cocoaModem
cocoaModem SITOR-B Receiver

Kok Chen, W7AY [w7ay (at) arrl (dot) net]
Last updated: Mar 29, 2006


Index (User's Manual - SITOR-B Receiver)

General Information
Aural Monitor
Accessibility (Incremental Speak and Voice Assist)
Macros
RTTY Interfaces
PSK Interface
MFSK Interface
Hellschreiber Interface
CW Interface
ASCII Interface
SITOR-B Receiver
  • Hellschreiber Control Panel
  • Receiver Section
  • Transmitter Section
HF-FAX Receiver
Synchronous AM Receiver
Versions
Part II



SITOR-B Receiver


cocoaModem supports a receive-only interface for the SITOR-B protocol for reception of NAVTEX and W1AW AMTOR-FEC bulletins. There are two independent receivers, each of which can be connected to a different audio interface.

(International 518 kHz NAVTEX schedules can be found in Annex 7 of this document. Details for USA station can be found here.)

Fig 1 shows the SITOR-B interface (518 kHz NAVTEX broadcast from Pt. Reyes, California):

pastedGraphic
SITOR-B Interface


SITOR-B is an implementation of CCIR 476-5 specifications, using the same 170 Hz shift as Amateur RTTY, but at 100 baud and using synchronous transmission (no start or stop bits). Character synchronization is from occasional sync patterns and from the structure of the character stream itself. FEC is achieved through the use of a 7-bit code, which implements essentially the same character set as 5 bit Baudot, allowing single error bits per character to be completely recoverable. In addition, each character is repeated once in an interleaved character stream, delayed from the original character by 35 bits. This repeated character gives another layer for error detection and correction.

In cocoaModem, SITOR-B squelch is not derived from the signal strength for the FSK signal but from how may errors need to be corrected. With maximal squelch (the squelch slider pushed all the way towards the left), characters are printed only the repeated and the original bytes are identical and is zero Hamming distance away from a coded character. The next level of squelch allows single error bits to be corrected and printed if one of the characters of the repeated pair has no error and is identical to the corrected character. The next level allows both characters to be correctable as long as the two corrected characters are identical. The next squelch level allows printing if either character is a single Hamming distance away from a true encoding, etc.

The Locked indicator on the right shows the quality of the received signal. When there is no decodable signal, the locked indicator remains gray. The indicator turns yellow when a synchronization sequence is received. It is customary for a NAVTEX station to send many seconds of the synchronization sequence when it first starts transmitting. As long as the indicator remains solid yellow, you are tuned to a good signal.

The indicator turns green when it is printing correctly. If the print is not squelched, the indicator flashes orange when any correctable error that is printed (with very high probability of being correct). When printing error occurs, the indicator will flash red. You will for example, not see any red or orange flashes if the squelch control is pushed all the way to the left since both correctable errors and uncorrectable errors are squelched away.

The tuning mechanism is similar to the Wideband RTTY Interface. You can use the default tone pairs, and tune the transceiver itself so that the signal is lines up with the green tone pair markers, or you can bring the signal within the passband of the waterfall display and click on the mark tone of the signal. The scroll wheel of a mouse can be used to fine tune the signal when the cursor is within the waterfall and the cocoaModem window is the active window.

The input device configuration is also similar to the other modes (except there is no transmit section).



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