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author | Eric S. Raymond <esr@thyrsus.com> | 2010-07-02 03:50:26 -0400 |
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committer | Eric S. Raymond <esr@thyrsus.com> | 2010-07-02 03:50:26 -0400 |
commit | a8c0ea93f1dd96ee4492b62846e422bee711d988 (patch) | |
tree | 7ab944fc4828069f07bcf5d18b9ab5c23f651756 /www | |
parent | d3799b3c5d870b70883a9a7c41b95d04b85fee04 (diff) | |
download | gpsd-a8c0ea93f1dd96ee4492b62846e422bee711d988.tar.gz |
Minor clarification about string encoding.
Diffstat (limited to 'www')
-rw-r--r-- | www/AIVDM.txt | 16 |
1 files changed, 9 insertions, 7 deletions
diff --git a/www/AIVDM.txt b/www/AIVDM.txt index c2683b1e..8708e61b 100644 --- a/www/AIVDM.txt +++ b/www/AIVDM.txt @@ -323,13 +323,15 @@ the message descriptions where this has already occurred. It is good practice for a decoder to make reserved fields available to client applications as uninterpreted bitfields. -Character-string fields within AIS messages are encoded in a -special way, referred to as "six-bit" in the tables below. Each -six-bit nibble maps to an ASCII character. Nibbles 0-31 map to the -characters "@" ( ASCII 64) through "\_" (ASCII 95) respectively; -nibbles 32-63 map to characters " " (ASCII 32) though "?" (ASCII -63). Lowercase ASCII letters, the backtick, right and left curly -brackets, pipe bar, tilde and DEL cannot be encoded. +Character-string fields within AIS messages are encoded in a special +way, referred to as "six-bit" in the tables below. First, chop the +string field into consecutive six-bit nibbles without padding (each +span of three 8-bit bytes includes 4 of these). Each six-bit nibble +maps to an ASCII character. Nibbles 0-31 map to the characters "@" ( +ASCII 64) through "\_" (ASCII 95) respectively; nibbles 32-63 map to +characters " " (ASCII 32) though "?" (ASCII 63). Lowercase ASCII +letters, the backtick, right and left curly brackets, pipe bar, tilde +and DEL cannot be encoded. .Sixbit ASCII [width="25%",frame="topbot"] |