[Accessibility] Updated requirements document
pdm at brailcom.org
Mon Nov 15 14:42:50 PST 2004
Here is an updated version of the requirements document. It is based on
a rewrite of the document by Willie Walker (we also had discussion about
the requirements in private, resulting in some changes and additions to
them). It should reflect the current state of the whole discussion, so
if something is wrong or missing, tell me.
For now, I put everything into a single document, it's simpler for me.
If there are other parties who might be interested in the TTS interface,
someone should inform them about the mailing list. Please note I'm not
going to do it!
Common TTS Driver Interface
Document version: 2004-11-15
The purpose of this document is to define a common way to access speech
synthesizers on Free Software and Open Source platforms. It contains a
list of general requirements on the speech synthesizer interface drivers
implementing this specification and the definition of a low-level
interface that can be used to access the speech synthesizer drivers.
This section defines a set of requirements on speech synthesizer
drivers that need to support assistive technologies on free software
1. Design Criteria
The Common TTS Driver Interface requirements will be developed
within the following broad design criteria:
1.1. Focus on supporting assistive technologies first. These
assistive technologies can be written in any programming language
and may provide specific support for particular environments such
as KDE or GNOME.
1.2. Simple and specific requirements win out over complex and
1.3. Use existing APIs and specs when possible.
1.4. All language dependent functionality should be covered here, not
1.5. Requirements will be categorized in the following priority
order: MUST HAVE, SHOULD HAVE, and NICE TO HAVE.
The priorities have the following meanings:
MUST HAVE: All conforming drivers must satisfy this
SHOULD HAVE: The driver will be usable without this feature, but
it is expected the feature is implemented in all drivers
intended for serious use.
NICE TO HAVE: Optional features.
Regardless of the priority, full interface must be always
provided, even when the given functionality is actually not
implemented behind the interface.
1.6. Requirements outside the scope of this document will be
labelled as OUTSIDE SCOPE.
- Should an application be able to determine if SHOULD HAVE and NICE
TO HAVE features are supported or not?
2. Synthesizer Discovery Requirements
2.1. MUST HAVE: An application will be able to discover all speech
synthesizer drivers available to the machine.
2.2. MUST HAVE: An application will be able to discover all possible
voices available for a particular speech synthesizer driver.
2.3. MUST HAVE: An application will be able to determine the
supported languages, possibly including also a dialect or a
country, for each voice available for a particular speech
Rationale: Knowledge about available voices and languages is
necessary to select proper driver and to be able to select a
supported language or different voices in an application.
2.4. MUST HAVE: Applications may assume their interaction with the
speech synthesizer driver doesn't affect other operating system
components in any unexpected way.
2.5. OUTSIDE SCOPE: Higher level communication interfaces (like IPC
services or text protocols) to the speech synthesizer drivers.
Note: It is expected they will be implemented by particular
projects (gnome-speech, KTTSD, Speech Dispatcher) as wrappers
around the low-level communication interface defined below.
3. Synthesizer Configuration Requirements
3.1. MUST HAVE: An application will be able to specify the default
voice to use for a particular synthesizer, and will be able to
change the default voice in between `speak' requests.
3.2. SHOULD HAVE: An application will be able to specify the default
prosody and style elements for a voice. These elements will match
those defined in the SSML specification, and the synthesizer may
choose which attributes it wishes to support. Note that prosody
and style elements specified in SSML sent as a `speak' request
will override the default values.
3.3. SHOULD HAVE: An application should be able to provide the
synthesizer with an application-specific pronunciation lexicon
addenda. Note that using `phoneme' element in SSML is another way
to accomplish this on a very localized basis, and will override
any pronunciation lexicon data for the synthesizer.
Rationale: This feature is necessary so that the application is
able to speak artificial words or words with explicitly modified
pronunciation (e.g. "the word ... is often mispronounced as ...
by foreign speakers").
3.4. MUST HAVE: Applications may assume they have their own local
copy of a synthesizer and voice. That is, one application's
configuration of a synthesizer or voice should not conflict with
another application's configuration settings.
3.5. MUST HAVE: Changing the default voice or voice/prosody element
attributes does not effect a `speak' in progress.
4. Synthesis Process Requirements
4.1. MUST HAVE: The speech synthesizer driver is able to process
plain text (i.e. text that is not marked up via SSML) encoded in
the UTF-8 character encoding.
4.2. MUST HAVE: The speech synthesizer driver is able to process
text formatted using SSML and encoded in UTF-8. The synthesizer
may choose to ignore markup it cannot handle or even to ignore all
markup as long as it is able to process the text inside the
4.3. SHOULD HAVE: The speech synthesizer driver is able to properly
process the SSML markup subset defined in the part B. of this
4.4. MUST HAVE: An application must be able to cancel a synthesis
operation in progress. In case of hardware synthesizers, this
means cancelling the audio output as well.
