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Thomann's Cool Online Guides: Choosing a PA System

2. Sound Sources & Signal Paths

You have to begin with the source of the sound - the instruments, voices and other devices that you want to present in an authentic and
BalancedTerm used to describe a system of audio connection that employs phase inversion noise cancellation. Using two signal wires and an earth, the audio is split and phase inverted in the second signal wire. On reaching the destination the second signal is re-inverted and combined with the first. Any electromagnetic interference is picked up equally by each signal cable and so will be cancelled out by the phase inversion. A balanced line requires balanced equipment at both ends.
balanced
mix to the audience out there in the arena. The great advantage of a using a PA is the ability it gives you to balance sounds that might otherwise drown each other out. You can have a brass section vamping away, an acoustic guitar filling out the sound, and a drummer thrashing out the beat, while still delivering loud and clear vocals. Better still, you can enrich each part with
EffectsGeneral term applied to audio processors for dynamics, time, ambience and equalisation whether in the form of 19" rack units, guitar floor pedals, or software plug-ins.
effects
, dynamics processing and equalisation to achieve a front-line sound designed to make the most of the acoustics of the room.

When broken down, PA technology is about sound transformation and re-transformation. You collect the sound at the source, and deliver it to the audience in as close a rendition of the original as possible. We’re going to consider each stage in turn:

The signal path of a typical small PA system:

  1. Sound transformation
    Firstly, the original sound is changed into an electrical signal, usually via a
    MicrophoneA device which converts airborne sound into an electrical signal.
    microphone
    . Note however, that although keyboards and
    BassThe lowest part of the audio frequency range; in popular music, a (generally) rhythmic, low frequency melodic line emphasising the root notes of the chord progression.
    bass
    guitars have electrical outputs, they are usually connected to the PA system via a DI box (Direct Injection) which amplifies the signal and ‘balances’ it to protect it from interference - see our separate guide on this subject.

     

  2. Cables
    The sound signal must then be carried to the mixing and amplification systems. Usually,
    ShieldedDenotes the fact that a device or cable is protected from electromagnetic interference by an electrical shield. In cables, this usually consists of an inner sleeve of braided copper. This, in theory, picks up any interference signal and conducts it to earth, while allowing the cable to be flexible. In electronic devices, shielding usually consists of a thin metal or foil enclosure around sensitive components, for the same purpose.
    shielded
    cables are used to protect against interference, though wireless systems are becoming ever more popular.

     

  3. Mixing and processing
    The whole sound is
    BalancedTerm used to describe a system of audio connection that employs phase inversion noise cancellation. Using two signal wires and an earth, the audio is split and phase inverted in the second signal wire. On reaching the destination the second signal is re-inverted and combined with the first. Any electromagnetic interference is picked up equally by each signal cable and so will be cancelled out by the phase inversion. A balanced line requires balanced equipment at both ends.
    balanced
    at the mixing desk, and any
    EffectsGeneral term applied to audio processors for dynamics, time, ambience and equalisation whether in the form of 19" rack units, guitar floor pedals, or software plug-ins.
    effects
    required are also added. The desk will incorporate different input types to accommodate different types of signal, such as those coming from
    MicrophoneA device which converts airborne sound into an electrical signal.
    microphones
    and those that are ‘directly’ connected.

     

  4. Reinforcement
    From the
    ConsoleA device through which audio signals are routed for mixing, monitoring, processing and re-routed for either recording, amplification or both. A console contains a number of channel strips and a selection of auxiliary, monitoring and main outputs.
    mixer
    , the sound is carried to the
    AmplifierA device for increasing the power of a signal by taking power from a supply and shaping the output to match the input signal.
    amplifier
    , sometimes known as the output stage. This is where the sound is made loud enough to fill the room comfortably without
    DistortionIn most cases distortion is an undesirable alteration to a signal which occurs when a piece of equipment is driven with a input level that is too high for its operating level. Sometimes, as in the case of guitar distortion, this can be an intentional and desirable effect.
    distortion
    . Smaller Pas sometimes combine the
    ConsoleA device through which audio signals are routed for mixing, monitoring, processing and re-routed for either recording, amplification or both. A console contains a number of channel strips and a selection of auxiliary, monitoring and main outputs.
    mixer
    and the
    AmplifierA device for increasing the power of a signal by taking power from a supply and shaping the output to match the input signal.
    amplifier
    into a single unit, although increasingly these days, amps are built into speakers.

     

  5. Transformation - again...
    The final crucial stage is the change back to an acoustic signal. This is the task of the speakers, which receive the electrical signal from the
    AmplifierA device for increasing the power of a signal by taking power from a supply and shaping the output to match the input signal.
    amplifier
    and deliver it as waves of audible sound.
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