Word clock
Word clock is a basic clock signal in digital audio technology , which is necessary to enable the transmission of audio data streams between digital audio devices. Word clock is used to synchronize the processing of the digital data of all devices involved. The word clock has exactly the period of a sample at a given sampling rate and is independent of the dynamic range used or the sampling depth .
Every digital audio device has an internal clock generator which, in standalone mode, provides it with the basic clock with which it processes the audio data. In the network of two or more digital audio devices, one device must specify the word clock (master) and all other devices (slaves) must adopt this clock. To do this, each digital device can be switched from its internal to an externally supplied clock signal. It is not enough to simply set the internal clock generator to the same clock frequency for every device in the network - small but unavoidable inaccuracies would cause the clock signals to run unsynchronized next to one another and drift apart. This asynchrony of the digital data would sometimes lead to clearly audible pop and click noises up to a complete signal failure.
Standardized clock and sampling frequencies (see sampling rate ) are 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz and 192 kHz. With most digital audio formats, the word clock can be derived from the data stream of the received audio data (e.g. S / PDIF , AES / EBU , ADAT , MADI , but not with TDIF ). If the word clock signal is transmitted separately from the digital audio signal, this is usually done using coaxial 75-ohm cables with BNC connectors . For this, the connected devices must have dedicated BNC word clock connections, which is only the case with professional devices.
The word clock master clock signal may be either by a daisy chain are guided like a chain to the next and from that on to the next device, etc. (must line then on the last device in the chain with a matching termination resistor are terminated), or the clock signal is in a star shape from the master distributed directly to all other devices. Since small throughput delays add up for each additional device in the chain when the clock signal is passed on in series, the clocking from device to device becomes less precise and signal dropouts can occur. Therefore, star-shaped word clock distribution is to be preferred, in which the clock master sends its synchronization signal directly to each device to be synchronized.
In principle, any digital audio device that has a digital output can act as a word clock master. Consumer devices, especially CD players, which naturally have no audio input, must always be word clock masters. On the other hand, some professional CD players are equipped with word clock inputs for external clocking.
Independent word clock generators are often used in the professional sector. These are devices that only generate word clock signals and have several output connections for star-shaped distribution.
If a source is to be connected to a system that is not synchronized with it without external clocking (e.g. CD player), a sampling rate converter must be available at the relevant input which converts the sampling rate of the unsynchronized source to the system word clock of the rest of the system .