Speech Application Programming Interface

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This is about the Speech API. For other meanings, see the disambiguation page SAPI.

The Speech Application Programming Interface or SAPI is an API developed by Microsoft to allow the use of Speech Recognition and Speech Synthesis within Windows applications. To date a number of versions of the API have been released, which have shipped either as part of a Speech SDK, or as part of the Windows OS itself. Applications that use SAPI include Microsoft Office, Microsoft Agent and Microsoft Speech Server.

In general all versions of the API have been designed such that a software developer can write an application to perform Speech Recognition and Synthesis by using a standard set of interfaces, accessible from a variety of programming languages. In addition, it is possible for a 3rd-party company to produce their own Speech Recognition and Text-To-Speech engines or adapt existing engines to work with SAPI. In principle, as long as these engines conform to the defined interfaces they can be used instead of the Microsoft-supplied engines.

In general the Speech API is freely-redistributable component which can be shipped with any Windows application that wishes to use speech technology. Many versions (although not all) of the speech recognition and synthesis engines are also freely redistributable.

There have been two main 'families' of the Microsoft Speech API. SAPI versions 1 through 4 are all similar to each other, with extra features in each newer version. SAPI 5 however was a completely new interface, released in 2000. Since then several sub-versions of this API have been released.


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[edit] Basic architecture

Broadly the Speech API can be viewed as an interface or piece of middleware which sits between applications and speech engines (recognition and synthesis). In SAPI versions 1 to 4, applications could directly communicate with engines. The API included an abstract interface definition which applications and engines conformed to. Applications could also use simplified higher-level objects rather than directly call methods on the engines.

In SAPI 5 however, applications and engines do not directly communicate with each other. Instead each talk to a runtime component (sapi.dll). There's an API implemented by this component which applications use, and another set of interfaces for engines.

Typically in SAPI 5 applications issue calls through the API (for example to load a recognition grammar; start recognition; or provide text to be synthesized). The sapi.dll runtime component interprets these commands and processes them, where necessary calling on the engine through the engine interfaces (for example, the loading of a grammar from a file is done in the runtime, but then the grammar data is passed to the recognition engine to actually use in recognition). The recognition and synthesis engines also generate events while processing (for example, to indicate an utterance has been recognized or to indicate word boundaries in the synthesized speech). These pass in the reverse direction, from the engines, through the runtime dll, and on to an event sink in the application.

In additional to the actual API definition and runtime dll, other components are shipped with all versions of SAPI to make a complete Speech Software Development Kit. The following components are among those included in most versions of the Speech SDK:

  • API definition files - in MIDL and as C or C++ header files.
  • Runtime components - e.g. sapi.dll.
  • Control Panel applet - to select and configure default speech recognizer and synthesizer.
  • Text-To-Speech engines in multiple languages.
  • Speech Recognition engines in multiple languages.
  • Redistributable components to allow developers to package the engines and runtime with their application code to produce a single installable application.
  • Sample application code.
  • Sample engines - implementations of the necessary engine interfaces but with no true speech processing which could be used as a sample for those porting an engine to SAPI.
  • Documentation.

[edit] Versions

[edit] SAPI 1-4 API family

[edit] SAPI 1

The first version of SAPI was released in 1995, and was supported on Windows 95 and Windows NT 3.51. This version included low-level Direct Speech Recognition and Direct Text To Speech APIs which apps could use to directly control engines, as well as simplified 'higher-level' Voice Command and Voice Talk APIs.

[edit] SAPI 2

SAPI 2.0 was released in 1996.

[edit] SAPI 3

SAPI 3.0 added limited support for dictation speech recognition (discrete speech, not continuous), and additional sample apps and audio sources.

[edit] SAPI 4

SAPI 4.0 was released in 1998. This version of SAPI included both the core COM API; together with C++ wrapper classes to make programming from C++ easier; and ActiveX controls to allow drag-and-drop Visual Basic development. This was shipped as part of an SDK that included recognition and synthesis engines. It also shipped {with synthesis engines only} in Windows 2000.

The main components of the SAPI 4 API {which were all available in C++, COM, and ActiveX flavors} were:

  • Voice Command - high-level objects for command & control speech recognition
  • Voice Dictation - high-level objects for continuous dictation speech recognition
  • Voice Talk - high-level objects for speech synthesis
  • Voice Telephony - objects for writing telephone speech applications
  • Direct Speech Recognition - objects for direct control of recognition engine
  • Direct Text To Speech - objects for direct control of synthesis engine
  • Audio objects - for reading to and from an audio device or file


[edit] SAPI 5 API family

The Speech SDK version 5.0, incorporating the SAPI 5.0 runtime was released in 2000. This was a complete redesign from previous versions and neither engines nor applications which used older versions of SAPI could use the new version without considerable modification.

The design of the new API included the concept of strictly separating the application and engine so all calls were routed through the runtime sapi.dll. This change was intended to make the API more 'engine-independent', preventing applications from inadvertently depending on features of a specific engine. In addition this change was aimed at making it much easier to incorporate speech technology into an application by moving some management and initialization code into the runtime.

