Signal flow and the public address system.
This section discusses the components of a typical sound system and the related terminology. The function of these components is discussed as it relates to the signal flow through the system, illustrating the part that each component plays in a mix. An understanding of these concepts is an enormous asset when communicating your needs to a live sound technician.
Signal flow refers to the path that electronic signals follow through the components of a PA system. This article is organized to follow that path from the input devices to the output devices and through all of the stages in between. The following diagram illustrates the signal flow through a typical sound system, as described in this article.
An input device is the interface between a sound source and the sound system. In most cases, it will consist of either a microphone or a direct input (DI) box. These devices convert the sound or the electrical signal from an instrument or voice into an electrical signal that is compatible with the components of a sound system. The characteristics and features of these devices vary widely between different manufacturers and models.
- - Microphones are generally selected to complement the qualities of the sound source that they will be used to detect (more on this in the section about microphones and loudspeakers).
- - DI boxes are essentially just transformers. The instrument’s signal passes through a wire which induces a similar current in another wire that is wound (many times) around it. Most DI boxes have a “thru” output that allows an instrument’s signal to continue (unaffected) to an amplifier after passing through the transformer’s windings. DI boxes are available with a number of features that can include: a ‘pad’, ground lift, and phase inversion. A pad is basically a heavy-duty volume control to permit the input of amplified sources. A ground lift allows the sound system’s common ground (shared by all the components in the sound system) to be isolated from the source signal’s ground. The phase inversion switch (Ø) reverses the polarity of the transformer’s output to the sound system (more on this in the acoustics section).
The cables used to connect components of a sound system have three wires (conductors), similar to the three prong plug used to supply power to grounded electrical equipment. The electrical circuit used to pass a signal between sound equipment is ‘balanced’. This means that two of the three conductors are dedicated to carrying the ‘hot’ and ‘cold’ path for the circuit carrying the signal, and the third conductor connects the ground of each component. The ground conductor is woven into a mesh surrounding the other two wires which provides an electromagnetic shield to reduce the cables’ susceptibility to electromagnetic noise.
The mixing console (mixer) is the heart of the sound system. It is the device that controls how the input signals are routed to various signal processors and output devices (loudspeakers) connected to the system. They are available in analog and digital flavours, but they are all designed with the same architecture. Mixers can appear intimidating with their dense array of knobs, buttons, faders, lights and displays. Their operation is, however, very straightforward and logical if you keep the purpose of their various functions in perspective.
- - The channel strip is at the core of the mixing console’s functionality. Each input device is plugged into a separate channel in the mixer which controls its routing through all other components of the sound system. Channel strips run vertically on most mixers and their controls are usually arranged as follows: an input gain knob at the top, followed by the knobs that control equalization, a set of knobs controlling auxiliary sends, a pan knob, and a fader.
- - The input gain knob controls the volume of the signal coming into the mixer. At this stage, the signal is set at an appropriate level for the rest of its path through the mixer and the other components of the PA system.
- - The equalizer (EQ) on the channel strip is used to modify the tone of each input signal. Equalization at this stage is most commonly used to adjust the desirable and undesirable qualities of each instrument or voice as well as those of the input device (microphone or DI box).
- - The auxiliary sends are a set of alternate mixes that are generally sent to monitor speakers and effect processors. One auxiliary send is usually designated for each monitor speaker on stage, allowing the technician to tailor the output of the various monitors to the needs of each performer. Auxiliary mixes are often assigned to effects such as reverb and delay, so that one effect processor can be used on several channels in adjustable proportions.
- - The pan knob and the fader control how a channel will be added to the main mix, which is sent to the ‘front of house’ (FOH) speakers that face the audience. The pan knob controls the relative volume that the signal will be sent to the left and right outputs, and the fader controls the overall volume of the channel in the main mix.
Beyond these core functions, there are many routing options that can vary substantially between different models of mixers. That said, the important thing to remember is that the mixing console is used to control – a) the tone of each channel and – b) the combination of input signals into a number of separate mixes, each with its own output from the mixer.
Signal processors are tools that allow you to modify a signal in real-time. This topic will be addressed in depth in another section of this series, however it is important to understand where various effects fit into the signal flow of the system. There are three types of signal processors commonly used in live sound production: filters, dynamics processors, and effects.
- - Filters refer to processors that are used to modify the tone of a signal. Filters are the core components of equalizers. There are two common types of equalizers used in a PA system. They are distinguishable by the type of controls used to apply filters to the signal.
- - The equalizers in the channel strip on a mixer are called parametric equalizers. These typically have two or three control knobs for each of a few frequency-specific ‘bands’. Each ‘band’ refers to an adjustable range of frequencies that can be boosted or reduced by a specified amount.
- - Graphic equalizers are generally self-contained units. This type of equalizer has many fixed-frequency bands, usually with a bank of faders to boost or reduce each band by a specified amount. These are typically used on any signal that is being sent to an output device (loudspeaker) in a sound system. Equalization at this stage compensates for the frequency response of the speakers and the acoustic response of the room.
- - Dynamics processors react to and modify the volume of a signal. There are three types of dynamics processors commonly used in a PA system: noise gates, compressors, and limiters.
- - Noise gates suppress a signal until it’s volume exceeds a certain threshold. These are typically used as an insert on a single channel in the mixer. An ‘insert’ sends the channel’s signal to the gate after it passes through the input gain stage in the channel strip. The signal is processed, and returns to proceed normally through the rest of the channel strip.
- - Compressors reduce the volume of a signal in proportion to how much it’s input volume exceeds a certain threshold. Like gates, these are also commonly applied as inserts on individual channels.
- - Limiters prevent a signal from exceeding a certain threshold. These operate similarly to compressors, however they are designed to instantly reduce the volume of a signal by exactly the amount that it’s volume exceeds the threshold. These are most often used to protect a system’s output devices from being damaged and/or distorting. Occasionally they are useful as inserts on individual channels as well.
- - Effects processors come in a many different flavours, the most common being reverb and delays. These types of effects generate a simulation of the echoes (and/or ‘reverberance’) that the input signal would create in an acoustic setting. Effects processors usually receive their signal from an auxiliary mix, and return the effected signal to an auxiliary input (‘aux return’) on the mixer. It is then combined with the main mix.
Output devices are the noisy part of the PA system. These components will be covered in greater depth in the section about loudspeakers. This part of the sound system consists of crossovers, amplifiers, and loudspeakers. Powered speakers typically contain all three components, as well as a limiter. Each of these components are, however, available as standalone units with adjustable controls to permit greater flexibility in their application.
- - Crossovers are specialized filters that split an input signal into a few different output signals based on the specific frequency range that a speaker’s drivers (tweeters, woofers, and subwoofers) are designed to handle. Once the signals are emitted from their respective drivers as sound waves, they recombine acoustically to form the original signal.
- - Amplifiers convert the electrical signal used amongst the sound system components into a much more powerful one to drive the loudspeakers.
- - Loudspeakers are the moving parts that push and pull the surrounding air in order to transmit the waves that we perceive as sound.
Speakers that are located on stage are referred to as monitors. These are generally connected to separate auxiliary mixes from the mixing console. In this configuration, each monitor’s mix can be tailored to the needs of performers within its range.
The speakers that face the audience are called ‘front of house’ (FOH). They are usually connected to the main outputs of the mixing console.