Appendix 4 –Resolution Enhancement

50

 

 

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.

dB spl

100

80

60

40

20

016

18

-2020

10Hz

100Hz

1kHz

10kHz

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.

518 User Guide