A separate clock signal which is sent from a master device to all slaves to maintain tape position synchronization. In this process, the master clock device sends a signal from a dedicated word-clock output to a dedicated word-clock input on all slave devices over a separate cable, typically terminated with a BNC connector. Master clock generators are available at various accuracies, measured in ppm (parts per million): The AES defines Grade 1 clocks as having a long-term accuracy of ±1ppm and Grade 2 clocks as ±10ppm. The IEC specifications are Level I ±50ppm for “pro” equipment, Level II (consumer) accuracy at ±1,000ppm, and Level III has inaccuracy measured in days. It is important to be aware of the master clock spec on digital mixers, as some are specified with IEC Level II clocks, making attachment to high-resolution recorders problematic, and in this case, an external clock would be necessary to provide sufficient timing accuracy.
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