Editing Word clock

{{Short description|Clock signal used to synchronise digital audio devices}}
{{Refimprove|date=September 2008}}

In [[digital audio]] electronics, a '''word clock''' or '''wordclock''' (sometimes '''sample clock''', which can have a broader meaning){{elucidate|date=July 2012}}<!--please add an explanation of this assertion to the article body--> is a [[clock signal]] used to [[synchronise]] other devices, such as [[digital audio tape]] machines and [[compact disc players]], which interconnect via [[digital audio signal]]s. Word clock is so named because it clocks each [[audio sample]]. Samples are represented in [[data word]]s. 

[[S/PDIF]], [[AES/EBU]], [[MADI]], [[ADAT]], and [[TDIF]] are some of the formats that use a word clock. Various [[audio over Ethernet]] systems use [[communication protocol]]s to distribute word clock.  The device which generates the word clock is the clock source for all the other audio devices.

The signal is used for synchronizing [[digital audio]] signals between devices, such as CD players, audio I/O cards, etc.<ref>{{Cite web|date=27 April 2007|title=What is digital audio synchronization? How does it differ from time code synchronization?|url=https://www.sweetwater.com/sweetcare/articles/what-digital-audio-synchronization-differ-time-code-synchronization/#:~:text=%E2%80%9CSynchronization%E2%80%9D%20is%20an%20all%20encompassing,from%20the%20%E2%80%9Cmaster%E2%80%9D%20device.|access-date=31 March 2021|website=[[Sweetwater Sound|Sweetwater]]}}</ref> It allows all the components in the signal path to process the data and remain synchronized with each other.<ref>{{Cite journal|last=Shelton|first=Tim|date=1 May 1989|title=Synchronization of Digital Audio|url=https://www.aes.org/e-lib/browse.cfm?elib=5460|access-date=31 March 2021|website=[[Audio Engineering Society]]}}</ref>

==Comparison to timecode==
Word clock should not be confused with [[timecode]]; word clock is used entirely to keep a perfectly timed and constant [[bitrate]] to avoid timing errors that can cause data transmission errors.  Timecode is [[metadata]] ''about'' the media data being transmitted. Time code can be used as an initial phase reference for [[jam sync]] using the word clock as the frequency reference.

==Over coax cable==
Professional digital audio equipment may have a word clock input or output to synchronize timing between multiple devices. Although the electrical characteristics of the word clock signal have not been completely standardized, some characteristics should always apply. Items that should remain consistent are [[Transistor–transistor logic|TTL]] level, a 75ohm output impedance, 75ohm cables and a 75ohm terminating resistor at the end of a chain or cable.

Proper termination of the word clock signal with a 75ohm resistor is important. It prevents the clock signal from reflecting back into the cable and causing false detection of extra 1's and 0's. Some digital equipment includes a switchable terminator, some include a hardwired terminator and others have no terminator at all. An unfortunate aspect is that some equipment manuals do not indicate whether a hardwired terminator is included.<ref>[http://ardour.org/files/manual/ch-synchronization.html Section 9.1.3 of Ardour manual]</ref>

A ''chain'' connection from the source through the receivers may increase jitter. Using clock distributing devices for parallel transmission is a better method. The length and quality of coaxial cables are important.

==Over AES3==
The [[AES11]] standard defines a means for carrying a word clock over an [[AES3]] connection. In this context, the word clock is known as a [[Digital Audio Reference Signal]] (DARS).

In annex B, the AES11 standard also describes common practice in transmitting and receiving a plain word clock signal. This is not an attempt to standardize it, the annex is informative only.

==See also==
* [[Phase-locked loop]]

==References==
{{reflist}}

[[Category:Digital audio]]