Weiss Engineering INT204, Руководство пользователя

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3
Chapter 1
Advanced Digital and
Analog Audio Concepts
Explained
1.1 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. In an analog
to digital converter (A/D) the analog signal is sampled (measured) at regular
time intervals; in the case of a CD, 44 100 times a second or every 22.675737 µ s
(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. 96 kHz to 44.1 kHz, or e.g. for putting a 96 kHz recording
onto a 44.1 kHz CD.
You may now argue that in High-End Hi-Fi there are such things as
”
over-
samplers“ or
”
upsamplers“. Yes, those are in essence sampling rate converters,