Wells JASON Owner's Manual page 7

OWNERS MANUAL FOR WEISS JASON CD TRANSPORT
Yes, those are in essence sampling rate converters, however in a well designed system these
converters employ a synchronous design, where jitter does not play any role. Of course a
conversion between 96kHz and 44.1kHz as in the example above, can be done in a synchronous
manner as well. An ASRC in fact is only required either where one or both of the sampling
frequencies involved are changing over time („varispeed" mode of digital audio recorders) or where
it is unpractical to synchronize the two sampling frequencies.
So basically in Hi-Fi jitter matters where there are A/D or D/A converters involved. CD and DVD
players are by far the most numerous type of equipment employing D/A converters. And of course
stand-alone D/A converters. Jitter, being an analog quantity, can creep in at various places. The
D/A converter built into CD or DVD players can be „infected" by jitter through various crosstalk
mechanisms, like power supply contamination by power hungry motors (spindle / servo) or
microphony of the crystal generating the sampling clock or capacitive / inductive crosstalk between
clock signals etc.
In the standalone D/A converter jitter can be introduced by inferior cables between the source
(e.g. CD player) and the D/A converter unit or by the same mechanisms as described above except
for the motors of course.
In the case of a stand-alone D/A converter (as the MEDEA), one has to take two different jitter
contamination pathes into account.
One is the internal path where internal signals can affect the jitter amount of the sampling clock
generator. Here, all the good old analog design principles have to be applied. Such as shielding
from electric or magnetic fields, good grounding, good power supply decoupling, good signal
transmission between the clock generator and the actual D/A chip.
The other path is the external signal coming from the source to which the sampling clock has to be
locked. I.e. the D/A converter has to run synchronous to the incoming digital audio signal and thus
the frequency of the internal sampling clock generator has to be controlled so that it runs at the
same sampling speed as the source (CD player). This controlling is done by a Phase Locked Loop
(PLL) which is a control system with error feedback. Of course the PLL has to be able to follow the
long term fluctuations of the source, e.g. the sampling rate of the source will alter slightly over
time or over temperature, it will not be a constant 44.1kHz in the case of a CD. But the PLL should
not follow the short term fluctuations (jitter). Think of the PLL as beeing like a very slow-reacting
fly-wheel.
7
Page: Date: 10/04 /dw