1 February 2018

Baudot encoding with 1.5 stop bits (NATO maritime)

All NATO maritime transmissions (BCST, Ship to Shore and MRL) as well as being encrypted, use the standard 5 N 1.5 (1 start bit, 5 ITA2 data bits, and 1.5 stop bits), clearly a not synchronous standard (modem side). In particular on the BCST, by its nature, the clock is not fixed but varies from time to time according to certain algo. Synchronous transport (ie the cipher-modem chain) is used to recover it.
Although a single 7.5 bits length frame is quite easy to see in the time-domain,  its bit-oriented view is impossible at least in the tools at my disposal (unless to aggregate two consecutive frames and then get an integer number of 15 consecutive bits).
This difficulty about the representation of 7.5 bits frames may lead to misinterpretation and erros, as I did it (!) using SA/Bee to analyze what seemed a 150bps/500 Hz FSK burst transmission copied on 16155.0 Khz (cf). Speed was misured using the AM detector tool (Fig. 1)

Fig. 1
and once demodulated, each burst exhibits a 15-bit period (Fig. 2)

Fig. 2

But the ACF tells a different story: the signal  is structured in 7.5 bits lenght frames each consisting of 1 start bit, 5 data bit and 1.5 stop bits and duration of ~100 msec (Fig. 3). This means ITA-2 coding, commonly referred to as Baudot, and a speed of 75 bps!

Fig. 3
As said, the impossibility to represent an "half bit" makes that two consecutive frames are considered and then either the speed and the period are misrepresented!
The transmission can be decoded using a common Baudot decoder and setting the right speed (75bps) and shift (500Hz): it consists of ten groups of 5 figures per burst, as shown in Fig. 4. Removing the five extra-bits (start e stop bits) from the 15-bit period stream we just get the 61 ITA2 5-bit characters of each burst.

Fig. 4
It's worth noting that things are worse trying to force the speed to 75 bps: one bit is lost  (Figs 5,6)

Fig. 5
By the way, the 1.5 bit lenght of the stop signal is derived from the design of early teletype machines, which was designed this way because the electromechanical technology of its day was not precise enough for synchronous operation: thus the systems needed to be re-synchronized at the start of each character while the stop duration gave the system time to recover before the next start signal.

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