PA/Studio-Peripherie

PA/Studio-Pheriphery

Despite digital and virtual sound refiners, 19"-equipment is not threatened with extinction. Especially concerning effects - it just has to be "the real thing"! Neat 19"-hardware can be operated directly and - according to taste - be put together in a rack independent of the manufacturers. In the 19"-world you differentiate between dynamics (compressors & gates), effects (echos, delays & multieffects) and equalizers (EQs) (graphic & parametric). If you own an analogue mixer, the basic features should consist of compressors and gates (e.g. dbx 166xl, Drawmer 404).

Concerning the effects an echo (e.g. Lexicon MX-400) and a tap-delay (e.g. TC Electronic D-Two) are a good starter. In order to fight room-frequencies or feedback effectively, every channel (PA & monitor) should provide a third-octave-band EQ. With this nothing can go wrong! Still digital-console-users also upgrade their consoles with high-quality hardware-echos (e.g. Lexicon PCM-91, TC Electronic M-3000] or other 19"-sound-features (e.g. Empirical Labs Distressor, SPL Transient Designer) - whatever you like and can afford.


Signal Adaption and Effect Equipment

"Unvarnished" sound can be refurbished by "effects". Adaptions and effects are equipment which account for signal adaption. Next to the crossover and equalizer there are delays, multieffects, compressors, limiters, noise gates etc.

The connection is made through the mixer´s AUX-channel. If the input of an effect is connected to the AUX-send-1, all other inputs can be heared through the AUX 1-controller and the effect connected. The output of the effect returns to the mixer through the AUX-return-access. Due to this parallel channeling, e.g. the proportion of dry (unworked) signals and echoed (worked) signals can be cut. The normal volume control knob regulates the unworked proportion, the AUX-controller the worked proportion. The AUX-return-access of a mixer is a simple channel that can be used to import other signals. Alike the output of the effects can be returned through the normal input-line of the mixer.

Primarily effects that edit the signal level can be connected to the insert-access. Equipment that is connected to such, can only be used with the appropriate channel. That´s why inserts are ideal for dynamic-processors like e.g. compressors or gates which influence the signaling exclusively. Equalizers and crossovers are usually found in the total output. They are available for mixed signals. The equalizer arranges the PA according to the acoustic conditions, especially concerning the room acoustics which means that you are dealing with a complicated sound regulation that only works if the PA itsself is not responsible for the bad sound quality - what happens quite often! The offered equipment can be devided into parametric and graphic EQs.

The graphic EQ can be seen more often. This is because of it´s simple operation and overview. Using faders which are set horizontally next to each other, the correction course of the frequence can be seen at once. The number of single frequence-bands differs from model to model and range from 10 - 31. Graphic equalizers with octave distances have 10 frequences, equipment with third-band (1/3 octave) have over 27 to 31 bands.

Parametric equalizers do not only adjust the frequence and intencity of the amplifying or cushioning, but also vary the broad band. This is one of it´s advantages. It has a very clear adjustment and therefore allows an effectful correction of the frequency-course. These numerous adjustment possibilities as well as the potentiometres can complicate or confuse the operation for a beginner. Therefore parametric EQs can be found in studios or fixed installations. In the mobile PA-area the graphic equalizer is preferred.


Compressors

Dynamic processor provide the signal adaption with drive through the input-signal. In comparison to effects they do not add anything to the original sound. A compressor, e.g. functions like an "automatic" controller: If the input-signal exceeds a certain threshold, then the amplification of the compressor is reduced, the output-signal is cushioned. In this context the compressor-rationality shows which increase of the input-signal leads to around 1dB of the output-signal. A rationality of 4:1 shows that the increase of the input-signal above the threshold around 4dB leads to an increase of only 1dB in the output. Threshold and rationality of a compressor define the gain reduction and show how much the input signal is cushioned.

Two more important parametres of a compressor are the "attack time" and the "release time". They define with which delay the compressor reacts to the overrun or shortfall of the threshold. The attack time defines the period needed to reach two-thirds of the final gain reduction. Contrawise the release time defines the period that passes after the threshold has been exceeded until the gain reduction has recaptured two-thirds of its original value.


