In
digital audio
Digital audio is a representation of sound recorded in, or converted into, digital form. In digital audio, the sound wave of the audio signal is typically encoded as numerical samples in a continuous sequence. For example, in CD audio, sa ...
electronics, a word clock or wordclock (sometimes sample clock, which can have a broader meaning) is a
clock signal
In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits.
A clock sign ...
used to
synchronise other devices, such as
digital audio tape machines and
compact disc players, which interconnect via
digital audio signals. Word clock is so named because it clocks each
audio sample
In sound and music, sampling is the reuse of a portion (or sample) of a sound recording in another recording. Samples may comprise elements such as rhythm, melody, speech, sounds or entire bars of music, and may be layered, equalized, sped up or ...
. Samples are represented in
data word
In computing, a word is the natural unit of data used by a particular processor design. A word is a fixed-sized datum handled as a unit by the instruction set or the hardware of the processor. The number of bits or digits in a word (the ''word si ...
s.
S/PDIF
S/PDIF (Sony/Philips Digital Interface) is a type of digital audio interface used in consumer audio equipment to output audio over relatively short distances. The signal is transmitted over either a coaxial cable (using RCA or BNC connectors) ...
,
AES/EBU
AES3 is a standard for the exchange of digital audio signals between professional audio devices. An AES3 signal can carry two channels of pulse-code-modulated digital audio over several transmission media including balanced lines, unbalanced l ...
,
MADI Madi may refer to:
Places
* Madi, Chitwan, a municipality in Chitwan District in Nepal
* Madi Municipality, Sankhuwasabha, a municipality in Sankhuwasabha District in Nepal
* Madi Rural Municipality, Rolpa, a rural municipality in Rolpa Distr ...
,
ADAT, and
TDIF are some of the formats that use a word clock. Various
audio over Ethernet
In audio and broadcast engineering, Audio over Ethernet (sometimes AoE—not to be confused with ATA over Ethernet) is the use of an Ethernet-based network to distribute real-time digital audio. AoE replaces bulky snake cables or audio-specif ...
systems use
communication protocol
A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer scien ...
s to distribute word clock. The device which generates the word clock is the clock source for all the other audio devices.
Comparison to timecode
Word clock should not be confused with
timecode
A timecode (alternatively, time code) is a sequence of numeric codes generated at regular intervals by a timing synchronization system. Timecode is used in video production, show control and other applications which require temporal coordinatio ...
; word clock is used entirely to keep a perfectly timed and constant
bitrate
In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time.
The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
to avoid timing errors that can cause data transmission errors. Timecode is
metadata
Metadata is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including:
* Descriptive metadata – the descriptive ...
''about'' the media data being transmitted. Time code can be used as an initial phase reference for
jam sync Jam sync refers to the practice of applying a phase hit to a system to bring it in synchronization with another. The term originates from the use of this technique to replace defective time code
A timecode (alternatively, time code) is a sequen ...
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
TTL
TTL may refer to:
Photography
* Through-the-lens metering, a camera feature
* Zenit TTL, an SLR film camera named for its TTL metering capability
Technology
* Time to live, a computer data lifespan-limiting mechanism
* Transistor–transistor lo ...
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.
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 The AES11 standard published by the Audio Engineering Society provides a systematic approach to the synchronization of digital audio signals. AES11 recommends using an AES3 signal to distribute audio clocks within a facility. In this application, ...
standard defines a means for carrying a word clock over an AES3
AES3 is a standard for the exchange of digital audio signals between professional audio devices. An AES3 signal can carry two channels of pulse-code-modulated digital audio over several transmission media including balanced lines, unbalanced l ...
connection. In this context, the word clock is known as a Digital Audio Reference Signal The AES11 standard published by the Audio Engineering Society provides a systematic approach to the synchronization of digital audio signals. AES11 recommends using an AES3 signal to distribute audio clocks within a facility. In this application, ...
(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
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
References
{{reflist
Digital audio