Word Clock and Jitter
Digital boxes share one clock, and jitter nudges sample points in time until the top end turns harsh.
Related
The problem
Once two or more digital devices are wired together, they have to agree on who keeps time. If each runs on its own crystal, sample points drift apart and the combined sound blurs. If the clock itself is unstable, sample points shift back and forth in time during conversion and produce sidebands that should not exist.
How it works
Sampling takes points at equal intervals set by the clock. Jitter is the timing error between neighbouring sample points, from tens of picoseconds to a few nanoseconds. A converter turns timing error into amplitude error while rebuilding the waveform, and the error grows with frequency, so the top end suffers first. In a multi-device system the slave tracks the master through a phase-locked loop whose bandwidth decides how much jitter it filters and how much drift it follows. Word clock travels as a square wave over BNC cable, and AES11 defines the reference-clock specification. S/PDIF and ADAT carry clock inside their optical signals, but a dedicated clock line is usually steadier.
Settings and practice
Pick one master clock and have everything else follow it, never the reverse. Use 75-ohm BNC cable, and prefer a distribution amplifier over daisy-chaining. Watch termination on long runs. USB and Thunderbolt interfaces generate clock internally or from the host, so confirm who leads when several are patched together. Put the converter on the master side rather than letting the cheapest box set the system timebase. When jitter is suspected, swap the cable and the master before blaming a device.
What to listen for
On the funk kit, listen for a dry, brittle edge on the hi-hat tail; on the Debussy étude, for a shaky attack in quiet high notes; on the combined organ tracks, for an image that will not settle. Jitter is usually subtle, but it starts to sound unclean as tracks stack up.
Listen in the library
All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).
Sources
- 时钟分配与抖动对转换的影响，依音频接口厂商技术文档核对，未抄文本
- AES11 数字音频参考时钟的用途，依公开标准说明核对，未抄原文
Used for fact-checking only; all prose is original.