Varicode

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Varicode is an entropy coding , which means that the code of frequently occurring characters is shorter than that of rarer characters - similar to Morse code . This increases the data throughput. Varicode is used for data transmission on shortwave with the PSK31 mode in the amateur radio service .

Emergence

Peter Martinez (G3PLX) developed Varicode in 1997 to transmit text more efficiently, and therefore examined large quantities of English text according to their letter frequency .

construction

Since the codes only consist of 0 and 1, it is a binary code . To meet the Fano condition , three rules have been established.

  • After each message “00” follows as a separator.
  • "00" does not appear in any message.
  • Each message begins with a "1" and ends with a "1".

According to these rules, the most frequent ASCII characters were assigned the shortest codes. Starting with the space, where the 1 from the start and end coincide. This is followed by “e” (11), “o” (111), “t” (101) and mainly lower case letters, followed by upper case letters, numbers, special characters and control characters.

variants

Originally only ASCII was supported, German umlauts were e.g. B. not representable. In 1998, the first programs used the 8-bit ISO 8859-1 code , whereby the new 128 characters were taken from the standard one after the other and not sorted according to frequency. Almost all programs for (de) modulating PSK31 / PSK63 now support 256 characters (8 bit).

use

The digital operating mode PSK31 and the faster variant PSK63 use Varicode to compress the messages.

literature

Web links