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SALT Enables Telephony and Multimodal Applications.

In March 2002, the SALT Forum—founded by Cisco, Comverse, Intel, Microsoft, Philips, and SpeechWorks—published a specification for speech tags called Speech Application Language Tags (SALT).

The following illustrates a SALT implementation of the same application shown previously. The same color scheme is used as follows, so you can easily compare the SALT implementation with the VoiceXML implementation. With both approaches, the developer specifies prompts (green), grammars (red), and event handlers (blue); and submits results to a back end database (aqua).

<!-- HTML --> <html xmlns:salt="urn:saltforum.org/schemas/020124">
<body onload="RunAsk()">
  <form id="travelForm">
    <input name="txtBoxOriginCity" type="text" />
    <input name="txtBoxDestCity" type="text" />
  <!-- Speech Application Language Tags -->
  <salt:prompt id="askOriginCity"> Where would you like to leave from? </salt:prompt>
  <salt:prompt id="askDestCity">  Where would you like to go to?   </salt:prompt>
  <salt:prompt id="sayDidntUnderstand" 
     Sorry, I didn't understand.  
  <salt:reco id="recoOriginCity"
    <salt:grammar src="city.xml" />
  <salt:reco id="recoDestCity"
     <salt:grammar src="city.xml" />

  <!--- script -->
    function RunAsk() {
      if (travelForm.txtBoxOriginCity.value=="") {
    } else if (travelForm.txtBoxDestCity.value=="") {
   function procOriginCity() {
      travelForm.txtBoxOriginCity.value = recoOriginCity.text;
   function procDestCity() {
      travelForm.txtBoxDestCity.value = recoDestCity.text;


Unlike VoiceXML, SALT has no Forms Interpretation Algorithm. Instead, SALT tags must be embedded within a host language such as XHTML, SMIL, or a scripting language. (The preceding example uses XHTML.) Developers must specify all control and coordination functions using the host language. This approach uses a procedural style of programming, as opposed to VoiceXML's declarative style. In effect, developers implement their own version of the Forms Interpretation Algorithm, which is embedded within the code specified by the host language.

SALT can be used to develop telephone applications—applications that enable callers to use their existing telephones and cell phones. However, SALT can also be used to develop multimodal applications by embedding SALT tags into languages for building GUIs such as XHTML. SALT tags can be embedded into existing GUI applications, enabling them to speak and listen to callers, as well as display text and graphics while accepting input from a keyboard or keypad, and a mouse or stylus, which results in a multimodal application.

While the VoiceXML browser generally executes on a speech server connected to a telephone, different versions of SALT browsers execute on a server or on a client, such as a handheld computer or an advanced cell phone. SALT Forum members are expected to announce the availability of SALT browsers, development tools, and other products, now that the SALT 1.0 specification has been made public this summer.

The following table summarizes the principles characteristics of both VoiceXML and SALT.





Authoring system-driven and mixed-initiative voice dialogs over telephones and cell phones

Authoring system-driven, user-driven, and mixed-initiative voice dialogs and multimodal applications

Target Device

Interpreted on a speech server for use by telephones and cell phones

Executed on either a client or server

Language Style

Simple, high-level dialog markup language + XCMAScript

Lightweight markup tag extension of existing (X)HTML, WML, and scripting languages

Major Language features

<menu>, <form>, <grammar>, and control flow elements (<if>, <if-else> <next>)

<listen>, <prompt>, <dtmf>, <smex>, call control and prompt queue objects

Control Flow

Implicit by Forms Interpretation Algorithm

Developer specified using HTML + ECMAScript, WML, SML, and so on

Number of tags



Grammar Languages

Speech Recognition Grammar Specification, proprietary languages

Speech Recognition Grammar Specification, proprietary languages

Speech Synthesis

Speech Synthesis ML, proprietary languages

Speech Synthesis ML, proprietary languages

Semantic Representation

Natural Language Semantics ML

Natural Language Semantics ML

Call Control

Call Control XML (CCXML)

Call Control XML (CCXML), call control object <SMEX>

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