Limiters

A limiter "limits" a signal to a certain extent due to hinder the overcontrol of the following componants (e.g. amplifier, speakers). It operates like a compressor and has the same operation features and display instruments. But it´s rationality is min. 10:1 and it´s threshold is placed in the area of top-level. The attack- and release-times of this equipment are so short that they can hardly be heared or not at all. A limiter shouldn´t change the dynamic signal considerably. This hardly happens if only single points are arranged and only account for the felt dynamic of a signal.


Expanders

Expanders are - in comparison to compressors and limiters - only active if the input-signal is under the threshold. They function oppositely - they increase the whole dynamic area and use smaller rationality-values. A limiter with the full rationality (1:20) would completely "shave" the sound. An expander with a high-level rationality (also 1:20) would turn off the channel beneath the attuned level. The example of the bass-drum-sound can explain these two effects clearly: The attack is the moment when the loud and strong blow of the bass drum is created. A limiter allows this sound to run up to a certain point as quick as a flash and then decelerates it with the adjusted rationality. The limiter cannot influence later lingering of the bass drum, as the level is below it´s threshold. Due to the low threshold and short distance between the beginning of the signal and the peak level, the increase becomes quick and hard. At the same time it runs through the region below the threshold so quickly, that it cannot be registered by the expander. After the attack when the lingering of the drum begins, the signal decreases to a level that is close to the threshold of the expander. Then the expander starts to reduce the lingering, so that the back-sound of the bass drum gets shorter. This way large vibrations of the bass drum can be avoided.


Gates

Gates are electronic switches which only allow an electronic signal to pass if it has a certain level. This level is chosen so that undesired signals bellow this value are not let through (e.g. humming or buzzing). The value of a possible gain reduction can be adjusted individually.


De-Essers

A de-esser can limit and compress the frequence of sibilant sounds (s, sh, st etc.) To do so a filter is attached infront of the limiter which filters the limited area out of the frequence spectrum. The filtered signal is lead through a limiter with defined delay time - generally de-essers work with very fast attack- and release-times. A de-esser can also be put into effect by a compressor with an equalizer bedded into the side chain. The equalizer is adjusted to emphasize the sibilant sounds extremely (2-10kHz). This way the compressor regulates down when sibilants crop up.


Feedback Killers

A big problem when using PA-systems is that the sound source and the person listening are usually in the same room. What´s worse are the monitors that the singers and musicians need to hear themselves. A large part of the monitor´s reproduced signals is feed back into the microphone what enevitably leads to acoustic feedbacks in the higher sound levels. Technically talking we differentiate between a positive and a negative feedback. During a negative feedback the input-signal is reduced due to the output signal being sent back. This is also known as a degenerative feedback. The positive feedback adds both the input- and output signals together which leads to a self-excitation of the system, especially if the returned signal is not cushioned strongly enough.

This phenomenon occurs more often in PA-systems which lead to acoustic feedbacks. The output signal that is reproduced by the speaker is recorded by a microphone, reamplified, converted through the speakers to higher signals, re-recorded by the mic and so on and so on and so on.... This chain is called a "loop reinforcement" and finally results in a loud whistling noise of the PA-system. As the connected amplifiers operate at full capacity when this feedback occurs, the connected preamps and speakers can be damaged.

To get a grip on this type of feedback there was really no choice up until the invention of Sabine´s Feedback Exterminator: A 31-band graphic EQ had to be used which was good to correct the sound, but no good for compressing frequences, as not only the feedback-frequence was compressed, but the complete sound also changed. An example: The filter of a third-octave-band equalizer is one whole octave wide. If the 1.000 Hz-fader of this equalizer is reduced to 12dB, then the capacity of the whole system is reduced to 500 - 2.000Hz by 46%, which will change the sound completely.

Not till the application of narrow-band digital filters did feedbacks become effectively evaded. The American manufacturer Sabine has been committed to this field for the past years and has become the industrial standard against feedbacks worldwide.


Multieffects

Effects are the basic ingredients of a clean mix. With the circulation of digital effect-processors nothing can get in their way not even small mixers. Multieffects offer a combination of different sounds which can be used solitary or in connection with each other. Multieffects are used for vocals and instruments likewise.