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15.8 Audio Formatting — Generalizing Aural CSS

A key idea in Audio System For Technical Readings (AsTeR) was the use of various voice properties in combination with non-speech auditory icons to create rich aural renderings. When I implemented Emacspeak, I brought over the notion of audio formatting to all buffers in Emacs by creating a voice-lock module that paralleled Emacs’ font-lock module. The visual medium is far richer in terms of available fonts and colors as compared to voice parameters available on TTS engines — consequently, it did not make sense to directly map Emacs’ face properties to voice parameters. To aid in projecting visual formatting onto auditory space, I created property personality analogous to Emacs’ face property that could be applied to content displayed in Emacs; module voice-lock applied that property appropriately, and the Emacspeak core handled the details of mapping personality values to the underlying TTS engine.

The values used in property personality were abstract, i.e., they were independent of any given speech engine. Later in the fall of 1995, I re-expressed these set of abstract voice properties in terms of Aural CSS; the work was published as a first draft toward the end of 1995, and implemented in Emacs-W3 in early 1996. Aural CSS was an appendix in the CSS-1.0 specification; later, it graduated to being its own module within CSS-2.0.

Later in 1996, all of Emacs’ voice-lock functionality was re-implemented in terms of Aural CSS; the implementation has stood the test of time in that as I added support for more TTS engines, I was able to implement engine-specific mappings of Aural-CSS values. This meant that the rest of Emacspeak could define various types of voices for use in specific contexts without having to worry about individual TTS engines. Conceptually, property personality can be thought of as holding an aural display list — various parts of the system can annotate pieces of text with relevant properties that finally get rendered in the aggregate. This model also works well with the notion of Emacs overlays where a moving overlay is used to temporarily highlight text that has other context-specific properties applied to it.

Audio formatting as implemented in Emacspeak is extremely effective when working with all types of content ranging from richly structured mark-up documents (LaTeX, org-mode) and formatted Web pages to program source code. Perceptually, switching to audio formatted output feels like switching from a black-and-white monitor to a rich color display. Today, Emacspeak’s audio formatted output is the only way I can correctly write else if vs elsif in various programming languages!

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