Chapter 6: Practical Audio Interfacing

6.1 The Importance of Synchronization

Unlike analog audio, digital audio has a discrete-time structure, because it is a sampled signal in which the samples may be further grouped into frames and blocks having a certain time duration. If digital audio devices are to communicate with each other, or if digital signals are to be combined in any way, then they need to be synchronized to a common reference in order that the sampling frequencies of the devices are identical and do not drift with relation to each other. It is not enough for two devices to be running at nominally the same sampling frequency (say both at 44.1kHz). Between the sampling clocks of professional audio equipment it is possible for differences in frequency of up to 10 parts per million (ppm) to exist and even a very slow drift means that two devices are not truly synchronous. Consumer devices can exhibit an even greater range of sampling frequencies that are nominally the same.

The audible effect resulting from a non-synchronous signal drifting with relation to a sync reference or another signal is usually the occurrence of a glitch or click at the difference frequency between the signal and the reference, typically at an audio level around 50 dB below the signal, due to the repetition or dropping of samples. This will appear when attempting to mix two digital audio signals whose sampling rates differ by a small amount, or when attempting to decode a signal such as an unlocked consumer source by a professional system which is locked to a fixed reference. This said, it is not always easy to detect asynchronous operation by listening, even though sample slippage is occurring, as it depends on the nature of audio signal at the time. Some systems may not operate at all if presented with asynchronous signals.

Furthermore, when digital audio is used with analog or digital video, the sampling rate of the audio needs to be locked to the video reference signal and to any timecode signals which may be used. In single studio operations the problem of ensuring lock to a common clock is not as great as it is in a multi-studio centre , or where digital audio signals arrive from remote locations. In distributed system cases either the remote signals must be synchronized to the local sample clock as they arrive , or the remote studio must somehow be fed with the same reference signal as the local studio. A number of approaches may be used to ensure that this happens and they will be explained in this chapter.

Another topic related to synchronization will also be examined here, and that is the importance of short- term clock stability in digitally interfaced audio systems. It is important to distinguish between clock stability requirements in interfacing and clock stability in convertors, although the two are related to some extent.



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

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