The new API was initially a pure COM API and could be used easily only from C/C++. Support for VB and scripting languages were added later. Operating systems from Windows 98 and NT 4.0 upwards were supported.

Major features of the API include:

  • Shared Recognizer. For desktop speech recognition applications, a recognizer object can be used that runs in a separate process (sapisvr.exe). All applications using the shared recognizer communicate with this single instance. This allows sharing of resources, removes contention for the microphone and allows for a global UI for control of all speech apps.
  • In-proc recognizer. For apps that require explicit control of the recognition process the in-proc recognizer object can be used instead of the shared one.
  • Grammar objects. Speech grammars are used to specify the words that the recognizer is listening for. SAPI 5 defines an XML markup for specifying a grammar, as well as mechanisms to create them dynamically in code. Methods also exist for instructing the recognizer to load a built-in dictation language model.
  • Voice object. This performs speech synthesis, producing an audio stream from text. A markup language {similar to XML, but not strictly XML} can be used for controlling the synthesis process.
  • Audio interfaces. The runtime includes objects for performing speech input from the microphone or speech output to speakers {or any sound device}; as well as to and from wave files. It is also possible to write a custom audio object to stream audio to or from a non-standard location.
  • User lexicon object. This allows custom words and pronunciations to be added by a user or application. These are added to the recognition or synthesis engine's built-in lexicons.
  • Object tokens. This is a concept allowing recognition and TTS engines, audio objects, lexicons and other categories of object to be registered, enumerated and instantiated in a common way.


[edit] SAPI 5.0

This version shipped in late 2000 as part of the Speech SDK version 5.0, together with version 5.0 recognition and synthesis engines. The recognition engines supported continuous dictation and command & control and were released in U.S. English, Japanese and Simplified chinese versions. In the U.S. English system, special acoustic models were available for children's speech and telephony speech. The synthesis engine was available in English and Chinese. This version of the API and recognition engines also shipped in Microsoft Office XP in 2001.

[edit] SAPI 5.1

This version shipped in late 2001 as part of the Speech SDK version 5.1. Automation-compliant interfaces were added to the API to allow use from Visual Basic, scripting languages such as JScript, and managed code. This version of the API and TTS engines were shipped in Windows XP. This API was also shipped, together with a substantially improved version 6 recognition engine in Office 2003 and Windows XP Tablet PC Edition.

[edit] SAPI 5.2

This was a special version of the API for use only in the Microsoft Speech Server which shipped in 2004. It added support for SRGS and SSML mark-up languages, as well as additional server features and performance improvements. The Speech Server also shipped with the version 6 desktop recognition engine and the version 7 server recognition engine.

[edit] SAPI 5.3

This is the version of the API that is expected to be shipping in Windows Vista together with new recognition and synthesis engines.

[edit] New features in SAPI 5.3:
  • Support for W3C XML speech grammars for recognition and synthesis. The Speech Synthesis Markup Language (SSML) version 1.0 provides the ability to mark up voice characteristics, speed, volume, pitch, emphasis, and pronunciation.
  • The Speech Recognition Grammar Specification (SRGS) supports the definition of context-free grammars, with two limitations:
    • It does not support the use of SRGS to specify dual-tone modulated-frequency (touch-tone) grammars.
    • It does not support Augmented Backus-Naur Form (ABNF).
  • Support for semantic interpretation script within grammars. SAPI 5.3 enables an SRGS grammar to be annotated with JavaScript for semantic interpretation to supplement the recognized text.
  • User-Specified shortcuts in lexicons, which is the ability to add a string to the lexicon and associate it with a shortcut word. When dictating, the user can say the shortcut word and the recognizer will return the expanded string.
  • Additional functionality and ease-of-programming provided by new types.
  • Performance improvements, improved reliability and security.

[edit] Compatibility

The Speech API has had compatibility issues with some versions of Microsoft Visual Studio. For example, in Visual Studio 2005, some reports[citation needed] indicate that various headers included in the Speech API fail to compile. No rectification for this problem has been forthcoming from Microsoft.

The Speech API is compatible with the following operating systems[1]:

[edit] Managed code Speech API

A managed code API is being developed by Microsoft which has similar functionality to SAPI 5 but will be more suitable to be used by managed code applications. The new API is part of .NET Framework 3.0 and will be available on Windows XP, Windows Server 2003 and Windows Vista.

The existing SAPI 5 API can also be used from managed code to a limited extent by creating COM Interop code (helper code designed to assist in accessing COM interfaces and classes). This works well in some scenarios however the new API should provide a more seamless experience equivalent to using any other managed code library.

[edit] Major applications using SAPI

[edit] Speech functionality in Windows Vista

Windows Vista will include a number of new speech-related features including:

  • Speech control of the full Windows GUI and applications
  • New tutorial, microphone wizard, and UI for controlling speech recognition
  • New version of the Speech API runtime: SAPI 5.3
  • Built-in updated Speech Recognition engine
  • New Speech Synthesis engine Microsoft Anna
  • Managed code API
  • Supports many languages such as German, Spanish and Chinese

[edit] See also

[edit] External links

[edit] References

  1. ^ [Corporation]. "SAPI System Requirements". MSDN Library. Retrieved on 2006-04-12.
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