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Meridian 518 Manual

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Additional technical information 38
518 User Guide
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
… 38 …


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