This unique book is intended for sound and digital audio engineers. It explains, in detail, the principle of perceptual enhancement of reproduced sound, from an apparatus limited in resources (such as a loudspeaker), or from audio material limited in frequency (such as compressed audio). This enhancement is based on the capabilities of human hearing, namely, that we infer, or in a way “reconstruct,” missing components from judiciously formed clues. In the low-frequency arena, this is accomplished mostly with nonlinearities, which produce frequency components that help the ear to infer the missing bass, not unlike some clever old designs for tube amplifier/output transformer assemblies. For high frequencies, the main principle is based on extrapolating from the available spectral shape.

The book contains eight chapters and one appendix. It begins with a brief introduction to the subject of bandwidth extension, mainly explaining what the meaning of bandwidth extension is in the audio framework. Chapter 1 provides concise and up-to-date information about signal processing, auditory perception, and loudspeaker particularities relevant to the design and implementation of bandwidth extension methods. The next few chapters delve into the core subject. Chapter 2 is concerned with psychoacoustic bandwidth extension for low-frequency signals, including a discussion of virtual bass and ultra bass experiments. The next chapter is concerned with physical bass enhancement, namely, when the loudspeaker can reproduce low-frequency contents. Also discussed is the combination of physical and psychoacoustical low-frequency enhancement. Chapter 4 presents design issues in loudspeakers for low-frequency bandwidth extension. The theme of chapter 5 is high frequency bandwidth! extension. Included is a discussion of nonlinear/filtering methods, similar to the ones used in some software synthesizers, as well as spectral band replication, the method adopted for the new generation of MP3 coding, and also as a tool in MPEG-4 audio. A slight criticism is that more pages should have been devoted to spectral band replication (SBR).

Chapter 6 details bandwidth extension methods for speech, particularly for coded speech. Chapter 7 is an interesting short chapter on combating noise with the help of bandwidth extension methods. Chapter 8 presents a quite detailed list of patents related to audio bandwidth extension. The appendix presents a mathematical framework for attempting to quantify the outcomes of qualitative experiments. In conclusion, this book is well written, and full of information on bandwidth extension for audio signals that is difficult to find in other sources; it is recommended to anyone involved in audio or digital audio design.