Main page Continuous FSK signals have a characteristic "jangling" sound and it is possible with experience to tell the difference between some of the types that can be heard. There are broadly two categories of signal that can be heard:
Asynchronously coded
Asynchronously coded signals are frequently used by radio amateurs and include systems such as 5-element radio teletype (RTTY). This system has an alphabet of just 32 characters: A-Z, CR, LF, Letters, Figures, Space. Numerals, parenthesis, punctuation and various other characters are obtained using the Letters/Figures keys to extend the basic character set.
RTTY is a very simple system in which each baudot character is constructed from 5 2-level symbols (bits in fact). Start and stop bits are added to each character by the transmitter so that the receiver can extract them from the incoming bit stream. This means of maintaining a proper timing relationship between transmitter and receiver is inefficient since you have to send symbols simply to stay in sync.
Typical RTTY systems may be 45.45, 50 or 75 baud and use 1 start bit and 1 ½ stop bits per 5-element character. That means 1/3 of the bits used to send each character are needed simply for synchronisation purposes. However, the advantage of asynchronous systems is that simple electro-mechanical terminals (teleprinters) can be used without the need for computers. They can also cope well with "one finger" typists (like me) where there is an undeterminable delay between each character.
Synchronously coded
Synchronously encoded signals do not transmit start and stop bits but rely on embedded symbol structures sent infrequently to provide long-term synchronisation. Apart from occasional synchronisation (which may be only at the beginning of each message) only data symbols are sent making synchronous system potentially much more efficient of bandwidth than asynchronous ones.
The penalty with synchronous systems is that they require stable symbol clocks and generally need more complex equipment. However, because the structure of the signal is better defined it is possible to apply error detection and correction codes.
An example of a synchronous system commonly used by radio amateurs is AMTOR mode B (also known as FEC). This system illustrates the principles behind synchronously coded signals very well.
FEC stands for forward error correction, and with AMTOR mode B involves each character being sent twice inter-spaced by four other characters. This means that (at 100 baud) each 7-element character has a 280 ms gap between each time it is sent so that any noise that may have corrupted one instance of the character may have cleared by the time the next one is sent. Errors are detected by looking for a 4:3 symbol ratio where valid characters in the alphabet supported by the system must have four marks and three spaces (for 2-level symbols). A receiver can check for the right ratio of marks to spaces in each character and select those that are correct. There are two chances of getting it right, but this system will not detect when noise or interference has corrupted characters so that the 4:3 ratio is still true. There is also no direct means of requesting a repeat automatically using ARQs.