Pre-mastering

Today's digital audio workstations edit material with a high word width. As a last step the reduction takes place on the 16 bits of the audio CD. Because after limiting the peaks a level boost is possible to get information from the lower bits of the high-resolution source signal in the usable dynamic range of the CD. For this reason, it is useful to have level limit and word length reduction as the last step of the pre-mastering.

Devices and plug-ins

Analog classics like the UREI 1176 LN or the Universal Audio LA2A have reached cult status. In recording studios today they are used mainly for their characteristic sound, because technically speaking, the new digital devices are better.

In the radio range limiter of Jünger Audio are widely used in music production, the TC Finalizer made ​​at its launch stir. This works with a chain of effects that last step is a controversial among experts Softclipper. Nevertheless, the device is still widespread today, and his Runkfunk counterpart named DB Max is considered cost-effective form of transmission processing.

In the early 1990s, the Israeli company Waves presented with a plug-in called L1 before the new secret weapon in the Loudness War. Her successor L2 was so successful that it gave him as hardware. Today there is a wide range of plug-in limiters for the interfaces TDM, AU and VST. In addition to the native versions of the market also offers those with a higher need for computing power, need the then DSP -based add-on hardware from TC ( PowerCore ) and Universal Audio ( UAD ). For these platforms are also plug-ins available, can emulate the legendary analog devices by mimicking addition to the desired and the undesired properties of the technology at that time and thus come very close to the sonic character of the original.

The best-known sound level limiters are SPL2 and SPL2 TS of DATEQ. They have tamper-proof cabinet or modify the dynamics of the audio signal, but only the maximum level.