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Meridian 518 Manual

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Appendix 2 – Hints on Mastering 46
518 User Guide
Appendix 2 – Hints on Mastering with noise-shaping
There are a few difficulties associated with employing noise-shaping
schemes to maximise the subjective dynamic-range of a digital audio
channel. These are now reviewed.
Vulnerability of the signal
The available dynamic range in a channel is only as wide as the
narrowest step. No matter how much care is taken with a recording, or to
maintain the maximum word-size right up to a noise-shaped dithered
truncation to e.g. 16 bits, the whole benefit can be thrown away if at that
stage a non-dithered signal process is carried out on the material. This is
not unheard of, a mastering house may e.g. choose to pass the signal
through an undithered dc filter!
This caution simply says that every process on the signal leading up to
delivery to the customer has to be reviewed for its impact on the noise-
floor and transparency. Such errors will be cumulative, but just one will
destroy any dynamic-range enhancements.
Audible results of digital errors and editing
Normal digital channels have provision for error detection and correction.
In sophisticated apparatus, short bursts of erroneous samples are
interpolated to reduce the audible effect. It is becoming more common in
CD replay systems, that the only error handling is a first-order hold; that is
when the system detects an erroneous sample, the previous valid sample
is output in its place.
This trend has partly been a response to the fact that errors on CD tend to
be single missing samples at an acceptably low rate. There are however
marked variations in this performance between players.
On normal music this strategy results in a minor modification to the
waveform and it usually passes unnoticed. The introduced sample is of
similar size to the signal, and is therefore readily incorporated or masked.
On normal material we also notice that errors occurring in silent passages
result in near-zero additions which tend to be inaudible.
Let us now consider the circumstance where a signal has been passed
through a noise-shaper with a function like Shape D. In loud passages
errors will be masked just as the normal circumstance. However channel
errors occurring in quiet or silent passages will tend to result in an audible
click (since the HF noise is boosted) with little or no material to mask it.
A similar problem pertains to editing ‘shaped’ material in quiet passages.
The requirement exists to maintain apparent continuity in the shaped
noise. We have observed clicks due to replay errors when shaped
material is re-played on cheap CD players, but no difficulty at all on
reasonable or up-market units.
Requirements for D/A conversion
Noise-shaping can be used to extend the audible dynamic range of a
channel beyond that which would be expected for a normal digital channel
of the same word-size. The essential principle by which this is achieved is
representing numbers below the LSB as an average of successive
bracketing numbers. For this system to work, the channel must exhibit
differential linearity equal to the resolution required.
We can illustrate this best by example. Let us assume that a signal has
been encoded with 20-bit precision and that a shaper type D has been
used to convey this signal onto a 16 bit CD channel. Measurement in the
digital domain shows very clearly the success with which the subjective
… 46 …


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