TTS Events Explanation

Events are structures that pass information from the TTS engine back to the application. When the audio data is output, SAPI fires corresponding events. Applications react to audio output as it occurs. Examples of reactions include animating a face appropriately as viseme events are received, or highlighting text as it is spoken. See the sample application, TTSApp, for an example of each.

Applications call ISpEventSource::SetInterest to inform SAPI about the types of events that they are interested in receiving. Applications can also call this through ISpVoice, because it inherits from ISpEventSource. Applications can then call ISpEventSource::GetEvents to retrieve fired events from SAPI.

The following is a set of event types generated by TTS engines (this is a subset of the SPEVENTENUM enumeration):

typedef enum SPEVENTENUM
{
    //--- TTS engine
    SPEI_START_INPUT_STREAM     = 1,
    SPEI_END_INPUT_STREAM       = 2,
    SPEI_VOICE_CHANGE           = 3,   // LPARAM_IS_TOKEN
    SPEI_TTS_BOOKMARK           = 4,   // LPARAM_IS_STRING
    SPEI_WORD_BOUNDARY          = 5,
    SPEI_PHONEME                = 6,
    SPEI_SENTENCE_BOUNDARY      = 7,
    SPEI_VISEME                 = 8,
    SPEI_TTS_AUDIO_LEVEL        = 9
} SPEVENTENUM;

The SPEVENT structure contains varying information depending on which of these event types it represents.

typedef struct SPEVENT
{
    WORD         eEventId;
    WORD         elParamType;
    ULONG        ulStreamNum;
    ULONGLONG    ullAudioStreamOffset;
    WPARAM       wParam;
    LPARAM       lParam;
} SPEVENT;

You can analyze the various fields of the SPEVENT structure for the event types they correspond to. For all event types, ulStreamNum corresponds to the stream number returned using ISpVoice::Speak or ISpVoice::SpeakStream.

The SPEI_START_INPUT_STREAM event indicates that the output object has begun receiving output for a specific stream number. The rest of the fields are not of interest to this event type.

The SPEI_END_INPUT_STREAM event indicates that the output object has finished receiving output for a specific stream number. The rest of the fields are not of interest to this event type.

The SPEI_VOICE_CHANGE event indicates that the voice responsible for speaking the input text (or stream) has changed because of a <Voice> XML tag. It is fired at the beginning of each Speak call. For more information on using object tokens, see the Object Tokens and Registry Settings white paper.

SPEVENT Field	Voice Change event
eEventId	SPEI_VOICE_CHANGE
elParamType	SPET_LPARAM_IS_TOKEN
wParam
lParam	Object token of the new voice.

The SPEI_TTS_BOOKMARK event indicates that the speak stream has reached a bookmark. Bookmarks can be inserted into the input text using the <Bookmark> XML tag.

SPEVENT Field	Bookmark event
eEventId	SPEI_TTS_BOOKMARK
elParamType	SPET_LPARAM_IS_STRING
wParam	Value of the bookmark string when converted to a long (_wtol(...) can be used).
lParam	Null-terminated copy of the bookmark string.

The SPEI_WORD_BOUNDARY event indicates that it has reached the beginning of a word.

SPEVENT Field	Word Boundary event
eEventId	SPEI_WORD_BOUNDARY
elParamType	SPET_LPARAM_IS_UNKNOWN
wParam	Character offset at the beginning of the word being synthesized.
lParam	Character length of the word in the current input stream being synthesized.

The SPEI_SENTENCE_BOUNDARY event indicates that the speak stream has reached the beginning of a sentence.

SPEVENT Field	Sentence Boundary event
eEventId	SPEI_SENTENCE_BOUNDARY
elParamType	SPET_LPARAM_IS_UNKNOWN
wParam	Character offset at the beginning of the sentence being synthesized.
lParam	Character length of the sentence in the current input stream being synthesized.

The SPEI_PHONEME event indicates that the speak stream has reached the phoneme.

SPEVENT Field	Phoneme event
eEventId	SPEI_PHONEME
elParamType	SPET_LPARAM_IS_UNKNOWN
wParam	The high word is the duration, in milliseconds, of the current phoneme. The low word is the PhoneID of the next phoneme.
lParam	The low word is the PhoneID of the current phoneme. The high word is the SPVFEATURE value associated with the current phoneme.

The SAPI 5 American English phoneme set can be found here. The SAPI 5 Chinese phoneme set can be found here. The SAPI 5 Japanese phoneme set can be found here.

SPVFEATURE contains two flags: SPVFEATURE_STRESSED and SPVFEATURE_EMPHASIS. SPVFEATURE_STRESSED means that the phoneme is stressed relative to the other phonemes of a word (stress is usually associated with the vowel of a stressed syllable). SPVFEATURE_EMPHASIS means that the phoneme is part of an emphasized word. That is, stress is a syllabic phenomenon within a word, and emphasis is a word-level phenomenon within a sentence.

The SPEI_VISEME event indicates that it has reached the viseme.

SPEVENT Field	Viseme event
eEventId	SPEI_VISEME
elParamType	SPET_LPARAM_IS_UNKNOWN
wParam	The high word is the duration, in milliseconds, of the current viseme. The low word is the code for the next viseme.
lParam	The low word is the code of the current viseme. The high word is the SPVFEATURE value associated with the current viseme (and phoneme).

See SPVISEMES for a listing of the SAPI 5 viseme set.

The SPEI_TTS_AUDIO_LEVEL event indicates the audio has reached the level of the synthesis at any given point.

SPEVENT Field	Audio Level event
eEventId	SPEI_TTS_AUDIO_LEVEL
elParamType	SPET_LPARAM_IS_UNDEFINED
wParam	TTS audio level (ULONG).
lParam	NULL

For an example of how to use TTS events in an application, see the Text-to-Speech Tutorial.

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