In this lesson we will discuss how we alter and manipulate our audio signals in order to create a desirable effect
¶ Audio Signal Processing
Audio Signal Processing is the intentional alteration or manipulation of audio signals, or sound, often through an audio effect or effects unit. (as defined by Wikipedia)
As audio signals may be electronically represented in either digital or analogue format, signal processing may occur in either domain. Analogue processors operate directly on the electrical signal, while digital processors operate mathematically on the digital representation of that signal.
¶ Equalization
Equaliztion is the process of adjusting the balance between frequency components within an electronic signal.
The circuit or equipment used to achieve equalization is an equalizer. These devices strengthen (boost) or weaken (cut) the energy of specific frequency bands or “frequency ranges”.
Equalization (EQ) is probably the most frequently used tool in an audio engineer’s tool box. One way to think about this is, that an equalizer filter is actually a frequency specific amplifier/attenuator.
Your earliest use of equalization was likely in the form of “Tone Control”. This is usually describing the controls on a car stereo also referred to as “treble, mid and bass”. These words refer to different frequency bands, you can adjust by using these controls. These simple controls were also common in the earlies integrated audio amplifiers.
The two adjustments for equalization are Attenuating (cutting or reducing) and amplifying (boosting). This is referring to what type of adjustment you are making, either adding gain to a frequency or subtracting gain from a frequency. When adding gain to the “treble” of a car stereo you will begin to hear high frequencies clearer than lower frequencies. This is because the treble control is boosting the higher frequencies of the signal.
¶ Equalization Filters
- Fixed band
- Parametric
- Quasi-parametric or Semi-parametric
- Graphic
- Shelf
- Band-pass
- Hi-pass
- Lo-pass
Each of these different of filters have a place in your audio toolbox. In that our discussion today is simply overview of equalization, we will only hit the high points. When shadowing one of our audio engineers, pay close attention to the process they use to shape each source.
¶ Reverberation or Reverb
Reverb is created when a sound or signal is reflected causing a large number of reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space – which could include walls, furniture, people and air.(as defined by Wikipedia)
In the digital realm we frequently use devices that mimic or emulate a given acoustic environment. Today’s reverb units are most commonly digital emulations for any given room type or special effect.
¶ Delay Effect
Delay is an audio effect which records an input signal to an audio storage medium, and then plays it back after a period of time. (as defined by Wikipedia)
The delayed signal may either be played back multiple times, or played back into the recording again, to create the sound of a repeating, decaying echo. In early recording studio settings, this may have been accomplished using a tape machine. In our modern world of processing, this is accomplished using digital recording mediums within a digital emulation device.
¶ Compressors and Limiters
Compression is an electronic effect unit that reduces the volume of loud sounds or amplifies quiet sounds by narrowing or "compressing" an audio signal's dynamic range.
Some compression parameter adjustments are; Input gain, threshold, ratio, attack, hold, release, knee, and makeup gain.
¶ Audio Limiters
A limiter is a type of compressor designed for a specific purpose — to limit the level of a signal to a certain threshold. Whereas a compressor will begin smoothly reducing the gain above the threshold, a limiter will almost completely prevent any additional gain above the threshold. Any ratio from 10:1 and up, is considered limiting.
¶ Noise Gates
A Noise Gate or gate is an electronic device or software emulation that is used to control the volume of an audio signal.
Gating is the use of a gate. Comparable to a compressor, which attenuates signals above a threshold, noise gates attenuate signals that register below the threshold.
¶ Expander
An Audio Expander is a gating circuit which expands a signal in a downward, or reducing, direction when the signal falls below an established threshold.
Some expanders will additionally lift a signal that is above the defined threshold. Expanders are generally used to make quiet sounds even quieter by reducing the level of an audio signal that falls below a set threshold level.
One common use of an expander is on a vocal microphone. We commonly will place an expander on a headset microphone. When the signal falls below the established threshold (usually between words), the ambient noise level picked up by the microphone is reduced by a given ratio and a predetermined level. The signal below the threshold may be expanded, the end result is increased dynamic range by ‘expanding’ the distance between unwanted noise level and the desired signal.
¶ Black-Boxes
Black Boxes can be either an analogue or digital processing unit. Regardless of the format.
A black-box has no exact definition, in that the term is actually a variable. In other words, the term can refer to any given piece of equipment. The main thing to understand that a black-box unit is external of an audio console. With a black-box device, a piece of gear is required for each application. In other words, you must have a different piece of gear for each desired process.
In the case of an analogue device; the connection between the console and the device is most commonly achieved with a copper wire connection. In the case of a digital unit, either an analogue or a digital connection may be required. The digital protocol may be Firewire, USB, Thurderbolt or even some type of networked connection like Dante. In the case of analogue; once again copper wire would likely be the choice.
¶ Plug-ins
A Plug-In is a software based component that adds a specific feature to an existing computer program.
In the case of a digital audio console, a plug-in may be an emulation of a specific processing device. In the case of plug-ins, the digital emulation may be used as many times as your computer’s processing power will allow.
The utilization of plug-ins has greatly expanded our ability to process signals in the digital domain.