The Technical Stuff
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Clock jitter in digital transmission
can be caused by a bad source clock, inferior cabling or
improper cable termination, and/or signal-induced noise (called “pattern-jitter” or “symbol-
jitter.”) Digital signal formats like AES/EBU, S/PDIF, and ADAT all embed a clock in the digital
signal so the receiving device can synchronize to the transmitted data bits correctly. The clock
used for data recovery is extracted from the signal using a clock synchronization circuit called a
phase-locked-loop
(PLL). This data-recovery PLL must be designed to respond very quickly to
attenuate high-frequency jitter and avoid bit errors during reception. This clock from the data-
recovery PLL cannot be used to generate the clocks used for digital conversion without further
clock conditioning! This is a very common design flaw in most low- and mid-range digital
converters.
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Clock jitter in digital conversion
is what most people refer to when they discuss jitter. It’s easily
observed in a digital signal by looking at its spectrum in the frequency domain. A jittery signal will
have “side-lobes” around each frequency and/or spurious tones at random, inharmonic
frequencies. Usually, the jitter will be worse with higher signal frequencies. You can test your
converters by sampling a high-quality 10kHz sine wave, and viewing it in the frequency domain
(available with any good wave editing software package).
All modern over-sampling digital converters require a clock (called “m-clock”) that is many times (typically
several MHz) higher than the sample clock. M-clock is easy to generate when the converter is the clock
master, but quite difficult to generate correctly when the converter needs to sync to an external clock.
External clock typically comes from a dedicated word clock input, or is extracted from the incoming digital
AES/EBU, S/PDIF or ADAT signal. Word clock cannot be used by the converters until it is multiplied up to the
m-clock rate. This requires a PLL or other frequency multiplier circuit which will either be cheap and jittery,
or expensive and clean, depending on who makes the converter. As we said earlier, the clock recovered from
the digital inputs is unsuitable for use as the converter’s m-clock, but because it’s conveniently at the
same frequency, many designers don’t bother cleaning up this signal.
Since the clock recovery, clock multiplier, and clock conditioning circuitry define the jitter for analog
conversion, no external clock source can clean up the jitter introduced by these circuits, regardless of how
perfect the external source clock is. The best they can do is avoid making it any worse, but this is hardly
worth the cost: It’s much better (and less expensive) to get a good converter than it is to try and fix a bad
one with an expensive master clock. The only reason to spend money on a high-quality master clock is to
ensure that multiple devices are synchronized correctly. This is essential for working with audio for
film/video, or when synchronizing multiple high-quality converters. A poor master clock can also affect
imaging and clarity in a multi-track environment.
The 2192 provides high-quality analog conversion for recording and/or playback, master clock generation,
resynchronization and distribution, and digital transcoding (format conversion). With its pristine audio
path, high-quality clocking, and simple front panel controls, it makes the perfect master audio interface
for every digital studio, and thus provides a very cost effective way to improve overall sound quality.
