Stage Tec Entwicklungsgesellschaft für professionelle Audiotechnik mbH Berlin: NEXUS

NEXUS Digital Audio Routing and Interconnect System

Characteristics

NEXUS Single-unit

Idea & Principle

The TrueMatch Technology
	Compared with conventional A/D converters, TrueMatch provides enhanced dynamics. In modern studios converters with 64 or 128 times oversampling are used more or less exclusively. They feature excellent linearity that is principally only dependent on the accuracy of the down-sampling filters employed and can therefore be considered to be practically perfect. However, linearity curves in A/D-converter specifications still show errors that increase as levels reduce. These errors are essentially caused by the following factors: › Noise A positive variation indicates that the digitized value corresponds to an input voltage higher than the actually applied voltage. The quantization noise is added to the input voltage. › Insufficient bid width Both positive and negative deviations from linear results indicate truncation errors: The digitized value does not adequately represent the input signal. Such errors are also referred to as »quantization noise«. Consequently, current conversion technology is perfectly suitable for digitizing high-level audio signals. Conventional converters hypothetically achieve their best accuracy just below the clipping threshold. Today, THD&N values of better than -100 dBFS (i.e. 0.001%) are almost standard.
	Low-level Signals However, significantly worse results are achieved with low-level signals. For example, when a signal level is 60 dB below the clipping threshold, conventional converters will demonstrate THD&N values no better than 0.1 to 0.5% (i.e. -60 to -45 dB @ -60 dBFS), meaning that such signals are digitized with at most 8 or 10 bits utilised. Hence, it is not only the high levels that need to be considered. (No engineer will complain about a harmonic distortion of 1% at full-scale level since the speakers already produce this percentage.) Low-level signals must also be as low-distortion and low-noise as possible.
	Dynamic Enhancement As our real-world acoustic environment has a large dynamic range, and loud signals must never exceed the clipping threshold (sufficient headroom must be considered), audio technology has always pursued improved dynamics — or, in terms of the digital age, more bits. Consequently, techniques with better and better resolutions have been developed but have always been based on theoretical dynamic values: 16-bit A/D converters have a THD&N of -96 dBFS, 20-bit converters of -120 dBFS, and 24-bit converters even -144 dBFS (truncated values). However, analogue technology (or, to be more precise, the fact that harmonic distortion increases with rising levels due to analogue input stages) has never been properly taken into account.
	Converters in Real Life The illustration shows the interdependence between THD&N and level. The theoretical THD&N value of a 24-bit converter at full-scale level (0 dBFS) is approx-imately -144 dBFS. A signal with a very low level of less than -144 dBFS cannot be processed by such a converter since it is subdued by the quantization noise (see the broken line). The right-hand part of the illustration shows the additional distortion produced by the real converter as the level increases.

Stage Tec Entwicklungsgesellschaft für professionelle Audiotechnik mbH, D-12459 Berlin, Tabbertstr. 10
Phone: +49 30 639902-0, Fax: +49 30 639902-32, , © 2002-2008 Stage Tec Berlin

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