Appendix 4 –Resolution Enhancement | 49 |
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Appendix 4 –Resolution Enhancement
Resolution enhancement in 518 depends on six techniques.
1.De-jittering
2.Dynamic-range optimisation with Gain
3.Dynamic-range optimisation with Pre-emphasis.
4.Dynamic-range optimisation with Noise-shaped Dither.
5.DAC enhancement by using high-frequency dither.
6.DAC or channel enhancement by word-length matching.
Jitter and de-jittering
Digital audio works by representing the acoustic waveform by a series of evenly-spaced sample words. In digital-audio systems, it is quite possible
–in fact usual – for the data to pass without error. Unfortunately, where the data is passed from process to process, the exact even timing of the samples may become disturbed so that the samples arrive occasionally slightly early or late. inaccuracy in the timing of the incoming stream is called 'jitter'–and Meridian have been skilled at dealing with this since 1983.
When a jittered digital stream is fed to a DAC, the sound is degraded –
typically a listener will describe it as 'harsh', 'glassy' or 'bright' or lacking in 'spaciousness'.
The de-jittering element is of great importance to some CD players, and of enormous significance to any LaserDisc player as they tend to have relatively high-jitter output. If you want to use a LaserDisc player for serious CD listening, then the de-jittering in 518 is absolutely necessary.
Another interesting aspect of jitter is that the current AES/EBU and SPDIF interfaces use coding that causes some signal-dependent jitter to be introduced. So, we can de-jitter a stream, but if it is then passed through a series of digital interfaces then it will almost certainly benefit from de- jittering again. Now, jitter does no harm in the digital domain: it is only important that the stream be correct going into the final converter – which in a Meridian system may be a DAC like 563, a Surround Processor like 565 or a DSP loudspeaker.
Figure 1. Showing jitter spectrum of twin PLL.
