|
Option USOS |
The USOS (Unshift On Space) option is one of the most important auxiliary functions for RTTY decoding. You should leave it enabled in almost all cases, especially in amateur radio.
Here’s an explanation of why and when it’s useful:
What does USOS do?
Since RTTY (Baudot code) only uses 5 bits, there are two levels: LTRS (letters) and FIGS (numbers/special characters). When the decoder receives a “FIGS” control character, it remains in number mode until it receives an “LTRS” control character.
USOS causes the decoder to automatically switch back to letter mode (LTRS) every time a space character is received.
When should you enable it?
In poor signal conditions (noise/fading):
This is the main reason. If the “LTRS” character is lost due to interference, the decoder would output the entire remaining text as gibberish without USOS. Since almost every word ends with a space, USOS automatically corrects this at the beginning of the next word.In the amateur radio standard:
In modern RTTY radio communication, it is standard practice for transmitters to send a “FIGS” character after a space if they want to send further numbers. Therefore, USOS is the safest setting here.Avoiding “garbage strings”:
It prevents a single incorrect decoder decision from rendering the entire following text unreadable.
When should you DISABLE it?
There are very few situations in which you should disable USOS:
Receiving pure data lists:
When you receive a station that sends long columns of numbers separated by spaces, and the receiving station does not send a new “FIGS” character after the space. Without USOS, the numbers would be passed through; with USOS, you would have to hope that the receiving station switches back to “FIGS” after each space.Historical commercial services:
Some very old weather or news services (which hardly exist anymore) did not adhere to the USOS standard.