HTML Speech Incubator Group
TTS Speech API Work Items
 (Internal Draft)

3.1.1 Strong Interest

A requirement was classified as having "strong interest" if at least 80% of the group believed it needs to be addressed by any specification developed based on the work of this group. These requirements are:

• FPR40. Web applications must be able to use barge-in (interrupting audio and TTS output when the user starts speaking).
• FPR50-TTS. Web applications must not be prevented from integrating output from multiple modalities.
• FPR52. The web app should be notified when TTS playback finishes.
• FPR60. Web application must be able to programatically abort tts output.
• FPR38-TTS. Web application must be able to specify language of synthesis.
• FPR51. The web app should be notified when TTS playback starts.
• FPR53. The web app should be notified when the audio corresponding to a TTS element is played back.
• FPR39. Web application must be able to be notified when the selected language is not available.
• FPR18. It must be possible for the user to revoke consent.
• FPR11. If the web apps specify speech services, it should be possible to specify parameters.
• FPR12. Speech services that can be specified by web apps must include network speech services.
• FPR3. Implementation must support SSML.
• FPR46. Web apps should be able to specify which voice is used for TTS.
• FPR7. Web apps should be able to request speech service different from default.
• FPR9. If browser refuses to use the web application requested speech service, it must inform the web app.

3.1.2 Moderate Interest

A requirement was classified as having "moderate interest" if less than 80% but at least 50% of the group believed it needs to be addressed by any specification developed based on the work of this group. These requirements are:

• FPR33. There should be at least one mandatory-to-support codec that isn't encumbered with IP issues and has sufficient fidelity & low bandwidth requirements.
• FPR41. It should be easy to extend the standard without affecting existing speech applications.
• FPR36-TTS. User agents must provide a default interface to control speech synthesis.
• FPR61. Aborting the TTS output should be efficient.
• FPR32. Speech services that can be specified by web apps must include local speech services.
• FPR30. Web applications must be allowed at least one form of communication with a particular speech service that is supported in all UAs.
• FPR55. Web application must be able to encrypt communications to remote speech service.
• FPR58. Web application and speech services must have a means of binding session information to communications.
• FPR6. Browser must provide default speech resource.
• FPR20. The spec should not unnecessarily restrict the UA's choice in privacy policy.

3.1.3 Mild Interest

A requirement was classified as having "mild interest" if less than 50% of the group believed it needs to be addressed by any specification developed based on the work of this group. These requirements are:

• FPR29. Speech synthesis implementations should be allowed to fire implementation specific events.
• FPR31. User agents and speech services may agree to use alternate protocols for communication.
• FPR10. If browser uses speech services other than the default one, it must inform the user which one(s) it is using.
• FPR8. User agent (browser) can refuse to use requested speech service.

