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1) Audio Quality

1.3 Audio Processes: limitation, unintended change and intended change

An audio system changes the characteristics of an audio signal by applying its audio process. The audio process is divided into three sub-processes: limitation, unintended change and intended change.

Limitation
a system's limits in representing signals in level, frequency, and time.

Unintended change
the change of an audio signal caused by unintended processes in an audio system.

Intended change
the change of an audio signal caused by intended processes in an audio system.

An audio system's limitation poses physical limits to level, frequency and time. For example, the high-end limitation in an audio system's level range is any incapability to reach 120dB(SPL) at a listeners position, while an audio system's low-end level limitation often presents itself as a constant level error signal higher than 0dB(SPL) at the listeners position such as a noise floor. Frequency limitation includes any low and high frequency bandwidth limits within the 20Hz-20kHz frequency range, while timing limitation includes any response time or time coherence incapability of more than 6 microseconds (eg. network latency), or any time coherence problem generating audible level errors (eg. jitter). Chapter 4 presents details on the limits of the human auditory system.

An audio system's unintended change poses changes to the audio signal such as equalising, distortion, compression. These changes are not intended by the designers or operators of the audio system - they are included in the audio process because they could not be avoided due to technological, financial, time and/or expertise constraints of the designer and/or operator. Unintended changes can be represented as error signals that are (partly) linear with the audio signal, sometimes summarized by a percentage (eg. %THD) or level ratio (eg. dB gain of a filter). Most commonly, unintended changes are regarded as having a negative impact on sound quality. But in some cases, if a system's initially unintended change is perceived to have a positive effect on sound quality, the product manufacturer or system designer can actively decide to not take countermeasures - thus turning the unintended change into an intended change.

An audio system's intended change poses changes to the audio signal by intention of the designer and/or operator of the system - most commonly to improve sound quality to the opinion or expectation of the designer/operator (on behalf of an audience), or to change the sound to match an external context (eg. video postproduction). An intended change can be designed into products and systems by manufacturers and system designers as a fixed process, or offered to system operators (sound engineers) to apply as a variable parameter. More on fixed and variable intended change (‘coloured sound vs natural sound') in chapter 3: Performance & Response.

>>1.4 Quality

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