Meridian America Meridian 518 manual Noise Shaping with dither, Dynamic Range, Pre and De-emphasis

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Additional technical information

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Unfortunately, although this process is generally understood, it is not common practise to employ correct dithering procedures at these stages. In fact, practising engineers are often under the misapprehension that dither is only needed once in a system, or that if there is sufficient noise in a recording then further dither will not be needed. This is an important misunderstanding that has so often led to lost quality in recordings.

518 provides two additive triangular probability-distribution dither forms – flat and high-pass. The essential difference between them is that H.P. dither has a lower subjective noise-floor, being about 3.5dB less audible than additive white dither.

Both spectra are illustrated in Appendix 1.

Noise Shaping with dither

Noise-shaping is a technique in which a filtered version of the quantisation error plus dither is fed back and subtracted from the quantiser input. By this technique, the total noise can be redistributed, moving the noise from spectral regions where the listener is sensitive to other regions where they are less audible.

In 518 we provide four different shapers; they are carefully selected for their different properties, each optimised for a different application.

Dynamic Range

The dynamic range of a digital channel is defined by the maximum level – often referred to as full-scale or 0dBFS – and by the quantisation noise floor which is itself determined by the sample word-length. A normal TPDF dithered quantisation introduces a benign uncorrelated addition to the noise-floor.

A background to this and a method of calculating dynamic range is given in the paper [2] referred to on page 31 of this manual.

From a users point of view, the noise-floor is determined by the number of bits; the subjective noise-floor is determined by the spectrum of the noise, i.e. the shaper used.

The most important application where maximising subjective dynamic- range occurs is in a word-size reduction. Here the correct strategy for maximising dynamic range is to use the Gain feature of 518 to raise the overall recording so that it nearly reaches full-scale. (This can be done by playing the recording and monitoring with a peak-hold indicator the maximum level; then replay increasing the gain sufficient to raise the peak to say -1dBFS).

Further maximising of the subjective dynamic-range can occur by minimising the impact of the added noise using a shaper. For example, if a 20-bit original recording achieved a peak level of -4dBFS, then transferring it to a CD using Gain +3dB and Shape B could result in a 19- bit subjective dynamic range on the CD. Transferring it without 518, i.e. by truncation, will reduce the recording to 15 bit with unpleasant low-level artefacts.

Pre and De-emphasis

The use of pre and de-emphasis as signal-processing means of optimising the subjective dynamic range of analogue channels, should be quite familiar ideas to audio engineers. In particular, pre and de-emphasis have been used in channels where the analogue noise level showed an increasing level with frequency, e.g. magnetic tape, shellac or vinyl grooves and Frequency Modulation. In all these cases, a well- documented property of music and speech is exploited, namely that for material microphoned at normal listening positions, and of acoustic

518 User Guide

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Contents 1992, 1993 Contents Appendix 2 Hints on Mastering Simple rules for usingAppendix 4 -Resolution Enhancement Designed & manufactured in the UK by Sales and service in the UKSales and service in the USA AcknowledgementsAbout the 500 Series IntroductionAbout this manual Conventions used in this manualUnpacking Installing yourGeneral precautions ConnectionsPlanning setup Whats inside a 518?Overview as Preamp Type What about analogue sources? What else do I need to know?Meridian DSP speaker improvement Overview as Resolution Enhancement Processor TypeCD or LaserDisc playback improvement Meridian Surround improvement Overview as Mastering Processor Type Overview as Multimedia Processor TypeCustomising features ∙ Normal ∙ Standby ∙ Type ∙ ConfigBasic operation Switching onStarting off with StandbySelecting a source Adjusting the volumeOperating 518 as a Control Unit Type OutShape a DitherFlat Shape BDisplay Advanced operation using the Remote Control Muting the sound Operating 518 as Resolution Enhancer TypeVolume control Menu KeysMemory With the Remote ControlOperation summary Config control settings Restoring default settingsConfig an overview Customising 518, an overviewConfig sources and inputs How to customiseCustomising, general procedure How to change to Type or ConfigMaking changes to an existing setup Customising using TypeHow to change back to Normal Setting Type ControllerConfig Customising using ConfigHow to use Config Selecting TypesSetting the product address for Selecting the controller modeSetting the system address for Setting the Standby mode forSetting the Volume mode for Setting the Blank display option forSetting the Emphasis option for Setting the Volume displayVol. dB Setting up sourcesChoosing source logos Choosing the address of a source product Changing the input for a sourceChoosing the Comms type for a source product Digital input connections ConnectionsDigital audio connections Digital output connectionTo connect to other Meridian 500 Series equipment Connecting to Meridian Digital speakersCommunications connections ConPart Mastering ProcessorMeridian 500 Comms warning DescriptionAs a Mastering or Multimedia Processor Type D1 +2Emph +2 ShpB +2Making Recordings Transfer to editor Transfer to the final carrierNoise shaper characteristics Shaper Advantage 44.1kHz 48kHz Normal Pre-emphasisDigital Audio and Quantisation Operating BackgroundTransparency in digital systems DitherPre and De-emphasis Noise Shaping with ditherDynamic Range No change in sample size Simple rules for usingStep-down in sample word size Step up in dynamic rangeSpecification Additional technical informationHelp Cleaning Maintenance & ServicePart Appendix 1 Noise Shapes NoiseAbove noise spectrum for Shape B at 44.1kHz 0Hz 5kHz 10kHz 15kHz 20kHz Appendix 2 Hints on Mastering with noise-shaping Vulnerability of the signalAudible results of digital errors and editing Requirements for D/A conversionAppendix 2 Hints on Mastering GoTo Functions Appendix 3 RS232Handset Functions Appendix 4 -Resolution Enhancement Jitter and de-jitteringDynamic-range FrequencyDynamic-range optimisation with Gain Dynamic-range optimisation with Pre-emphasisShowing CD pre- and de-emphasis curves Dynamic-range optimisation with Noise-shaped Dither DAC enhancement by using high-frequency dither DAC or channel enhancement by word-length matchingContents IndexIndex User Guide User Guide