6.3 Distribution of Sync References

6.3 Distribution of Sync References

It is appropriate to consider digital audio as similar to video when approaching the subject of sync distribution, especially in large systems. Consequently, it is advisable to use a central high quality sync signal generator (the equivalent of a video sync pulse generator, or SPG), the output of which is made available widely around the studio centre , using digital distribution amplifiers (DDAs) to supply different outlets in a 'star' configuration. In video operations this is usually called 'house sync'. Each digital device in the system may then be connected to this house sync signal and each should be set to operate in the external sync mode. Because long cable runs or poor quality interfaces can distort digital signals, resulting in timing jitter (see section 6.4.3), it may be advisable to install local reference signal generators slaved to the house sync master generator as a means of providing a 'clean' local reference within a studio. Until such time as AES11 reference inputs become available on audio products one might use word clock or a video reference signal instead.

Using the technique of central sync-signal distribution (see Figure 6.2) it becomes possible to treat all devices as slaves to the sync generator, rather than each one locking to the audio output of the previous one. In this case there is no 'master' machine and the sync generator acts as the 'master'. It requires that all machines in the system operate at the same sampling rate, unless a sampling frequency convertor or signal synchronizer is used (see section 6.5).

image from book
Figure 6.2: All devices within a studio may be synchronized by distributing an AES11 reference signal to external AES sync inputs (if present).

Alternatively, in a small studio, it may be uneconomical or impractical to use a separate SPG. In such cases one device in the studio must be designated as the master. This device would then effectively act as the SPG, operating in the internal sync mode, with all other devices operating in the external sync mode and slaving to it (see Figure 6.3). If a digital mixer were to be used then it could be used as the SPG, but alternatively it would be possible to use a tape recorder, disk system or other device with a stable clock. In such a configuration it would be necessary either to use AES/EBU interfaces for all interconnection (in which case it would be possible to operate in the 'genlock' mode described above, where all devices derive a clock from their digital audio inputs) or to distribute a separate word clock or AES sync signal from the master. In the genlock configuration, the danger exists of timing errors being compounded as delays introduced by one device are passed serially down the signal chain.

image from book
Figure 6.3: (a) In small systems one of the devices, such as a digital mixer, may act as a sync reference for the others; alternatively, as shown in (b), devices may lock to their AES-format audio inputs, although they may only do so when recording. For synchronous replay, devices normally require a separate external sync input, unless they have been modified to lock to the digital audio input on replay as well.

In situations where sources are widely spread apart, perhaps even being fed in from remote sites (as might be the case in broadcast operations) the distribution of a single sync reference to all devices becomes very difficult or impossible . In such cases it may be necessary to retime external 'wild' feeds to the house sync before they can be connected to inhouse equipment and for this purpose a sampling frequency synchronizer should be used (see section 6.5).



Digital Interface Handbook
Digital Interface Handbook, Third Edition
ISBN: 0240519094
EAN: 2147483647
Year: 2004
Pages: 120

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