Using Multiple TTS Streams On The Emacspeak Audio Desktop

Emacspeak now uses multiple text-to-speech streams — as an example, this enables spoken notifications that do not interrupt ongoing spoken output. To make such notifications more perceivable, Emacspeak places notifications to the right of the user by leveraging Linux-ALSA features that allow one to scale the amplitude of the left and right audio channels.

Until now, Emacspeak has used a single instance of a Text-To-Speech (TTS) engine to produce all spoken feedback. An unfortunate consequence is that any spoken announcement necessarily interrupts ongoing speech; as an example, an incoming instant-message (e.g., Jabber notification) can interrupt what you're currently reading.

Emacs itself produces a large number of asynchronous messages depending on the number of processes running within Emacs; at present, all Emacs generated messages are equal though there are ongoing plans to improve this situation going forward, e.g., using package alert. With Emacspeak now able to use multiple TTS streams, arrival of package alert within Emacs should facilitate smarter handling of different categories of messages over time.

Playing multiple TTS streams simultaneously can make it hard to understand the resulting output; Emacspeak leverages underlying ALSA functionality to send notifications to a virtual ALSA device that places the auditory output mostly on the right channel. See the following paragraphs on setup/configuration. I'm presently using this on Linux with the linux-outloud voice — you need to have a copy of this TTS engine installed and working — see Voxin for details on obtaining that engine. Note: the Emacspeak espeak server does not use raw ALSA for its output — consequently, notifications produced by espeak play on both left and right channels, making it impossible to understand. The mac server may be able to support this functionality using something Mac-specific — patches welcome.

• Emacspeak now adds user-option emacspeak-tts-use-notify-stream. If this is set to t in the user's initialization file before Emacspeak is loaded, Emacspeak checks to see if the user's selected TTS engine supports multiple instances, and if so launches a second instance of the TTS engine for use as a Notification TTS Stream. See my tvr/emacs-startup.el in the Emacspeak Git Repository for an example setup.
• The Notification TTS Stream can be restarted via command dtk-notify-initialize bound to C-e d C-n. You should ordinarily not need to invoke this command.
• The Notification TTS Stream can be shut-down using command dtk-notify-shutdown bound to C-e d C-s. When the /Notification TTS Stream is not available, Emacspeak defaults to using a single TTS stream for all spoken output — i.e., no change.
• At present, emacspeak tries to use a separate Notification TTS Stream when the selected TTS engine is a software TTS running locally.
• File servers/linux-outloud/notify-asoundrc contains the .asoundrc that I am using on my thinkpad. To have Emacspeak place the Notification TTS Stream mostly on the right, the contents of that file (suitably modified for your sound card) need to be placed in file $HOME/.asoundrc. Warning: Handle with care — a broken .asoundrc can kill all audio output.
• The .asoundrc scales left and right amplitude to place the output mostly on the right — to change this behavior, you can edit the Transformation Table for virtual device tts_mono in the .asoundrc file.
• This set-up has not been tested with pulseaudio.

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