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The broad goal of Silvius is to generate keystrokes based on voice commands. In contrast to more mainstream voice recognition solutions, the focus of Silvius is to generate commands, rather than perform transcription of "long-form" speech input. You won't use Silvius to dictate an e-mail, but on the other hand it is very well suited to control other applications.

Silvius is an open-ended system: it can generate series of more or less arbitrary keystroke events based on speech input; and, with the grammar part being written in Python, it can easily be extended with new speech input patterns to generate new series of keystrokes.

This works particularly well for keyboard-focused applications, where you can make Silvius send series of commands to perform complex manipulations in response to speech commands. Essentially, Silvius provides "macro" functionality for the application that is in focus.

As an example, suppose you use "shell" as the "trigger word" for Bash actions. It's very easy to extend the Silvius grammar to recognize "shell list", and make it produce the keystroke series ls -la . For the command "shell change", you could make Silvius produce the keystroke series cd .

With your shell in focus, you can then say "shell list slap" to view the contents of the current directory ("slap" produces an 'enter' keystroke). Or say "shell change dot dot slap" to go up one directory level.

You can do the same thing, for example, for your most frequently used Git commands, and for frequently used Emacs or Vim commands, etc. In this way, you build up a repertoire of voice commands that are applicable across the applications that you use.

Silvius provides a bunch of basics, such as cursor navigation commands ("up", "down", "left", "right"), a command for typing each letter (like the NATO alphabet), a command for typing numbers ("number two hundred fifteen") etc.

Beyond that, you are encouraged to experiment and add your own commands to the grammar. This document explains how the grammar works, to enable you to do just that.

Silvius structure

Silvius is built up out of two main parts, in a client-server setup:

       client          server

   --> mic in ------------+ 
                    speech -> text
   <-- keystrokes --------+

The Silvius git repository contains the code for the client-side components. The mic-in part is located in the stream subdirectory, while the keystrokes part is located in the grammar subdirectory.

The remainder of this document will concentrate on generating keystrokes from the transcribed speech.

Transforming transcribed speech into keystrokes can be broken down in several steps:

--> scanner --> parser --> executor --> automator

Briefly, the scanner reads the input and augments it with additional information. The parser will match the input to grammar rules that we define, to determine the actions to take for the input. The executor accepts the actions, and turns them into low-level actions for the automator. The automator is platform-dependent, and will actually perform the low-level actions triggered by the executor.

Scanner & Parser


The scanner produces a list of tokens from the input. In the Silvius client, a Token is a simple Python object that contains three pieces of information (see file

  • a type
  • a wordno
  • optional extra information

The scanner in this project, which takes the input and transforms it into a list of tokens, is simple. It reads a single line from the input, splits it in words based on whitespace, and then creates a Token for each word.

The scanner has a list of keywords; the keywords correspond to words that have a special meaning in the parser, i.e., words that are associated with specific actions in the parser. For example, the word "arch" will be recognized as a letter in the parser, and it will cause the parser to add a keystroke for the letter "a" to the output.

If the scanner finds a keyword string in the input, the corresponding Token will have the keyword as its type, and no extra information.

If the input word is not a keyword, the corresponding Token will have the ANY as its type, and the word will be stored in the extra information.

In both cases, the wordno field is filled with a number that contains the position of the word in the input.

After all words have been processed, the scanner attaches a special token with type END to the token list.


"charlie delta space word silvius"

will be transformed into the following token list:

[ Token("charlie", 0), Token("delta", 1),
  Token("space", 2), Token("word", 3),
  Token("ANY", 4, "silvius") Token("END", 5) ]

Abstract Syntax Tree

The list of tokens, constructed from the speech input, is then fed into the Spark parser. The Spark parser will match its set of rules against the token list, and produce an Abstract Syntax Tree (AST).

In our case, the AST contains commands to be executed by Silvius' keystroke automator. Here is an example, of the AST tree produced by parsing the input "charlie delta space word silvius":

Input words:  charlie  delta  space  word  silvius
Token types: [charlie, delta, space, word, ANY,    END]
Resulting AST:
       chain {
           char ['c']
           char ['d']
           raw_char ['space']
           sequence ['silvius']

The AST consists of nodes that are connected together in a tree structure; this means that AST nodes can have other AST nodes as children. Each node will always have one parent node:

             ____ 'chain' ____
          __/     /     \     \__
     AST      AST       AST       AST
   'char'   'char'  'raw_char'  'sequence'
    'c'      'd'      'space'    'silvius'

Each node in the AST tree is a Python object of type AST (see file, which has a type, optional metainformation, and a list of childrenthat are also ASTnodes.

ASTtypes are different from Token types: the AST types describe the actions that the executor and automator should perform. These are the different possible action types:

  • chain Make a chain of keyboard actions. The actions are listed in the AST node's children. All actions in the chain will be performed in the order in which they appear in the list.
  • char The character that will be sent to the automator is the first character of the meta-information (AST.meta[0]). This type is used for "text" characters, i.e. characters that are to appear on the screen.
  • raw_char The character that will be sent to the automator is the first character of the meta-information (AST.meta[0]). "Raw" characters are any "non-text" characters: punctuation marks, different styles of parentheses, the "escape" character/key,...
  • mod_plus_key Insert a modifier+keystroke action in the parse tree. This allows you to perform keystrokes like "control-" with one command.
  • movement This is intended for movement keystrokes, such as the arrow keys or page-up/page-down/home/end keys.
  • sequence Insert a sequence of actions. These actions can be e.g. char or raw_char actions. The actions will be performed in the order in which they appear in the AST node's children list.
  • word_sequence Insert sequences of space-separated words. The characters in each word are sent as individual keystrokes, as if they were raw_chars.
  • null No action
  • repeat Repeat actions. The action(s) to repeat are stored in the AST nodes children. The number of times to repeat the action(s) is stored in the AST node's meta information (AST.meta[0]).

