Desktop Audio Technology

Digital audio and MIDI principles

Francis Rumsey

• Publisher: Routledge
• Year: 2003
• ISBN:
  9781136115813
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Akustiktechnik

In this thorough introduction to the technology behind audio workstations, Dr Francis Rumsey explains not only how digital audio works but also how to make best use of its capabilities. A combined revision of his two successful titles, MIDI Systems and Control and The Audio Workstation Handbook, this new book covers recent developments such as surround sound formats, direct stream digital, new audio project formats, new interfaces and alternatives to MIDI.Desktop Audio Technology begins by setting out principles of digital audio and how these are applied in recording, replay and editing within workstations. MIDI and synthetic audio control is then covered, looking at the means by which artificial sounds can be controlled and manipulated. This is followed by explanations of hardware, including storage devices, buses, computer interfaces and audio processing options. Dr Rumsey then focuses on transferring audio between systems, including coverage of audio interfaces, networking and file formats. The next section examines audio software, providing working examples of different commercial packages that exemplify some of the concepts previously described. The final chapter considers operational issues such as recent spatial reproduction formats, consumer format mastering and quality control issues, as well as troubleshooting and systems issues.If you are a student, lecturer or practitioner in the field of audio and are looking for an authoritative technical guide to the underlying principles of digital audio and MIDI, this book is for you.Dr Francis Rumsey is a Reader in Sound Recording at the University of Surrey (UK) and a Visiting Professor at the School of Music in Piteå (Sweden). He is a Fellow of the Audio Engineering Society and a regular contributor to the AES Journal. Dr Rumsey is also author of Spatial Audio and co-author of Sound and Recording (with Tim McCormick) and The Digital Interface Handbook (with John Watkinson), all published by Focal Press.

1  Introduction to desktop audio technology

1.1 About this book

Audio in computers and other modern desktop devices is inherently digital. This is a book about how digital audio works and how to make best use of its capabilities, including control technologies that are related to the MIDI protocol. The argument for digital audio has been well and truly made by now so there is no particular attempt in this book to justify the merits of it against analog audio. It is a fact that the resources of the computer technology described in this book would not be available unless audio information was converted into a digital form, so the case is closed. The future of audio is now digital, without question, and the devices that audio engineers use are increasingly just generic computing devices that happen to be suitable. Of course audio signals need to be analog at the point where they are converted into acoustic signals, in order for them to be transmitted through the air, but storage, transfer and processing are the topics of this book.

The technology covered in this book is divided into a number of areas. The book is based to some extent on earlier books that are now out of print, bringing together the most important information on digital audio and MIDI in one place. The two are so often combined in applications now that it seems sensible to present them in one book. It also introduces a lot of more recent information on these topics because the field has moved on considerably since those books were written. In this book, therefore, the reader will find coverage of recent developments such as surround sound formats, direct stream digital, new audio project formats, new interfaces and alternatives to MIDI.

The first main chapter, Chapter 2, is concerned with the principles of digital audio and Chapter 3 discusses specific aspects of how this is applied in recording, replay and editing within workstations. Chapter 4 is all about MIDI and synthetic audio control, looking at the means by which artificial sounds can be controlled and manipulated. Chapter 5 deals with hardware of various sorts, including storage devices, buses, computer interfaces and audio processing options. Chapter 6 then concentrates on the question of how to transfer audio between systems, including coverage of audio interfaces, networking and file formats. Chapter 7 deals with audio software or applications, giving examples of different commercial packages that exemplify some of the concepts previously described in practice. The book is not about specific commercial software, however, so readers or manufacturers should look elsewhere if they want detailed coverage of these. Chapter 8 concludes by considering operational issues that may not be familiar to some readers, such as recent spatial reproduction formats, consumer format mastering and quality control issues. It also covers troubleshooting and systems issues such as synchronisation.

Coverage is primarily aimed at professional operations but it is acknowledged that a good definition of this is hard to come by and that many people who use such technology are not professionals. However, the intention is to cover the systems and concepts that apply in operations such as production and post-production, broadcasting and music. To a large extent this book stops at the point where audio leaves the studio environment. Topics such as Internet streaming and consumer delivery of audio have intentionally been avoided as these are large topics in their own right.

