Bedienungsanleitung Meridian 518 (Seite 37 von 58) (Englisch)

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Additional technical information 37

518 User Guide

518 Operating Background

Transparency in digital systems

Digital audio gives a unique opportunity to deliver exceptional sound

quality in a recording or broadcast. In the first decade of this technology,

considerable progress was made developing the gateways – i.e. A/D and

D/A conversion. The promise of digital audio is only deliverable if the

digital audio signal is handled with complete integrity.

It is has been a common misconception among practising engineers that

the signal is robust once it is in the digital domain. This combined with a

general misunderstanding about dither, has led to working practises that

degrade the potential of the recording. Obviously, it is necessary to

understand the signal processes, and to have a way to process the signal

optimally.

The 518 allows you to achieve a transparent channel with the maximum

subjective dynamic range. This section explains the technical background.

Digital Audio and Quantisation

Digital audio is used because it is possible to transmit the data through

space (broadcast) or time (recording) without any loss of information or

generation degradation. This property of digital audio makes it extremely

attractive. The major development in digital audio has been in extending

our comprehension of the processes of digitisation and reconstruction of

the original analogue signal.

Dither

Dither is an essential lubricant if a digital audio channel that includes

processing is to remain transparent.

Transparency in a digital channel is the result of maintaining linearity at both

high and low levels and not falling prey to signal-processing distortions like,

for example, noise modulation.

It is well established that in order for linearity to be maintained, dither

must be applied at every non-trivial signal processing stage. By non-trivial,

we include all operations that modify a sample. With very few exceptions,

arithmetic operations that modify a sample result in an increase in the

number of bits representing the output result; in a fixed size channel, this

can therefore lead to truncation or quantisation effects.

In a recording process, the typical non-trivial operations will be:

• Truncation and requantisation if the A/D converter has a larger word

size than the recorder.

• Truncation and requantisation if the recorder has a larger word size

than the editor.

• Truncation, requantisation and many potential signal-processing

errors in the editor itself.

• Truncation and requantisation if the editor has a larger word size than

the final carrier.

• Truncation and requantisation mastering house processes the

material (e.g. gain shift, dc filtering etc.).

It is well established that applying dither with triangular probability

distribution of ± 1 LSB to a quantisation process results in perfect

linearity, contributes no noise modulation and permits resolution of signal

elements below the LSB. It is therefore the ideal tool to achieve

transparent processing.

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Meridian 518 Seite 37

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