Word Clock - Nuendo DD8 Operation Manual

9. Word Clock

9.1 Operation and Technical Background
In the analogue domain one can connect any device to another device, a
synchronization is not necessary. Digital audio is different. Correct inter-
pretation of digital audio data is dependent upon a definite sample fre-
quency. Signals can only be correctly processed or transferred between
devices if these all share the same clock, otherwise digital signals are mis-
interpreted, causing distortion, clicks/crackle and even dropouts.
AES/EBU, SPDIF and ADAT optical are self-clocking (seen from a non-
technical view TDIF also, as word clock is embedded into the TDIF ca-
ble), so an additional line for word clock could be considered redun-
dant. In practice however, using several devices at the same time can
cause problems. For example, if devices are connected in a loop with-
out there being a defined 'master' device, self-clocking may break
down. Besides, the clocks of all devices must be synchronized from a
single source. Devices without S/PDIF inputs (typically playback de-
vices such as CD players) cannot be synchronized via self-clocking. Fi-
nally there are 'problematic' devices, which are nearly un-usable
without a word clock attached.
In digital studios, synchronization requirements can be met by con-
necting all devices to a central sync source. For instance, the master de-
vice could be a mixing desk, sending a reference signal - word clock - to
all other devices. However, this will only work if all the other devices
have word clock or sync inputs (e.g. some professional CD players), al-
lowing them to run as slaves. This being the case, all devices will re-
ceive the same clock signal, so there is no fundamental reason for sync
problems when they are connected together.
But word clock is not only the 'great problem solver', it also has some
disadvantages. The word clock is based on a fraction of the really
needed clock. For example SPDIF: 44.1 kHz word clock (a simple square
wave signal) has to be multiplied by 128 or 256. This signal then re-
places the one from the quartz crystal. Because of the high multiplica-
tion factor the reconstructed clock will have great deviations called
jitter. The jitter caused by word clock is typically 15 times higher as
when using a quartz based clock.
NUENDO DD 8
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