Chapter 1: Introduction to Interfacing

1.1 The Need for Digital Interfaces

1.1.1 Transparent Links

Digital audio and video systems make it possible for the user to maintain a high and consistent sound or picture quality from beginning to end of a production. Unlike analog systems, the quality of the signal in a digital system need not be affected by the normal processes of recording, transmission or transferral over interconnects, but this is only true provided that the signal remains in the digital domain throughout the signal chain. Converting the signal to and from the analog domain at any point has the effect of introducing additional noise and distortion, which will appear as audible or visual artefacts in the programme material. Herein lies the reason for adopting digital interfaces when transferring signals between digital devices it is the means of ensuring that the signal is carried ' transparently ', without the need to introduce a stage of analog conversion. It enables the receiving device to make a 'cloned' copy of the original data, which may be identical numerically and temporally.

1.1.2 The Need for Standards

The digital interface between two or more devices in an audio or video system is the point at which data is transferred. Digital interconnects allow programme data to be exchanged, and they may also provide a certain capacity for additional information such as 'housekeeping data' (to inform a receiver of the characteristics of the programme signal, for example), text data, subcode data replayed from a tape or disk, user data, communications channels (e.g. low quality speech) and perhaps timecode. These applications are all covered in detail in the course of this book. Standards have been developed in an attempt to ensure that the format of the data adheres to a convention and that the meaning of different bits is clear, in order that devices may communicate correctly, but there is more than one standard and thus not all devices will communicate with each other. Furthermore, even between devices using ostensibly the same interface there are often problems in communication due to differences in the level or completeness of implementation of the standard, the effects of which are numerous . Older devices may have problems with data from newer devices or vice versa, since the standard may have been modified or clarified over the years .

As digital audio and video systems become more mature the importance of correct communication also increases , and the evidence is that manufacturers are now beginning to take correct implementation of interface standards more seriously, since more people are adopting fully digital signal chains. But it must be said that at the same time the applications of such technology are becoming increasingly complicated, and the additional data which accompanies programme data on many interfaces is becoming more comprehensive there being a wide range of different uses for such data. Thus manufacturers must decide the extent to which they implement optional features, and what to do with the data which is not required or understood by a particular device. Eventually it is likely that digital interface receivers will become more ' intelligent ', such that they may analyse the incoming data and adapt so as to accommodate it with the minimum of problems, but this is rare in today's systems and would currently add considerably to the cost of them.

1.1.3 Digital Interfaces and Programme Quality

To say that signal quality cannot be affected provided that the signal remains in the digital domain is a bold statement and requires some qualification, since there will be cases where signal processing in the digital chain may affect quality. The statement is true if it is possible to assume that the sampling rate of the signal, its resolution (number of bits per sample) and the method of quantization all remain unchanged (see Chapter 2). Further one must assume that the signal has not been subjected to any processing, since filtering, gain changing, and other such operations may introduce audible or visual side effects. Operations such as sampling frequency conversion and changes in resolution (say, between 20 and 16 bits per sample) may also introduce artefacts, since these can never be 'perfect' processes. Therefore an operation such as copying a signal digitally between two recording systems with the same characteristics is a transparent process, resulting in absolutely no loss of quality (see Figure 1.1), but copying between two recorders with different sampling rates via a sampling frequency convertor is not (although the side effects in most cases are very small).

Figure 1.1: (a) A 'clone' copy may be made using a digital interconnect between two devices operating at the same sampling rate and resolution. (b) When sampling parameters differ , digital interconnects may still be used, such as in this example, but the copy will not be a true 'clone' of the original.

Confusion arises when users witness a change in sound or picture quality even in the former of the above two cases, leading them to suggest that digital copying is not a transparent process, but the root of this problem is not in the digital copying process it is in the digital-to-analog conversion process of the device which the operator is using to monitor the signal, as discussed in greater detail in Chapter 2. It is true that timing instabilities and (occasionally) errors may arise when signals are transferred digitally between devices, but both of these are normally correctable or avoidable within the digital domain. Since data errors are extremely rare in digitally interfaced systems, it is timing instabilities which will have the most likely effect on the convertor. Poor quality clock recovery in the receiver and lack of timebase correction in the convertor often allow timing instabilities resulting from the digital interface to affect programme quality when it is monitored in the analog domain. This does not mean that the digital programme itself is of poor quality, simply that the convertor is incapable of rejecting the instability. Although it is difficult to ensure low jitter clock recovery in receivers, especially with the stability required for very high convertor resolutions (e.g. 20 bits in audio), it is definitely here that the root of the problem lies and not really in the nature of digital signals themselves , since it is possible to correct such instabilities with digital signals but not normally possible with analog signals. This is discussed further in Chapter 2 and in section 6.4.3, and for additional coverage of these topics the reader is referred to Rumsey ¹ and Watkinson ^{2 , 3} .