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<h3 class="section">7.3 Phonetic Code</h3>
<!-- @emph{(The following section was originally written by Bj@"orn Jacke, -->
<!-- bjoern.jacke at gmx de)} -->
<p>Aspell is in fact the spell checker that comes up with the best
suggestions if it finds an unknown word. One reason is that it does
not just compare the word with other words in the dictionary (like
Ispell does) but also uses phonetic comparisons with other words.
<p>The new table driven phonetic code is very flexible and setting up
phonetic transformation rules for other languages is not difficult but
there can be a number of stumbling blocks — that's why I wrote this
section.
<p>The main phonetic code is free of any language specific code and
should be powerful enough to allow setting up rules for any language.
Anything which is language specific is kept in a plain text file and
can easily be edited. So it's even possible to write phonetic
transformation rules if you don't have any programming skills. All
you need to know is how words of the language are written and how they
are pronounced.
<h4 class="subsection">7.3.1 Syntax of the transformation array</h4>
<p>In the translation array there are two strings on each line; the first
one is the search string (or switch name) and the second one is the
replacement string (or switch parameter). The line
<pre class="example"> version <var>version</var>
</pre>
<p class="noindent">is also required to appear somewhere in the translation array. The
version string can be anything but it should be changed whenever a new
version of the translation array is released. This is important
because it will keep Aspell from using a compiled dictionary with the
wrong set of rules. For example, if when coming up with suggestion
for <code>hallo</code>, Aspell will use the new rules to come up with the
soundslike say <code>H*L*</code>, but if `<samp><span class="samp">hello</span></samp>' is stored in the
dictionary using the old rules as <code>HL</code> instead of <code>H*L*</code>
Aspell will never be able to come up with `<samp><span class="samp">hello</span></samp>'. So to solve
this problem Aspell checks if the version strings match and aborts
with an error if they don't. Thus it is important to update it
whenever a new version of the translation array is released. This is
only a problem with the main word list as the personal word lists are
now stored as simple word lists with a single header line (i.e. no
soundslike data).
<p>Each non switch line represents one replacement (transformation) rule.
Words beginning with the same letter must be grouped together; the
order inside this group does not depend on alphabetical issues but it
gives priorities; the higher the rule the higher the priority. That's
why the first rule that matches is applied. In the following example:
<pre class="example"> GH _
G K
</pre>
<p class="noindent">`<samp><span class="samp">GH -> _</span></samp>' has higher priority than `<samp><span class="samp">G -> K</span></samp>'
<p>`<samp><span class="samp">_</span></samp>' represents the empty string “”. If `<samp><span class="samp">GH -> _</span></samp>' came
after `<samp><span class="samp">G -> K</span></samp>', the second rule would never match because the
algorithm would stop searching for more rules after the first match.
The above rules transform any `<samp><span class="samp">GH</span></samp>' to an empty string (delete
them) and transforms any other `<samp><span class="samp">G</span></samp>' to `<samp><span class="samp">K</span></samp>'.
<p>At the end of the first string of a line (the search string) there may
optionally stand a number of characters in brackets. One (only one!)
of these characters must fit. It's comparable with the `<samp><span class="samp">[ ]</span></samp>'
brackets in regular expressions. The rule `<samp><span class="samp">DG(EIY) -> J</span></samp>' for
example would match any `<samp><span class="samp">DGE</span></samp>', `<samp><span class="samp">DGI</span></samp>' and
`<samp><span class="samp">DGY</span></samp>' and replace them with `<samp><span class="samp">J</span></samp>'. This way you can
reduce several rules to one.
<p>Before the search string, one or more dashes `<samp><span class="samp">-</span></samp>' may be placed.
Those search strings will be matched totally but only the beginning of
the string will be replaced. Furthermore, for these rules no follow-up
rule will be searched (what this is will be explained later). The
rule `<samp><span class="samp">TCH-- </span></samp>'-> _ will match any word containing
`<samp><span class="samp">TCH</span></samp>' (like `<samp><span class="samp">match</span></samp>') but will only replace the first
character `<samp><span class="samp">T</span></samp>' with an empty string. The number of dashes
determines how many characters from the end will not be replaced.
After the replacement, the search for transformation rules continues
with the not replaced `<samp><span class="samp">CH</span></samp>'!
