Communications at Sea

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Support material for: 'Understanding Weatherfax' and 'Communications at Sea'

Communications at sea - corrections
Recognising fax transmissions
Radio data sample sounds

Recognising fax transmissions

Try this weather fax sound clip and see if you can identify the following parts :-

1. START TONE
This is a tone formed by fast, regular switching between the black and white frequencies and is used to start certain types of automatic receivers. The switching frequency, usually 300 Hz for most weather fax transmissions, is used to indicate the index of co-operation.

2. PHASING SIGNAL OR SYNC. PULSE
This follows briefly after the start tone and is heard as a repeated blip. On screen it appears a rectangular block that marks the beginning of a scan line. Ideally, this should be positioned at the extreme left, top corner of the screen. If it appears in the middle, the received image will be split into two parts which normally causes no difficulties as most software contains a facility for rejoining (rejustifying) the two halves.

3. TEST SCALE TONE BAR
Not all stations transmit a tone bar but, when present, it is heard as a repeated glissando whistle or steadily changing frequency. On a correctly tuned screen it appears as a narrow band of grey progressing from black to white.

4. IMAGE BODY
The image itself, the longest part of the transmission, which lasts between 8 and 15 minutes.

The sound clip lasts just under a minute but in a real transmission, when the body of the message is complete the signal ends with a STOP TONE which sounds rather like the start tone but is sent at 450 Hz and which tells the receiving equipment that the fax is complete. Some stations follow this with a few seconds or minutes of UNMODULATED CARRIER or 'black' tone signal.

Radio data transmission samples

Weather fax signals are just one of many encoded data formats that you'll encounter on the HF bands. Here are samples of some others that are in common use

	Family	Variant
	AMTOR	100 baud FEC
	AMTOR	100 baud ARQ
	GTOR	200 baud
	Pactor	100 baud
	Pactor	200 baud
	PT2	Unproto mode
	PT2	ARQ mode
	Slow Scan Television	Martin 1
	Slow Scan Television	Scottie 2
	Slow Scan Television	Scottie DX

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