Apollo Hardware Manual
Digital Clocking Basics
28
Clock jitter affects digital transmission and digital conversion differently, as follows:
•
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.
•
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 10
kHz 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 megahertz) 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 use a good converter like Apollo 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.
Apollo provides high-quality A/D and D/A conversion for recording and/or playback. With its pristine audio path,
high-quality clocking, and simple front panel controls, it makes a great master or slave audio interface for
every digital studio, and thus provides a very cost effective way to improve overall sound quality.