Parser implementation

The transformation of a token list into an AST requires two parts: rules, to match the input against; and code to manipulate the AST when a rule matches.

The parser starts at the top level rule; in the example below, we'll assume this is the rule single_command. (In the real parser, there are some additional rules, to handle consecutive commands etc. The start rule in the real parser is single_input.)

The Spark parser uses specially-crafted Python functions to implement both parts. Here are some examples:

def p_single_command(self, args):
        single_command ::= letter
        single_command ::= sky_letter
        single_command ::= word_phrase
    return args[0]

def p_letter(self, args):
        letter ::= arch
        letter ::= bravo
        letter ::= charlie
        letter ::= xray
        letter ::= expert
        letter ::= yankee
        letter ::= zulu
    if(args[0].type == 'expert'): args[0].type = 'x'
    return AST('char', [ args[0].type[0] ])

There are two specific things to note about these functions: the declaration of the rules in the function's docstring, and the code in the function body that can manipulate the parsing state.

Rule declaration

First of all, note that the function names are p_single_commandand p_letter. The p_ prefix tells Spark that these functions declare parsing rules. The single_command and letter parts matches the left-hand side of the rule declarations in the function's docstring:

def p_letter(self, args):
        letter::= arch

The docstrings, then, list all the different rules for single_command and letter. In these two examples, all rules happen to have a right-hand side that consists of a single terminal:

    letter ::= arch
    letter ::= bravo
    letter ::= charlie
    letter ::= yankee
    letter ::= zulu

In this example, letter will match any of the words "arch", "bravo",..., "zulu" in the input.

Rule matching

Suppose the input is the word "charlie". The parser always starts by looking for a match for the single_input rule. The way it does this is by looking one-by-one at the right-hand sides of the rules listed in the docstring of the function p_single_input.

The first rule listed in the docstring of p_single_input has right-hand side letter. If there would be no function p_letter (with rules letter ::= ... in its docstring), then this would produce a match, if the word "letter" would have been the input, instead of "charlie".

However, since a function p_letter exists, the parser will look at that function's doctring. The first rule (right-hand side) in the docstring is arch. "Arch" does not match "charlie", and there is also no function p_arch with "rewrite" rules for arch, so this is a dead-end. Likewise for bravo ("charlie" does not match "bravo", and there is no function p_bravo).

The third letter rule, with right-hand side charlie, matches input word "charlie". The parser knows that it can stop matching there, and that it can start running the function bodies to create the AST.

The rules matched, in order, are these ones:

single_input ::= letter
letter ::= charlie

So the function bodies executed, in reverse order, will be p_letterand p_single_input.

Function body

The p_letter function has the following declaration:

def p_letter(self, args):

The args argument of the function is a list, where the n-th element corresponds to the n-th symbol (terminal or not) in the right-hand side of the rule that matched the input.

In our example, where the input is "charlie", the rule that matches is letters ::= charlie. In this case, args[0] will contain a Token for charlie. A Tokencontains a type; charlie in this case.

The function body for p_letter looks like this:

    if(args[0].type == 'expert'): args[0].type = 'x'
    return AST('char', [ args[0].type[0] ])

In the first line, the token type is checked. If it happens to be "expert", it is replaced with just "x".

The returnstatement will return the AST node that will be inserted in the AST that is the result of the parse. In this case, a char node is created; the character itself is placed into the AST node's type field. The actual character is taken from the Token's type field; in the example this will be the first letter of "charlie", i.e. the letter 'c'.

(This explains the previous line in the function body: the first letter of "expert" is 'e', but we want to use "expert" to produce 'x' in the output. So, if we detect "expert", we replace its token type with 'x', before grabbing the first character of it and inserting it into the AST node's type. (This is only needed for "expert"; for all other words, the first letter is actually the one we want to insert in the output.)

Going up the stack of rules that matched, we next find single_input ::= letter. The function body for p_single_input is simply:

return args[0]

This means that the first element of the argument list is returned as-is, without additional manipulation. The final AST, in this case, is then a single AST node:


Executor & Automator


The executor (in walks the AST produced by the parser, and translates the commands (chain, raw_char, repeat,...) into lower-level commands for the automator.

Similar to the parser, the executor uses function prefix n_ to designate functions that implement a command:

def n_chain(self, node):

def n_char(self, node):


def n_repeat(self, node):

Depending on the platform, the executor will select a different automator to generate the actual keystrokes.


The automator exists in 3 flavors, for the 3 main operating systems. The general operating principle for each automator is that it uses an external, platform-specific tool to generate keystroke events. The keystrokes are then sent to the application that is in focus, as if you pressed these keys on the keyboard.

  • Linux:

    this implementation uses xdotool to implement the keystroke generation behavior. This tool requires X-windows to be installed.

  • Windows:

    this implementation uses nircmd to implement the keystroke generation behavior.

  • Mac OS X:

    this implementation uses CLI-Click to implement the keystroke generation behavior.