OPERATING INSTRUCTIONS FOR THE INT202 FIREWIRE INTERFACE
Page
6
Date: 03/10
ADVANCED DIGITAL AND ANALOG AUDIO
CONCEPTS EXPLAINED
Jitter Suppression and Clocking
What is jitter and how does it affect audio quality? In the
audio field the term jitter designates a timing uncertainty
of digital clock signals. E.g. in an Analog to Digital
Converter (A/D) the analog signal is sampled (measured)
at regular time intervals; in the case of a CD, 44100
times a second or every 22.675737.. microseconds.
If these time intervals are not strictly constant then one
talks of a jittery conversion clock. In practice it is of
course not possible to generate
exactly
the same time
interval between each and every sample. After all, even
digital signals are analog in their properties and thus are
influenced by noise, crosstalk, power supply fluctuations,
temperature etc.
Hence a jittery clock introduces errors to the
measurements taken by the A/D, resulting from
measurements being taken at the wrong time. One can
easily observe that the level of the error introduced is
higher during high audio frequencies, because high
frequency signals have a steeper signal form.
A good designer takes care that the jitter amount in
his/her design is minimized as well as possible.
What type of equipment can be compromised by jitter?
There are three types: The A/D Converter as described
above, then there is the D/A Converter where the same
mechanism as in the A/D Converter applies and the third
is the Asynchronous Sample Rate Converter (ASRC). The
ASRC is not something usually found in Hi-Fi systems. It
is used by Sound Engineers to change the sample rate
from e.g. 96kHz to 44.1kHz, or e.g. for putting a 96kHz
recording onto a 44.1kHz CD.
You may now argue that in High-End Hi-Fi there are such
things as „Oversamplers“ or „Upsamplers“.
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
