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1) Audio Quality

1.7 Discussing audio quality

The objective of using an audio system to process one or more sound sources is to achieve a better sound quality compared to not using an audio system. The main issue in the minds of product manufacturers, system designers and sound engineers is therefore sound. Assuming the sound source as a fixed parameter, the main tools to achieve a better sound quality are the audio system's intended changes - either built into the audio system as fixed characteristics, or available to the sound engineer as variable parameters.

However, a system's sound quality is significantly influenced by the system's audio quality. By definition, the more limitations and unintended changes the system imposes on the processed audio signals, the lower the sound quality will be. This white paper aims to provide insight in audio quality issues in networked systems to allow system designers and sound engineers to achieve the highest possible audio quality, allowing them to appreciate the system's intended changes as a basis for investments or rentals, and apply the available variable intended changes at their will to achieve the best possible sound quality.

Audio quality discussions can be conducted based on physical measurements of the audio signal and audio systems. Basing discussions on listening sessions however brings up the issue of disregarding the intended changes of the audio process - to leave only the limitations and unintended changes to discuss. Disregarding intended changes is easy if an audio system is built according to the ‘natural sound' philosophy - such a system passes audio signals as transparent (natural) as possible, and offers the system's intended changes to the sound engineer as variable components (colouring options, eg. equalizers, compressors), including the possibility to switch them off to allow audio quality assessment. Systems designed with a ‘coloured sound' philosophy have fixed intended changes, making it more difficult to assess audio quality issues because the intended changes can never be switched off.

As every system includes some amount of fixed intended changes, most prominently in the loudspeakers, listening session scripts can be used to focus on single parameters when comparing systems - the equivalent of the ceteris paribus approach in the economic sciences. If the compared systems all possess the same fixed intended changes, the listener can decide to concentrate on comparing a selected single audio quality parameter. More detailed information on this topic can be found in chapter 9: Quality assessment methods.

To facilitate system audio quality discussions, system audio quality characteristics can be represented by the characteristics of the difference between input and output of a system - the error signal. This error signal can be constant, linear with the level of a signal's frequency components, partially linear or nonlinear:

table 103: error signal types

difference type examples
constant limitation audio: HA noise, A/D quantization noise (unintended)
sound: masking noise (intended)
audio signal change linear with signal level audio: jitter noise, equalising (unintended)
sound: equalising (intended)
non linear change partially or
not linear with signal level
audio: amplifier clipping, zero-crossing distortion, compression (unintended)
sound: guitar amp distortion, compression (intended)

Figure 108A and 108B on the below presents a listing of audio quality and sound quality issues in a networked system. In figure 108A (audio quality issues in a networked audio system), a selection of limitation and unintended change error signals and their causes are presented as grey bars:

  • Constant error signals (eg. limitations) are shown with one arrow pointing to the average error level in dB(FS).
  • Linear error signals (eg. unintended changes) are shown with two arrows connected with a dotted line - one arrow pointing to the audio signal level at 0 dB(FS), the other to the error level to indicate that the error level depends on the signal level.

In figure 108B (sound quality issues in a networked audio system), a selection of available intended change processes are presented.

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