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5. Sampling issues

5.7 Word clock

All digital audio devices need a word clock to trigger all the device’s digital audio processes. If a device is used as a stand-alone component - eg. a CD player, or a digital mixing console used with only analogue connections - then the internal clock is most often a crystal oscillator, providing a stable and accurate clock signal. However, as soon as two or more digital audio devices are used in a system, their internal word clocks need to use the same frequency to assure that all samples sent by one device are actually received by the other. If for example one device’s internal word clock is running at 48,000 Hz, and it sends its data to a second device running at 47,999 Hz, each second one sample will go missing, causing a distortion in the audio signal. To prevent this, all devices in a digital audio system have to synchronize to a single, common ‘master’ word clock source - which can be any of the devices in the network, or a separate external word clock generator.

Digital audio devices can not simply be connected directly to a master word clock - any disruption in the master word clock (eg. induced by electronic circuit noise, power supply noise) or its cabling (time delays, electromagnetic interference) can spread around the system to all other devices, with the possibility to cause unstable situations. To ensure stability in a digital system, all devices in it are synchronised to the master word clock through Phase Lock Loop (PLL) circuits, following only slow changes in the master clock’s frequency, but ignoring fast disruptions. PLL’s use a Voltage Controlled Oscillator (VCO) or a more accurate Crystal VCO (VCXO) to generate the device’s internal sample clock to drive all its processes, with the frequency kept in phase with the master word clock by a phase comparator circuit that only follows the slow phase changes. VCXO PLL designs are suited for stable studio environments because they are very accurate. The downside of VCXO’s is that they can only support a limited range of external frequencies - losing synchronisation if the master clock’s frequency runs out of the specified range. Also, some VCXO based PLL designs can take a long time to synchronise. In broadcast and live systems, a broad range of sample rates have to be supported to enable the use of a variety of external synchronisation sources (eg. time code regenerated word clocks, digital tape recorders), with a fast synchronisation time to ensure a quick recovery in redundant systems. For this reason, VCO based PLL’s are often used. With the introduction of the Precision Time Protocol (PTP) in AVB networks, also used by Dante, a part of the synchronisation is taken care of by the network interfaces.

From a functionality point of view, synchronisation signals are distributed separately from the audio data. Packet switching network protocols, such as CobraNet and Dante, distribute the synchronisation signal physically on the same cabling as the audio packets, but logically as separate data packets (figure 513a). Serial digital protocols such as AES3 (AES/EBU), AES10 (MADI), and packet streaming network protocols such as EtherSound, include the synchronisation signal in the audio stream (figure 513b). At any time, the designer of the system can decide to use an alternative word clock distribution using an external master word clock generator in order to synchronise to an external source, for example a video signal. An external word clock can be connected to just one device in the networked audio system - distributed to all other devices through the original distribution method (figure 513c), or to all devices individually (figure 513d). Special care should be taken for live systems with long cable runs as the word clock signals transported over coaxial cables are prone to degeneration, potentially causing synchronisation instabilities in a system.

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