Meridian America Digital Audio Processor Meridian 518 - Manual (Page 50)

Appendix 4 –Resolution Enhancement 50
518 User Guide
The block diagram on page 7 shows the twin phase-lock-loop which 518
uses to smooth out the digital sample stream. Each loop acts as a
progressive 'flywheel' – following the average sample rate exactly – but
removing short-term speed unevenness.
Dynamic-range
The dynamic range of a channel or system is determined by the ratio
between the loudest signal it will carry without distortion and the
background noise.
For digital signals the loudest signal – full output of the channel – is called
full-scale and this level uses all the available bits.
In a system using digital components, the background noise may be
determined by:
•
the analogue input signal
•
the analogue output components like DAC, preamp or power amp
•
the smallest word-size in the digital chain.
For digital systems to be transparent, dither is needed – this is described
on page 37. Whether the dither is added at the analogue–digital
conversion stage or later, it produces a defined noise-floor.
10Hz 100Hz 1kHz 10kHz
-20
0
20
40
60
80
100
20
18
16
d
B
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p
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Frequency
Figure 2. Showing audibility of noise in dithered channels of
different wordsize at very high listening levels.
In the diagram above, an audio system has been modelled for the loudest
reasonable listening circumstance – where peak levels reach 120dB spl.
The 'U'-shaped curve is the threshold of human hearing – sounds below
this are inaudible. The three curves show the human-audible-significance
of the dither level in 16, 18 and 20 bit channels. The diagram shows that
for a digital audio system to reproduce the whole scope of human hearing
without noise – 19 bits are needed.
For interest and comparison, the diagram below compares this
circumstance with LP records where the playback is set for 115dB
maximum.