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Template Language

Voice commands are recognized by voice2json from a set of template sentences that you define in your profile. These are stored in an ini file (sentences.ini) whose section values are simplified JSGF grammars. The set of all sentences represented in these grammars is used to create an ARPA language model and an intent recognizer. See the whitepaper for details.


The combination of an ini file and JSGF is arguably an abuse of two file formats, so why do this? At a minimum, voice2json needs a set of sentences, grouped by intent, in order to train the speech and intent recognizers. A fairly pleasant way to express this in text (I think) is as follows:

[Intent 1]
sentence 1
sentence 2

[Intent 2]
sentence 3
sentence 4

Compared to JSON, YAML, etc., there is minimal syntactic overhead for the purposes of just writing down sentences. However, its shortcomings become painfully obvious once you have more than a handful of sentences or intents:

  1. If two sentences are nearly identical, save for an optional word like “the” or “a”, you have to maintain two nearly identical copies of a sentence.
  2. When speaking about collections of things, like colors or states (on/off), you need a sentence for every alternative choice.
  3. You cannot share commonly repeated phrases across sentences or intents.
  4. There is no way to annotate phrases so the intent recognizer knows the values for an intent’s named entities/slots (e.g., color).

Each of these shortcomings are addressed by considering the space between intent headings ([Intent 1], etc.) as a grammar that will represent a space of valid, annotated voice commands. voice2json specifically represents these spaces as finite state transducers, serving as input to opengrm to produce language models without ever generating a single sentence. The same representation is then used to recognize intents.

Optional Words

Within a sentence, you can specify optional word(s) by surrounding them [with brackets]. These represents at least two sentences: one with the optional word(s), and one without. So the following sentence template:

[an] example sentence [with] some optional words

represents 4 concrete sentences:

  1. an example sentence with some optional words
  2. example sentence with some optional words
  3. an example sentence some optional words
  4. example sentence some optional words


A set of items, where only one is present at a time, is (specified | like | this). For N alternatives, there will be N different sentences represented (unless you nest optional words, etc.). The template:

set the light to (red | green | blue)

will represent:

  1. set the light to red
  2. set the light to green
  3. set the light to blue


Rules allow you to reuse common phrases, alternatives, etc. Rules are defined by rule_name = ... alongside your sentences and referenced by <rule_name>. The template above with colors could be rewritten as:

colors = (red | green | blue)
set the light to <colors>

which will represent the same 4 sentences as above. Importantly, you can share rules across intents by prefixing the rule’s name with the intent name followed by a dot:

colors = (red | green | blue)
set the light to <colors>

is the light <SetLightColor.colors>

The second intent (GetLightColor) references the colors rule from SetLightColor.


The example templates above represent sentences for training the speech recognizer, but using them as-is to train the intent recognizer will not be satisfactory. The color in the SetLightColor intent, for example, is obviously important for a system that will fulfill it (i.e., actually change a light’s color). voice2json’s intent recognizer just needs a little extra information from you, the user.

Luckily, JSGF has a tag feature that lets you annotate portions of sentences/rules. voice2json interprets that the JSGF tag text (inside {...}) as slot/entity names and the tagged portions of the sentence (left of {...}) as slot/entity values. The SetLightColor example can be extended with tags like this:

colors = (red | green | blue){color}
set the light to <colors>

With the {color} tag attached to the (red | green | blue) alternative set, each color name will carry the tag. This is the same as typing ((red){color} | (green){color} | (blue){color}), but less verbose. This template now represents the following tagged sentences:

  1. set the light to [red](color)
  2. set the light to [green](color)
  3. set the light to [blue](color)

When the SetLightColor intent is recognized now, perhaps with “set the light to red”, the corresponding JSON event will have a color entity/slot with the value of red:

    "intent": {
        "name": "SetColor"
    "entities": [
        { "entity": "color", "value": "red" }
    "slots": {
        "color": "red"

The downstream system that will use this event to fulfill the user’s intent (e.g., Node-RED) now only has to inspect the color slot to decide what to do!

Word/Tag Substitutions

voice2json allows you to control how words are emitted in the final JSON event.

Consider the following example with two lights, a lamp in the living room and a light in the garage:

state = (on | off)
name = (living room lamp | garage light)
turn (<state>){state} [the] (<name>){name}

If the voice command “turn on the living room lamp” is spoken, voice2json will produce the expected JSON:

  "intent": {
    "name": "LightState"
  "slots": {
    "state": "on",
    "name": "living room lamp"

If the system that consumes this JSON does not know what on and living room lamp are, however, it will be unable to handle the intent. Suppose this hypothetical system knows only how to enable and disable either switch_1 or switch_2. We could ease the burden of interfacing with some minor modifications to sentences.ini:

state = (on:enable | off:disable)
name = (living room lamp){name:switch_1} | (garage light){name:switch_2}
turn (<state>){state} [the] (<name>)

The syntax on:enable tells voice2json to listen for the word on, but emit the word enable in its place. Similarly, the syntax (living room lamp){name:switch_1} tells voice2json to listen for living room lamp, but actually put switch_1 in the name slot:

  "text": "turn enable the switch_1",
  "intent": {
    "name": "LightState",
    "confidence": 1
  "entities": [
      "entity": "state",
      "value": "enable",
      "raw_value": "on",
      "start": 5,
      "raw_start": 5,
      "end": 11,
      "raw_end": 7
      "entity": "name",
      "value": "switch_1",
      "raw_value": "living room lamp",
      "start": 16,
      "raw_start": 12,
      "end": 24,
      "raw_end": 28
  "raw_text": "turn on the living room lamp",
  "tokens": [
  "raw_tokens": [
  "slots": {
    "state": "enable",
    "name": "switch_1"
  "intents": [],
  "recognize_seconds": 0.000274658203125

Notice that with substitutions, the text and raw_text properties of the recognized intent no longer match. Likewise, raw_value differs from value for each entity. The raw_ properties contain the left side of the : in each substitution.

