Word clock
Encyclopedia
A word clock or wordclock (sometimes sample clock, which can have a broader meaning) is a clock signal
(not the actual device
) used to synchronise other devices, such as digital audio tape
machines and compact disc
players, which interconnect via digital audio
. S/PDIF
, AES/EBU
, ADAT
, and TDIF are some of the formats that use a word clock. Various audio over Ethernet
protocols
use broadcast packets for the word clock. The device which maintains the word clock on a network is the master clock
.
Word clock should not be confused with Time Code
; word clock is used entirely to keep a perfectly-timed and constant bitrate
to avoid data errors
. The word clock generator, usually built-in to analog-to-digital converter
s, creates digital
pulse
s which contain no other data
, and is considered essential to avoid frequency drift
between the internal oscillators of each device. Timecode is actual data (technically metadata
) about the media
content
being transmitted, and is optional, being sent in a higher layer
.
Most professional digital audio equipment have a "Word Clock" input and/or output to synchronize timing between multiple devices. Word clock is similar to the left/right clock (LRCK) in a hardware audio codec
as they both run at the sample frequency. And although the electrical characteristics of the word clock signal have not been completely standardized some characteristics should always apply. Things that should remain consistent are 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 include a switchable terminator, some include a hardwired terminator and others have no terminator at all. The unfortunate part is that some equipment manuals do not indicate whether a hardwired terminator is included or not.
The phase
of the digital word clock signal is another important factor. The wrong phase can cause the order of the left and right inputs of a stereo pair or the order of paired input channels to be reversed.
Further technical references can be found on the Audio Engineering Society website and in AES Standard AES11-2009 (Revision of AES11-2003).
Clock signal
In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...
(not the actual device
Measuring instrument
In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...
) used to synchronise other devices, such as digital audio tape
Digital Audio Tape
Digital Audio Tape is a signal recording and playback medium developed by Sony and introduced in 1987. In appearance it is similar to a compact audio cassette, using 4 mm magnetic tape enclosed in a protective shell, but is roughly half the size at 73 mm × 54 mm × 10.5 mm. As...
machines and compact disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
players, which interconnect via digital audio
Digital audio
Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
. S/PDIF
S/PDIF
S/PDIF is a digital audio interconnect used in consumer audio equipment over relatively short distances. The signal is transmitted over either a coaxial cable with RCA connectors or a fiber optic cable with TOSLINK connectors. S/PDIF interconnects components in home theaters and other digital high...
, AES/EBU
AES/EBU
AES3 is the standard used for the transport of digital audio signals between professional audio devices. It is also known as AES/EBU and is published by the Audio Engineering Society and as part of IEC 60958. It was developed by the AES and the European Broadcasting Union and first published in...
, ADAT
ADAT
Alesis Digital Audio Tape or ADAT is a magnetic tape format used for the simultaneous digital recording of eight analog audio or digital audio tracks at once, onto a Super VHS tape that is used by consumer VCRs.- History :...
, and TDIF are some of the formats that use a word clock. Various audio over Ethernet
Audio over Ethernet
In audio engineering and broadcast engineering, Audio over Ethernet is the use of an Ethernet-based network to distribute real-time digital audio....
protocols
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
use broadcast packets for the word clock. The device which maintains the word clock on a network is the master clock
Master clock
A master clock is a precision clock that provides timing signals to synchronize slave clocks as part of a clock network. The master clock in such installations is controlled by an accurate quartz crystal oscillator, usually referenced to an external frequency standard such as MSF, which is part of...
.
Word clock should not be confused with Time Code
Time code
A timecode is a sequence of numeric codes generated at regular intervals by a timing system.- Video and film timecode :...
; word clock is used entirely to keep a perfectly-timed and constant bitrate
Bitrate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
to avoid data errors
Data corruption
Data corruption refers to errors in computer data that occur during writing, reading, storage, transmission, or processing, which introduce unintended changes to the original data...
. The word clock generator, usually built-in to analog-to-digital converter
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
s, creates digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
pulse
Pulse
In medicine, one's pulse represents the tactile arterial palpation of the heartbeat by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed against a bone, such as at the neck , at the wrist , behind the knee , on the inside of the elbow , and near the...
s which contain no other data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
, and is considered essential to avoid frequency drift
Frequency drift
In electrical engineering, and particularly in telecommunications, frequency drift is an unintended and generally arbitrary offset of an oscillator from its nominal frequency. Causes may include changes in temperature, which can alter the piezoelectric effect in a quartz crystal, or problems with...
between the internal oscillators of each device. Timecode is actual data (technically metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
) about the media
Digital media
Digital media is a form of electronic media where data is stored in digital form. It can refer to the technical aspect of storage and transmission Digital media is a form of electronic media where data is stored in digital (as opposed to analog) form. It can refer to the technical aspect of...
content
Content (media and publishing)
In media production and publishing, content is information and experiences that may provide value for an end-user/audience in specific contexts. Content may be delivered via any medium such as the internet, television, and audio CDs, as well as live events such as conferences and stage performances...
being transmitted, and is optional, being sent in a higher layer
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
.
Most professional digital audio equipment have a "Word Clock" input and/or output to synchronize timing between multiple devices. Word clock is similar to the left/right clock (LRCK) in a hardware audio codec
Audio codec
All codecs are devices or computer programs capable of coding or decoding a digital data stream or signal.The term audio codec has two meanings depending on the context:...
as they both run at the sample frequency. And although the electrical characteristics of the word clock signal have not been completely standardized some characteristics should always apply. Things that should remain consistent are 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 include a switchable terminator, some include a hardwired terminator and others have no terminator at all. The unfortunate part is that some equipment manuals do not indicate whether a hardwired terminator is included or not.
The phase
Phase
-In physics:*Phase , a physically distinctive form of a substance, such as the solid, liquid, and gaseous states of ordinary matter**Phase transition is the transformation of a thermodynamic system from one phase to another*Phase...
of the digital word clock signal is another important factor. The wrong phase can cause the order of the left and right inputs of a stereo pair or the order of paired input channels to be reversed.
Further technical references can be found on the Audio Engineering Society website and in AES Standard AES11-2009 (Revision of AES11-2003).
See also
- data word
- clockClockA clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
- MPEG transport stream
- digital-to-analog converterDigital-to-analog converterIn electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
- phase locked loop