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Synchronization of Digital Audio
The AES has taken a leading role in specifying digital audio sample frequencies and in defining the nature and format of the serial interface signal. Worldwide standards have been established. As an initial step in systems engineering, a series of recommendations on synchronization practice have been developed and are now being published. The paper will explain the concept of synchronization and describe why, with digital working in the recording studio and in broadcast operations, it becomes necessary to engineer precision control of signal timing. This must be applied to items of equipment inside the studio and for independent digital plants which may be interconnected in a national network. Synchronization is in essence the control of the phase and frequency of audio bitstream clocks for signals which need to be combined. Allowance is made for fixed and random timing differences in electronic processing and interconnections. Signals are synchronized either directly or through the control of sample clocks and other timing elements from which the digital audio pulse-train is constructed. The essential practical requirements are seen to be a defined accuracy for equipment clock generators and the provision of a standard reference sync whereby apparatus may be externally locked. Alternative approaches will be described as appropriate to the full range of studio operations and reference will be made to precision techniques which have evolved in color television. The paper will consider the requirements for digital audio timing in television premises, and the complementary roles of time code and the synchronization techniques which have been introduced. Finally, the Recommendations will be described, with emphasis on practical use and taking into account that the subject will be novel to many audio engineers. The performance realized in equipment will be considered, and guidance given on meeting some stringent needs for maintaining programme quality.
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