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3. Configuring BRLTTY

Most if not all of the settings of BRLTTY are configurable through the configuration menu. Those that are not (e.g. cursor routing performance) can be altered permanently by modifying the source and recompiling.

3.1 The Braille Translation Tables

BRLTTY uses two Braille translation tables to govern the mapping from character/attribute number to dot combination. The first, and most important, is the text translation table.

BRLTTY is initially configured to use the North American Braille Computer Code (NABCC). However, as well as the default text translation table file (us.tbl), BRLTTY comes with four alternative tables: french.tbl, german.tbl, simple.tbl and uk.tbl. See section Command Line Options for details of using alternative translation tables.

The attribute translation table is used when BRLTTY is in attribute display mode. There is currently only the default table, attrib.tbl. This was designed so that each dot represents one bit of the attribute byte, with the foreground colour on the right (dots 4, 5, 6, 8) and the background colour on the left. A foreground bit being on or a background bit being off triggers the corresponding dot: this odd logic in practice aids readability of the most common attributes.

These translation tables are 256-byte binary files. The character/attribute value is used as an index into the file, and the byte at that position gives the corresponding dot combination.

The mapping from bit number to physical dot position varies from display to display and can be fairly arbitrary. An attempt has therefore been made to standardise by defining the action of each bit in a table entry. The Braille display drivers then perform any necessary adjustment for the display.

Bit 0

Dot 1 (top left)

Bit 1

Dot 4 (top right)

Bit 2

Dot 2 (below 1)

Bit 3

Dot 5 (below 4)

Bit 4

Dot 3 (below 2)

Bit 5

Dot 6 (below 5)

Bit 6

Dot 7 (bottom left)

Bit 7

Dot 8 (bottom right)

In each case, a bit being set means that the corresponding dot is present.

Two small utilities, txt2tbl and tbl2txt are provided for easy manipulation of these translation files, see below.

The filenames of the compiled-in translation tables are defined in Makefile. Any changes you make to these will take effect on recompiling the software. However, you can use an alternative text translation table, see section Command Line Options.

3.2 Table Utilities

txt2tbl and tbl2txt are simple programs to convert between binary table files and a textual representation (as used by some DOS drivers). They can be used to create or modify text translation table files without the need to plunge into binary editors and hexadecimal numbers.

To use either of these programs, type:

   progname input output
There are no other command line options.

In the textual representation, each table entry is written as a line. The active part of the line is all the characters between the first pair of parentheses---these utilities ignore all other characters.

Between parentheses are numbers representing the setting of the corresponding Braille dot (see above). Offsets into the binary table file are allocated sequentially from 0 to 255.

For example, the 91th line (ASCII 90 = `Z') in the table might look like:

90 (1 3 567 )

convtable is a filter to convert Braille tables between different binary formats: BRLTTY standard, The standard used by Tieman B.V. and that used by both Alva B.V. and Telesensory Systems Inc. For instance, type

   convtable src dest < input > output
where src and dest can be s for standard (BRLTTY), t for Tieman or a for Alva/TSI mappings.


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