Over the last fifteen years or so, we have probably all gathered some awareness of the workings of digital audio systems through books and articles — indeed, SOS has carried numerous features on the subject in its time. The problem with the written word on audio‑related subjects, however, is that you can not hear the effects being described and, just as a picture says a thousand words, a good audible example can easily be worth as much.
With this in mind, Focal Press have recently published a new CD on the topic of digital audio, as the latest addition to their Music Technology Series (see box). The new release is available in two versions: you can either get a straight demonstration CD complete with comprehensive sleeve notes, or a 'Resource Pack' which combines around 40 pages of notes and overhead transparency masters with a copy of the CD.
The lecturers' notes are unlikely to be appropriate to the more casual reader, as a fairly high degree of prior knowledge is required to make sense of them. The stand‑alone CD, however, is far more affordable, and the comprehensive sleeve notes act as a more than adequate guide to the demonstration tracks. The CD carries no fewer than 93 listening examples, which reveal the effects of different bit resolutions, truncation, dithering and noise‑shaping; sampling clock jitter; word clock synchronisation errors; error correction and interpolation; pitch‑shifting; and zipper noise from operational controls. The final nine tracks are configured as a "critical listening test" for the user to evaluate their ability to detect typical digital artefacts — something which is far harder than might at first appear!
Many of the demonstrations on the disc are quite subtle and a good monitoring system (speakers or headphones) is sometimes required to hear the characteristic effects. The sleeve notes do not provide any advice on what to listen out for, but the choice of demonstration material is well suited to the relevant artefacts and the compilation has been produced to a very high standard by Markus Erne, chairman of the Swiss branch of the Audio Engineering Society.
If you are interested in developing your understanding of digital audio, want to be better able to recognise some of its quirks, or just like listening to the hi‑fi in your anorak, this very reasonably priced CD would certainly be a worthwhile purchase. Hugh Robjohns
The Focal Press Music Technology Series also includes books on the subjects of: MIDI Systems And Control, Sound and Recording (3rd edition), Sound Synthesis and Sampling, Acoustics and Psychoacoustics, and the Audio Workstation Handbook.