Slot Substitutions

This syntax also works inside slot files. When nothing is put on the right side of the :, the word is silently dropped, so:

[the:] light

will match both the light and light, but always emit just light. This technique is especially useful in English with articles like “a” and “an”. It is common to write something in sentences.ini like this:

turn on (a | an) ($colors){color} light

where slots/colors might be:


This will match turn on a red light and turn on an orange light as intended, but also turn on an red light and turn on a orange light. Using word replacement and a slot file, we can instead write:

turn on ($colors){color} light

where slots/colors is:

a: red
an: orange

This will only match the spoken commands turn on a red light and turn on an orange light, and the color slot will never contain “a” or “an”!

Extra Words

You can also use substitution to add words that are not present in the speech:

:please turn on the light

will accept the spoken sentence “turn on the light”, but emit “please turn on the light” in the recognized intent.

Slot References

In the SetLightColor example above, the color names are stored in sentences.ini as a rule:

colors = (red | green | blue)

Ths is convenient when the list of colors is small, changes infrequently, and does not depend on an external service. But what if this was a list of movie names that were stored on your Kodi Home Theater?

movies = ("Primer" | "Moon" | "Chronicle" | "Timecrimes" | "Coherence" | ... )

It would be much easier if this list was stored externally, but could be referenced in the appropriate places in the grammar. This is possible in voice2json by placing text files in the directory given in training.slots-directory from your profile (slots/ by default).

By putting the movie names above in a text file at slots/movies


you can now reference them as $movies in your your sentences.ini file! Something like:

play ($movies){movie_name}

will generate JSON events like:

    "intent": {
        "name": "PlayMovie"
    "entities": [
        { "entity": "movie_name", "value": "Primer" }
    "slots": {
        "movie_name": "Primer"

If you update the movies file, make sure to re-train voice2json in order to pick up the new movie names. Only intent grammars that reference $movies will be re-built.

Slot Programs

Slot lists are great if your slot values always stay the same and are easily written out by hand. If you have slot values that you need to be generated each time voice2json is trained, you can use slot programs.

Create a directory named slot_programs in your profile (e.g., $HOME/.config/voice2json/slot_programs):

mkdir -p "${slot_programs}"

Add a file in the slot_programs directory with the name of your slot, e.g. colors. Write a program in this file, such as a bash script. Make sure to include the shebang and mark the file as executable:

cat <<EOF > "${slot_programs}/colors"
#!/usr/bin/env bash
echo 'red'
echo 'green'
echo 'blue'

chmod +x "${slot_programs}/colors"

Now, when you reference $colors in your sentences.ini, voice2json will run the program you wrote and collect the slot values from each line. Note that you can output all the same things as regular slots lists, including optional words, alternatives, etc.

You can pass arguments to your program using the syntax $name,arg1,arg2,... in sentences.ini (no spaces). Arguments will be pass on the command-line, so arg1 and arg2 will be $1 and $2 in a bash script.

Like regular slots lists, slot programs can also be put in sub-directories under slot_programs. A program in slot_programs/foo/bar should be referenced in sentences.ini as $foo/bar.


By default, all named entity values in a recognized intent’s JSON are strings. If you need a different data type, such as an integer or float, or want to do some kind of complex conversion, use a converter:

set brightness to (low:0 | medium:0.5 | high:1){brightness!float}

The !name syntax calls a converter by name. voice2json includes several built-in converters:

You can define your own converters by placing a file in the converters directory of your profile. Like slot programs, this file should contain a shebang and be marked as executable (chmod +x). A file named converters/foo/bar should be referenced as !foo/bar in sentences.ini.

Your custom converter will receive the value to convert on standard in (stdin) encoded as JSON. You should print a converted JSON value to standard out stdout. The example below demonstrates converting a string value into an integer:

#!/usr/bin/env python3
import sys
import json

value = json.load(sys.stdin)

Converters can be chained, so !foo!bar will call the foo converter and then pass the result to bar.

Number Replacement

voice2json supports using number literals (75) and number ranges (1..10) directly in your sentence templates. During training, the num2words package is used to generate words that the speech recognizer can handle (“seventy five”).

For example:

set the temperature to 75

will be translated to:

set the temperature to seventy: five:75

During intent recognition, “seventy five” will be replaced with the integer 75.

Number Ranges

A number range example:

set brightness to (0..100){brightness}

The brightness property of the recognized SetBrightness intent will automatically be converted to an integer for you. You can optionally add a step to the integer range:

evens = 0..100,2
odds = 1..100,2

Under the hood, number ranges are actually references to a built-in slot program.


If one of your sentences happens to start with an optional word (e.g., [the]), this can lead to a problem:

[the] problem sentence

Python’s configparser will interpret [the] as a new section header, which will produce a new intent, grammar, etc. voice2json handles this special case by using a backslash escape sequence (\[):

\[the] problem sentence

Now [the] will be properly interpreted as a sentence under [SomeIntent]. You only need to escape a [ if it’s the very first character in your sentence.

Missing JSGF Features

The following features from the full JSGF specification are not supported in voice2json: