Spectrogram obtained using software by Richard S.Horne and AOR AR7030 Communications receiver.
Synchronous burst signals use on-off keying of the transmitter in regular time frames to break up the transmitted data stream into small segments. Each segment is of fixed duration and always begins at the same point in a fixed frame time. This gives them a characteristic regular chirp-chirp sound.
The information sending station (ISS) transmits a message using bursts containing data while the information receiving station (IRS) sends bursts back to the ISS which acknowledge correct ISS bursts or requests repeats of corrupted ones. This is an automatic repeat request (ARQ) system and offers error detection and correction capabilities which are not possible with continuous signals.
The classic example of a synchronous burst signal is the AMTOR mode A system used by radio amateurs. This is a 100 baud 2-level FSK signal in which 7-element characters are grouped together by the ISS into bursts lasting 210 ms. The IRS responds with an single 7-element character so that the total frame time (for 3 characters) is 450 ms.
The spectogram below shows an ISS (long bursts) and an IRS (short bursts) for a real off-air signal. You can clearly see the 2-level FSK structure and even read the binary elements being sent in each burst (if you assume a bit value for each tone).
Spectogram of TOR signal
Spectrogram obtained using software by Richard
S.Horne and AOR AR7030 Communications receiver.
Error detection is done using the fact that valid characters
in the alphabet supported by the system must have a 4:3 ratio of
marks to spaces. This means that the IRS can detect corrupted ISS
bursts by counting the number of marks and spaces received. If
they totals do not obey the 4:3 ratio rule the IRS requests a
repeat of the last ISS burst. The IRS is not able to correct
errors on the basis of what it has already received unlike with
forward error correction (FEC) schemes.
