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Jitter

cambri650bd-jitter

Jitter from a low jitter Blu-ray player, the Cambridge 650BD, a low amount.

WHAT IT TELLS US

Jitter arises from noise and timing errors in digital processing. It is a degradation of the digital signal itself, rather than a conversion problem and can fall through into the outside analogue world as noise and distortion (not necessarily of the harmonic variety). We measure jitter on a player’s basic digital signal, via its S/PDIF output. What it tells us is how clean the output stream is and whether there is any serious degradation here.


Relating jitter in its many forms to sound quality is difficult and contentious, but there does appear to be a link to bass quality – perhaps surprisingly – with low jitter players commonly having tidy and well expressed bass. This may be related to common low rate clock drift error that produces up to 1nS of jitter.


Low jitter digital players usually have a sense of pristine cleanliness in their sound, a slight lack of subliminal ‘stain’ as it were, in comparative listening tests we have carried out. Since signal related jitter commonly reaches 100pS on mediocre players, but can be less than 10pS on tightly re-clocked designs where a lot of attention has been paid to the reduction of jitter in all its forms (e.g. Cyrus CD8), there are large differences here.


Generally, sound quality degradation from jitter is subtle, except in bass quality where differences are greater if not especially upsetting.



rs-upl



HOW WE MEASURE IT

We measure jitter from S/PDIF, playing a 1kHz test tone at -60dB. This provides a measure of both random and signal related jitter, the two being quoted separately. Because jitter takes numerous forms and because low rate clock drift is commonly high when all other forms are low, we do not quote the band sum value the analyser provides as this does not distinguish between types.


sonybdps370-jitter

Signal related jitter at 1kHz and low rate clock drift below 100Hz, both common forms of jitter.

 

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