4.5. MUST HAVE: The speech synthesizer driver must be able to
process long input texts in such a way that the audio output
starts to be available for playing as soon as possible. An
application is not required to split long texts into smaller
4.6. SHOULD HAVE: The speech synthesizer driver should honor the
Performance Guidelines described below.
4.7. NICE TO HAVE: It would be nice if a synthesizer were able to
support "rewind" and "repeat" functionality for an utterance (see
related descriptions in the MRCP specification).
Rationale: This allows moving over long texts without the need to
synthesize the whole text and without loosing context.
4.8. NICE TO HAVE: It would be nice if a synthesizer were able to
support multilingual utterances.
4.9. SHOULD HAVE: A synthesizer should support notification of
`mark' elements, and the application should be able to align these
events with the synthesized audio.
4.10. NICE TO HAVE: It would be nice if a synthesizer supported
"word started" and "word ended" events and allowed alignment of
the events similar to that in 4.8.
Rationale: This is useful to update cursor position as a displayed
text is spoken.
4.11. NICE TO HAVE: It would be nice if a synthesizer supported
timing information at the phoneme level and allowed alignment of
the events similar to that in 4.8.
Rationale: This is useful for talking heads.
4.12. SHOULD HAVE: The application must be able to pause and resume
a synthesis operation in progress. In case of hardware
synthesizers, this means pausing and resuming the audio output
4.13. SHOULD HAVE: The synthesizer should not try to split the
contents of the `s' SSML element into several independent pieces,
unless required by a markup inside.
Rationale: An application may have better information about the
synthesized text and perform its own utterance chunking.
4.14. OUTSIDE SCOPE: Message management (queueing, ordering,
4.15. OUTSIDE SCOPE: Interfacing software synthesis with audio
- Still not clear consensus on how to return the synthesized audio
data (if at all). The main issue here is mostly with how to align
marker and other time-related events with the audio being played
on the audio output device.
- Not clear on how to (or if we even should) specify the audio
format to be used by a synthesizer.
- Implementation issue: Will the interaction with the driver be
synchronous or asynchronous? For example, will a call to `speak'
wait to return until all the audio has been processed? If not,
what happens when a call to "speak" is made while the synthesizer
is still processing a prior call to "speak?"
5. Performance Guidelines
In order to make the speech synthesizer driver actually usable with
assistive technologies, it must satisfy certain performance
expectations. The following text provides a clue to the driver
implementors to get a rough idea about what is needed in practice.
Typical scenarios when working with a speech enabled text editor:
5.1. Typed characters are spoken (echoed).
Reading of the characters and cancelling the synthesis must be
very fast, to catch up with a fast typist or even with
autorepeat. Consider a typical autorepeat rate 25 characters per
second. Ideally within each of the 40 ms intervals synthesis
should begin, produce some audio output and stop. To perform all
these actions within 100 ms (considering a fast typist and some
overhead of the application and the audio output) on a common
hardware is very desirable.
Appropriate character reading performance may be difficult to
achieve with contemporary software speech synthesizers, so it may
be necessary to use techniques like caching of the synthesized
characters. Also, it is necessary to ensure there is no initial
pause ("breathing in") within the synthesized character.
5.2. Moving over words or lines, each of them is spoken.
The sound sample needn't be available as quickly as in case of the
typed characters, but it still should be available without clearly
noticeable delay. As the user moves over the words or lines, he
must hear the text immediately. Cancelling the synthesis of the
previous word or line must be instant.
5.3. Reading a large text file.
In such a case, it is not necessary to start speaking instantly,
because reading a large text is not a very frequent operation.
One second long delay at the start is acceptable, although not
comfortable. Cancelling the speech must still be instant.
6. Related Specifications
(see requirements at the following URL:
B. SSML Subset in Use
This section defines the subset of the SSML markup and special
attribute values for use in input texts to the drivers.
Note: According to available information, SSML is not known to suffer
of any IP issues.
- Need to specify which SSML elements should (or must) be supported.
- Need to specify which SSML `voice' element attributes should (or
must) be supported.
- Need to specify which SSML `prosody' element attributes should (or
must) be supported. Especially pitch, rate and volume settings
should be supported.
- Definition of supported `say-as' attribute values. We probably want
to wait for a special `say-as' W3C specification that is expected to
come soon. Especially needed: speaking characters, speaking key
names, spelling mode, capital letter signalling mode, punctuation
- Perhaps we should identify an ordered priority list of the SSML
elements that should be supported?
C. Interface Description
This section defines the low-level TTS driver interface for use by all
assistive technologies on free software platforms.
1. Speech Synthesis Driver Discovery
2. Speech Synthesis Driver Interface
D. Copying This Document
Copyright (C) 2004 ...
This specification is made available under a BSD-style license ...
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