1.2 Audio workstations

I have called this book Desktop Audio Technology to highlight the coverage of digital audio as it applies in desktop devices such as computer-based audio workstations. There are many different definitions one could use here but for the purposes of this book it is convenient to describe an audio workstation as any computer-based device that stores and processes digital audio and/or control data (such as MIDI data). It is assumed that such devices use some form of direct access storage such as hard disks or solid-state memory, as opposed to tape, as the primary storage medium, so dedicated digital audio tape formats are not covered here.

Many devices that will be termed audio workstations here are also general-purpose multimedia workstations that may handle video and other media data. However the emphasis in this book is on the audio technology and principles involved. There is an increasing use of general purpose computing platforms for audio, both in professional and consumer environments, whereas previously there was wider use of dedicated hardware. This is largely because the processing power available on the average desktop PC is now more than adequate for dealing with multiple channels of audio recording and replay, whereas previously there was a need for hardware that was specially engineered for the purpose. So fast and capacious have desktop machines become that they can now accommodate digital signal processing of audio information (for effects, mixing, and so forth) using the main processor of the computer, at least for a number of channels and depending on the sophistication of the processing required.

The decline in popularity of dedicated audio workstations has had one notable disadvantage to the user – that being the loss of hardware dedicated to control system functions. Modern software packages running on PCs use screen-based interfaces, with keyboard and mouse controls, just like any other application, whereas dedicated devices often had physical controls dedicated to the functions required in editing and processing. This has enabled packages to be sold much more cheaply, but now there is a rapid growth in external physical controllers that restore some of the lost usability of dedicated systems. The result of this is that users can now decide whether they are content to operate a system using screen and mouse or whether they need something larger and more physical.

1.3 Audio and the computer industry

The fact that audio engineers now use general-purpose computers for much of their work highlights the position in which the field of audio engineering now finds itself. Once a clearly distinct field of endeavour with arcane dedicated equipment that needed lining up and careful handling by those in the know, there is a sense (a false one) that anyone can do audio these days. Just give them a computer and a bit of software and ‘Bob’s your uncle’, so to speak.

This situation is not unlike that encountered in the late 1980s by the typesetting, graphic design and publishing industry. Desktop publishing was taking the world by storm and all of a sudden anyone with a computer and a bit of software could produce camera ready artwork. Who needed typesetters or graphic designers any longer? We could do it all ourselves on our desks! Of course it rapidly became clear that there was still a need for people with the creative skills and the time to do it properly, they just had to learn to use the new equipment. If they insisted on sticking with hot metal or pen and paper they ran the risk of being branded as dinosaurs and losing out on a lot of new work. Sales executives, no matter how much they might fancy themselves as designers, are better off selling things and not wasting hours creating second rate brochures, for example.

The same or similar is true of audio, and there is a strong danger that the field will take some backward steps in terms of quality unless audio engineers continue to make clear what is good audio and what will not do. Audio is rapidly being swallowed up by the computer industry and many of the standard architectures and operating systems are incorporating audio features that will dictate what is possible in numerous future applications. Things like correct dithering, sampling frequency conversion, timing issues and so forth, are all crucial to achieving high quality. They are things that have been known about in relation to dedicated audio systems for years but they don’t always migrate into the computer industry, which now increasingly thinks it understands audio. So by all means we should take advantage of the economies of scale and the huge benefits that the computer industry brings to audio but we have a duty to ensure that high quality audio remains our key goal.

1.4 Audio and quality

Quality is going both ways in audio at the moment as depicted in Figure 1.1. When digital audio first appeared in consumer and professional forms it was normally fixed at a resolution of 16 bits and a sampling frequency of 44.1 or 48 kHz. This provided very good technical quality that, with good conversion hardware, arguably represented a noticeable improvement over existing analog formats in most respects. Since then there has been development both upwards and downwards in terms of quality.