<p>If a `<samp><span class="samp"><</span></samp>' is appended to the search string, the search for
replacement rules will continue with the replacement string and not with
the next character of the word. The rule `<samp><span class="samp">PH< -> F</span></samp>' for example
would replace `<samp><span class="samp">PH</span></samp>' with `<samp><span class="samp">F</span></samp>' and then again start to search for
a replacement rule for `<samp><span class="samp">F...</span></samp>'. If there would also be rules
like `<samp><span class="samp">FO </span></samp>'-> `<samp><span class="samp">O</span></samp>' and `<samp><span class="samp">F -> _</span></samp>' then words like
`<samp><span class="samp">PHOXYZ</span></samp>' would be transformed to `<samp><span class="samp">OXYZ</span></samp>' and any occurrences of
`<samp><span class="samp">PH</span></samp>' that are not followed by an `<samp><span class="samp">O</span></samp>' will be deleted like
`<samp><span class="samp">PHIXYZ -> IXYZ</span></samp>'. The second replacement however is not applied if
the priority of this rule is lower than the priority of the first rule.
<p>Priorities are added to a rule by putting a number between 0 and 9 at
the end of the search string, for example `<samp><span class="samp">ING6 -> N</span></samp>'.
The higher the number the higher is the priority.
<p>Priorities are especially important for the previously mentioned
follow-up rules. Follow-up rules are searched beginning from the last
string of the first search string. This is a bit complicated but I
hope this example will make it clearer:
<pre class="example"> CHS X
CH G
HAU--1 H
SCH SH
</pre>
<p>In this example `<samp><span class="samp">CHS</span></samp>' in the word `<samp><span class="samp">FUCHS</span></samp>' would be
transformed to `<samp><span class="samp">X</span></samp>'. If we take the word `<samp><span class="samp">DURCHSCHNITT</span></samp>' then
things look a bit different. Here `<samp><span class="samp">CH</span></samp>' belongs together and
`<samp><span class="samp">SCH</span></samp>' belongs together and both are spoken separately. The
algorithm however first finds the string `<samp><span class="samp">CHS</span></samp>' which may not be
transformed like in the previous word `<samp><span class="samp">FUCHS</span></samp>'. At this point the
algorithm can find a follow-up rule. It takes the last character of
the first matching rule (`<samp><span class="samp">CHS</span></samp>') which is `<samp><span class="samp">S</span></samp>' and looks for
the next match, beginning from this character. What it finds is
clear: It finds `<samp><span class="samp">SCH -> SH</span></samp>', which has the same priority
(no priority means standard priority, which is 5). If the priority is
the same or higher the follow-up rule will be applied. Let's take a
look at the word `<samp><span class="samp">SCHAUKEL</span></samp>'. In this word `<samp><span class="samp">SCH</span></samp>' belongs
together and may not be taken apart. After the algorithm has found
`<samp><span class="samp">SCH </span></samp>'-> `<samp><span class="samp">SH</span></samp>' it searches for a follow-up rule for
`<samp><span class="samp">H+</span></samp>'`<samp><span class="samp">AUKEL</span></samp>'. It finds `<samp><span class="samp">HAU--1 -> H</span></samp>', but does not
apply it because its priority is lower than the one of the first rule.
You see that this is a very powerful feature but it also can easily
lead to mistakes. If you really don't need this feature you can turn
it off by putting the line:
<pre class="example"> followup 0
</pre>
<p class="noindent">at the beginning of the phonetic table file. As mentioned, for rules
containing a `<samp><span class="samp">-</span></samp>' no follow-up rules are searched but giving such
rules a priority is not totally senseless because they can be
follow-up rules and in that case the priority makes sense again.
Follow-up rules of follow-up rules are not searched because this is in
fact not needed very often.
<p>The control character `<samp><span class="samp">^</span></samp>' says that the search string only
matches at the beginning of words so that the rule `<samp><span class="samp">RH -> R</span></samp>' will
only apply to words like `<samp><span class="samp">RHESUS</span></samp>' but not `<samp><span class="samp">PERHAPS</span></samp>'. You
can append another `<samp><span class="samp">^</span></samp>' to the search string. In that case the
algorithm treats the rest of the word totally separately from the
first matched string at the beginning. This is useful for prefixes
whose pronunciation does not depend on the rest of the word and vice
versa like `<samp><span class="samp">OVER^^</span></samp>' in English for example.
<p>The same way as `<samp><span class="samp">^</span></samp>' works does `<samp><span class="samp">$</span></samp>' only apply to words
that end with the search string. `<samp><span class="samp">GN$ -> N</span></samp>' only
matches on words like `<samp><span class="samp">SIGN</span></samp>' but not `<samp><span class="samp">SIGNUM</span></samp>'. If
you use `<samp><span class="samp">^</span></samp>' and `<samp><span class="samp">$</span></samp>' together, both of them must fit
`<samp><span class="samp">ENOUGH^$ -> NF</span></samp>' will only match the word
`<samp><span class="samp">ENOUGH</span></samp>' and nothing else.
<p>Of course you can combine all of the mentioned control characters but
they must occur in this order: `<samp><span class="samp">< - priority ^ $</span></samp>'. All
characters must be written in CAPITAL letters.
<p>If absolutely no rule can be found — might happen if you use strange
characters for which you don't have any replacement rule — the next
character will simply be skipped and the search for replacement rules
will continue with the rest of the word.
<p>If you want double letters to be reduced to one you must set up a rule
like `<samp><span class="samp">LL- -> L</span></samp>'. If double letters in the resulting phonetic
word should be allowed, you must place the line:
<pre class="example"> collapse_result 0
</pre>
<p class="noindent">at the beginning of your transformation table file; otherwise set the
value to `1'. The English rules for example strip all vowels from
words and so the word "GOGO" would be transformed to "K" and not to
"KK" (as desired) if <code>collapse_result</code> is set to 1. That's why
the English rules have <code>collapse_result</code> set to <code>0</code>.
<p>By default, all accents are removed from a word before it is matched to
the soundslike rules. If you do not want this then add the line
<pre class="example"> remove_accents 0
</pre>
<p>at the beginning of your file. The exact definition of an accent is
language dependent and is controlled via the character set file. If you
set remove_accents to '0' then you should also set "store-as" to "lower"
in the language data file (not the phonetic transformation file)
otherwise Aspell will have problems when both the accented and the
de-accented version of a word appearing in the dictionary; it will
consider one of them as incorrectly spelled.
<h4 class="subsection">7.3.2 How do I start finally?</h4>
<p>Before you start to write an array of transformation rules, you should
be aware that you have to do some work to make sure that things you do
will result in correct transformation rules.
<h5 class="subsubsection">7.3.2.1 Things that come in handy</h5>
<p>First of all, you need to have a large word list of the language you
want to make phonetics for. It should contain about as many words as
the dictionary of the spell checker. If you don't have such a list,
you will probably find an Ispell dictionary at
<a href="http://fmg-www.cs.ucla.edu/geoff/ispell-dictionaries.html">http://fmg-www.cs.ucla.edu/geoff/ispell-dictionaries.html</a> which
will help you. You can then make affix expansion via <samp><span class="command">ispell
-e</span></samp> and then pipe it through <samp><span class="command">tr " " "\n"</span></samp> to put one word on
each line. After that you eventually have to convert special
characters like `<samp><span class="samp">é</span></samp>' from Ispell's internal representation to
latin1 encoding. <samp><span class="command">sed s/e'/é/g</span></samp> for example would replace
all `<samp><span class="samp">e'</span></samp>' with `<samp><span class="samp">é</span></samp>'.
<p>The second is that you know how to use regular expressions and know
how to use <samp><span class="command">grep</span></samp>. You should for example know that:
<pre class="example"> grep ^[^aeiou]qu[io] wordlist | less
</pre>
<p class="noindent">will show you all words that begin with any character but `<samp><span class="samp">a</span></samp>',
`<samp><span class="samp">e</span></samp>', `<samp><span class="samp">i</span></samp>', `<samp><span class="samp">o</span></samp>' or `<samp><span class="samp">u</span></samp>' and then continue with
`<samp><span class="samp">qui</span></samp>' or `<samp><span class="samp">quo</span></samp>'. This stuff is important for example to
find out if a phonetic replacement rule you want to set up is valid
for all words which match the expression you want to replace. Taking
a look at the regex(7) man page is a good idea.
<h5 class="subsubsection">7.3.2.2 What the phonetic code should do</h5>
<p>Normal text comparison works well as long as the typer misspells a
word because he pressed one key he didn't really want to press. In
these cases, mostly one character differs from the original word.
