Audio Synchronization - Word Clock


Word clock is a synchronizing signal based on the sample rate. All digital audio equipment in one facility must be synchronized with each other for proper functionality (the internal clocks of all devices are synced together). Word clock versions are 44.1kHz, 48K, 96kHz and 192 kHz plus others. In a facility one word clock master must be defined and all other devices are slaves to this master.

The cycle duration of the wordclock signal is 100% identical with the used sample rate, i. e. 48kHz, 96 kHz etc.

top: ideal word clock input signal
down: real-world signal with slower rise and fall times and the
horizontal slope resulting from capacitive or transformer coupling

Word Clock is distributed with 75 Ohm coax cables. If distributed via AES-3, 110 Ohm twisted pair cables are used. Because no published standards for word clock exist, the compatibility between different devices is not guaranteed.

Clock Distribution Versions

Every device is connected in a star configuration. For each device an individual word clock output is needed (distribution amplifier). The star configuration is the preferred method to connect several devices to the word clock master. Word clock inputs for the star configuration should always be 75 Ohms terminated. If no internally termination is possible, an external 75 Ohm BNC terminator via BNC-T connector shall be installed.

In a parallel clock distribution multiple devices are connected to the same word clock output using BNC T-connectors in a chain as for elder Ethernet networks. Each BNC T-connector is feeding the signal to one word clock input and to the next device. All devices must use high-impedance inputs and the last point in the chain must be terminated.
Often the input impedance is not known and level problems can occur. Also with a high number of T-connectors in a chain, the reliability of the entire system is at risk.

In a serial clock distribution inputs are fed by the output of the former device in the chain. It should only be used for a limited number of devices and if possible only devices that have self-clocking interfaces.
Drawbacks are:
- clock skew, there each device can add a small amount of delay to the output signal
- signal degradation by passing the signal from one device to the next
- added jitter, there each device can add an amount of jitter from its internal clock to the signal in the chain

star topology                             parallel topology                             serial topology