3.5.1 General Design Decisions

1. There are three aspects to the solution which must be addressed: communication with and control of speech services, a script-level API, and markup-level hooks and capabilities.
2. The script API will be Javascript.
3. The scripting API is the primary focus, with all key functionality available via scripting. Any HTML markup capabilities, if present, will be based completely on the scripting capabilities.
4. Notifications from the user agent to the web application should be in the form of Javascript events/callbacks.
6. For TTS, there must be at least these two logical functions:
   1. play
   2. pause
   There is agreement that it should be possible to stop playback, but there is not agreement on the need for an explicit stop function.
7. It must be possible for a web application to specify the speech engine.
8. Speech service implementations must be referenceable by URI.
13. For TTS, SSML 1.1 is mandatory to support, as is UTF-8 plain text. These are the only mandated formats.
15. There must be no technical restriction that would prevent using only TTS or only ASR.
16. There must be no technical restriction that would prevent implementing only TTS or only ASR. There is *mostly* agreement on this.
17. There will be a mandatory set of capabilities with stated limitations on interoperability.
23. There should be a default user interface.
30. We expect to have the following six audio/speech events: onaudiostart/onaudioend, onsoundstart/onsoundend, onspeechstart/onspeechend. The onsound* events represent a "probably speech but not sure" condition, while the onspeech* events represent the recognizer being sure there's speech. The former are low latency. An end event can only occur after at least one start event of the same type has occurred. Only the user agent can generate onaudio* events, the energy detector can only generate onsound* events, and the speech service can only generate onspeech* events.
31. There are 3 classes of codecs: audio to the web-app specified ASR engine, recognition from existing audio (e.g., local file), and audio from the TTS engine. We need to specify a mandatory-to-support codec for each.
32. It must be possible to specify and use other codecs in addition to those that are mandatory-to-implement.
33. TTS: Support for streaming audio is required -- in particular, that TTS may begin playing before the synthesizer has finished synthesizing.
35. We will require support for http for all communication between the user agent and any selected engine, including chunked http for media streaming, and support negotiation of other protocols (such as WebSockets or whatever RTCWeb/WebRTC comes up with).
38. The scripting API communicates its parameter settings by sending them in the body of a POST request as Media Type "multipart". The subtype(s) accepted (e.g., mixed, formdata) are TBD.
39. TTS: If a TTS engine allows parameters to be specified in the URI in addition to in the POST body, when a parameter is specified in both places the one in the body takes precedence. This has the effect of making parameters set in the URI be treated as default values.
40. We cannot expect consistency in language support and performance/quality.
41. We agree that there must be API-level consistency regardless of user agent and engine.
42. We agree on having the same level of consistency across all four of the following categories:
    1. consistency between different UAs using their default engine
    2. consistency between different UAs using web app specified engine
    3. consistency between different UAs using different web specified engines
    4. consistency between default engine and specified engines
    With exception that #4 may have limitations due to privacy issues.
43. From this point on we will use "service" rather than "engine" because a service may be a proxy for more than one engine.
44. We will not support selection of service by characteristics.
45. Add to list of expected inconsistency (change from existing wording of interoperability): reco performance including maximum size on parameters, microphone characteristics, semantics and exact values of sensitivity and confidence, time need to perform ASR/TTS, latencies, endpoint sensitivity and latency, result contents, presence/absence of optional events, recorded waveform
47. If the user's device is emitting other sounds than those produced by the current HTML page, there is no particular requirement that the User Agent be required to detect/reduce/eliminate it.
48. If a web app specifies a speech service and it is not available, an error is thrown. No automatic fallback to another service or the default service takes place.
49. The API should provide a way to determine if a service is available before trying to use the service; this applies to the default service as well.
50. The API must provide a way to query the availability of a specific configuration of a service.
51. The API must provide a way to ask the user agent for the capabilities of a service. In the case of private information that the user agent may have when the default service is selected, the user agent may choose to answer with "no comment" (or equivalent).
52. Informed user consent is required for all use of private information. This includes list of languages for ASR and voices for TTS. When such information is requested by the web app or speech service and permission is refused, the API must return "no comment" (or equivalent).
53. It must be possible for user permission to be granted at the level of specific web apps and/or speech services.
54. User agents, acting on behalf of the user, may deny the use of specific web apps and/or speech services.
56. The API will support multiple simultaneous requests to speech services (same or different, ASR and TTS).
57. We disagree about whether there needs to be direct API support for a single ASR request and single TTS request that are tied together.
58. It must be possible to individually control ASR and TTS.
59. It must be possible for the web app author to get timely information about recognition event timing and about TTS playback timing. It must be possible for the web app author to determine, for any specific UA local time, what the previous TTS mark was and the offset from that mark.
60. It must be possible for the web app to stop/pause/silence audio output directly at the client/user agent.
61. When audio corresponding to TTS mark location begins to play, a Javascript event must be fired, and the event must contain the name of the mark and the UA timestamp for when it was played.
62. It must be possible to specify service-specific parameters in both the URI and the message body. It must be clear in the API that these parameters are service-specific, i.e., not standard.
63. Every message from UA to speech service should send the UA-local timestamp.
64. API must have ability to set service-specific parameters using names that clearly identify that they are service-specific, e.g., using an "x-" prefix. Parameter values can be arbitrary Javascript objects.
65. EMMA already permits app-specific result info, so there is no need to provide other ways for service-specific information to be returned in the result.
66. The API must support DOM 3 extension events as defined (which basically require vendor prefixes). See http://www.w3.org/TR/2009/WD-DOM-Level-3-Events-20090908/#extending_events-Vendor_Extensions. It must allow the speech service to fire these events.
67. The protocol must send its current timestamp to the speech service when it sends its first audio data.
68. It must be possible for the speech service to instruct the UA to fire a vendor-specific event when a specific offset to audio playback start is reached by the UA. What to do if audio is canceled, paused, etc. is TBD.
69. HTTPS must also be supported.
70. Using web app in secure communication channel should be treated just as when working with all secured sites (e.g., with respect to non-secured channel for speech data).
71. Default speech service implementations are encouraged not to use unsecured network communication when started by a web app in a secure communication channel
73. In Javascript will be able to set parameters as dot properties and also via a getParameters method. Browser should also allow service-specific parameters to be set this way.
78. Once there is a way (defined by another group) to get access to some blob of stored audio, we will support re-recognition of it.
79. No explicit need for JSON format of EMMA, but we might use it if it existed.
80. Candidate codecs to consider are Speex, FLAC, and Ogg Vorbis, in addition to plain old mu-law/a-law/linear PCM.
81. Protocol design should not prevent implementability of low-latency event delivery.
82. Protocol should support the client to begin TTS playback before receipt of all of the audio.
83. We will not require support for video codecs. However, protocol design must not prohibit transmission of codecs that have the same interface requirements as audio codecs.
84. Every event from speech service to the user agent must include timing information that the UA can convert into a UA-local timestamp. This timing info must be for the occurrence represented by the event, not the event time itself. For example, an end-of-speech event would contain timing for the actual end of speech, not the time when the speech service realizes end of speech occurred or when the event is sent.

3.5.2 Speech Service Communication and Control Design Decisions

This is where design decisions regarding control of and communication with remote speech services, including media negotiation and control, will be recorded.

3.5.3 Script API Design Decisions

This is where design decisions regarding the script API capabilities and realization will be recorded.

• It must be possible to define at least the following handlers (names TBD):
  • onspeechstart (not yet clear precisely what start of speech means)
  • onspeechend (not yet clear precisely what end of speech means)
  • onerror (one or more handlers for errors)
  • a handler for when the recognition result is available
  Note: significant work is needed to get interoperability here.

3.5.4 Markup API Design Decisions

This is where design decisions regarding the markup changes and/or enhancements will be recorded.

3.6.1 Protocol Proposal

TBD

3.6.2 Web Application API Proposal

Base on TTS section of the submission from Microsoft: a speech and tts proposal.