Quality has gone upwards with the introduction of higher sampling frequencies and resolutions, offering bandwidths into the hundreds of kilohertz if required, and a dynamic range that equals or exceeds that of human hearing. For those audiophiles that still exist in the world this will be appealing, but the economics and practicalities of supplying them with such delights have yet to be completely demonstrated. Spatial quality is also on the increase with the introduction of surround sound and other 3D audio formats. On the other hand, quality is being pushed downwards by the need to deliver audio over potentially very low rate links such as the Internet and mobile communications. Here the question is not how good the quality can be made but how bad it can be made without anyone noticing too much.

Figure 1.1 Audio quality is developing both upwards and downwards from the original reference point of ‘CD quality’. Note that lower data rates do not automatically lead to lower quality – this depends on the encoding method used. As the data rate gets lower the method of representation tends towards object representation (description of ‘scene’ elements, requiring resynthesis by a decoder) rather than natural audio coding (coding of the original audio waveform)

Technologies such as MPEG audio coding enable audio to be represented at much lower rates than previously, with minimal impact on audio quality. It is possible to trade off audio quality and bit rate to suit a particular context. Audio quality can also be scaled in MPEG 4 so that a decoder chooses the level of representation depending on the data rate and resources available. There is also a move towards representing audio in the form of objects and control information so that the sound can be resynthesised or rendered in the replay device. This effectively breaks the link between the source and destination in terms of technical quality, because the quality in such a case is dictated primarily by the resources available in the rendering engine, as discussed further in Chapter 8. It is also affected by the completeness of the description of the sound that is supplied. This type of representation is likely to be increasingly common in virtual and synthetic audio authoring environments, for games and the like.

2 Digital audio principles

This chapter explains the fundamental principles of digital audio as they apply in computers. The aim is to aid understanding of the inner workings of equipment so that appropriate operational and technical decisions can be made.

2.1 Analog and digital information

The human senses deal mainly with analog information but computers deal internally with digital information, resulting in a need for conversion between one domain and the other at various points.

Analog information is made up of a continuum of values of some physical quantity, which at any instant may have any value between the limits of the system. For example, a rotating knob may have one of an infinite number of positions – it is therefore an analog controller (see Figure 2.1). A simple switch, on the other hand, can be considered as a digital controller, since it has only two positions – off or on. It cannot take any value in between. The brightness of light that we perceive with our eyes is analog information and as the sun goes down the brightness falls gradually and smoothly, whereas a household light without a dimmer may be either on or off – its state is binary (that is it has only two possible states). A single unit of binary information is called a bit (binary digit) and a bit can only have the value one or zero (corresponding, say, to high and low, or on and off states of the electrical signal).

Electrically, analog information may be represented as a varying voltage or current. If the rotary knob of Figure 2.1 is used to control a variable resistor connected to a voltage supply, its position will affect the output voltage (see Figure 2.2). This, like the knob’s position, may occupy any value between the limits – in this case anywhere between 0 V and + V. The switch could be used to control a similar voltage supply and in this case the output voltage could only be either 0 V or + V. In other words the electrical information that resulted would be binary. The high (+ V) state could be said to correspond to a binary one and the low state to binary zero (although in many real cases it is actually the other way around). One switch can represent only one binary digit (or bit) but most digital information is made up of more than one bit, allowing digital representations of a number of fixed values.

Figure 2.1 (a) A continuously variable control such as a rotary knob is an analog controller. (b) A two-way switch is a digital controller

Figure 2.2 Electrical representation of analog and digital information. The rotary controller of Figure 2.1(a) could adjust a variable resistor, producing a voltage anywhere between the limits of 0 and +V, as shown in (a). The switch connected as shown in (b) allows the selection of either 0 or +V states at the output

Figure 2.3 When noise is added to an analog signal, as shown at (a), it is not possible for a receiver to know what is the original signal and what is the unwanted noise. With the binary signal, as shown at (b), it is possible to extract the original information even when noise has been added. Everything above the decision level is high and everything below it is low

Analog information in an electrical form can be converted into a digital electrical form using a device known as an analog-to-digital (A/D) convertor. This must be done if...