<p>In cases where the writer didn't know about the correct spelling of
the word, the word may have several characters that differ from the
original word but usually the word would still sound like the
original. Someone might think that `tough' is spelled `taff'. No
spell checker without phonetic code will come to the idea that this
might be `tough', but a spell checker who knows that `taff' would be
pronounced like `tough' will make good suggestions to the user. Another
example could be `funetik' and `phonetic'.
<p>From these examples you can see that the phonetic transformation should
not be too fussy and too precise. If you implement a whole phonetic
dictionary as you can find it in books this will not be very useful
because then there could still be many characters differing from the
misspelled and the desired word. What you should do if you implement
the phonetic transformation table is to reduce the number of used
letters to the only really necessary ones.
<p>Characters that sound similar should be reduced to one. In the English
language for example `Z' sounds like `S' and that's why the
transformation rule `<samp><span class="samp">Z -> S</span></samp>' is present in the
replacement table. “PH is spoken like “F and so we have a
`<samp><span class="samp">PH -> F</span></samp>' rule.
<p>If you take a closer look you will even see that vowels sound very
similar in the English language: `contradiction', `cuntradiction',
`cantradiction' or `centradiction' in fact sound nearly the same,
don't they? Therefore the English phonetic replacement rules not only
reduce all vowels to one but even remove them all (removing is done by
just setting up no rule for those letters). The phonetic code of
“contradiction” is “KNTRTKXN” and if you try to read this
letter-monster loud you will hear that it still sound a bit like
`contradiction'. You also see that “D” is transformed to “T”
because they nearly sound the same.
<p>If you think you have found a regularity you should <em>always</em> take
your word list and <samp><span class="command">grep</span></samp> for the corresponding regular
expression you want to make a transformation rule for. An example: If
you come to the idea that all English words ending on `ough' sound
like `AF' at the end because you think of `enough' and `tough'. If
you then <code>grep</code> for the corresponding regular expression by
<samp><span class="command">grep -i ough$ wordlist</span></samp> you will see that the rule you wanted
to set up is not correct because the rule doesn't fit to words like
`although' or `bough'. So you have to define your rule more precisely
or you have to set up exceptions if the number of words that differ
from the desired rule is not too big.
<p>Don't forget about follow-up rules which can help in many cases but
which also can lead to confusion and unwanted side effects. It's also
important to write exceptions in front of the more general rules
(`<samp><span class="samp">GH</span></samp>' before `<samp><span class="samp">G</span></samp>' etc.).
<p>If you think you have set up a number of rules that may produce some
good results try them out! If you run Aspell as <samp><span class="command">aspell
--lang=</span><var>your_language</var><span class="command"> pipe</span></samp> you get a prompt at which you can type
in words. If you just type words Aspell checks them and eventually
makes suggestions if they are misspelled. If you type in <code>$$Sw
</code><var>word</var> you will see the phonetic transformation and you can test
out if your work does what you want.
<p>Another good way to check that changes you make to your rules don't
have any bad side effects is to create another list from your word
list which contains not only the word of the word list but also the
corresponding phonetic version of this word on the same line. If you
do this once before the change and once after the change you can make
a diff (see <samp><span class="command">man diff</span></samp>) to see what <em>really</em> changed. To
do this use the command <samp><span class="command">aspell --lang=</span><var>your_language</var><span class="command">
soundslike</span></samp>. In this mode Aspell will output the the original word
and then its soundslike separated by a tab character for each word you
give it. If you are interested in seeing how the algorithm works you
can download a set of useful programs from
<a href="http://members.xoom.com/maccy/spell/phonet-utils.tar.gz">http://members.xoom.com/maccy/spell/phonet-utils.tar.gz</a>. This
includes a program that produces a list as mentioned above and another
program which illustrates how the algorithm works. It uses the same
transformation table as Aspell and so it helps a lot during the
process of creating a phonetic transformation table for Aspell.
<p>During your work you should write down your basic ideas so that other
people are able to understand what you did (and you still know about
it after a few weeks). The English table has a huge documentation
appended as an example.
<p>Now you can start experimenting with all the things you just read and
perhaps set up a nice phonetic transformation table for your language
to help Aspell to come up with the best correction suggestions ever
seen also for your language. Take a look at the Aspell homepage to
see if there is already a transformation table for your language. If
there is one you might also take a look at it to see if it could be
improved.
<p>If you think that this section helped you or if you think that this is
just a waste of time you can send any feedback to
<a href="mailto:bjoern.jacke@gmx.de">bjoern.jacke@gmx.de</a>.
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