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G3 FACSIMILE COMMUNICATIONS
6.2
Signal Types
In G3 facsimile communications, various signals are used in addition to
the previously described binary signals.
(1) Preamble
The preamble is the signal that prefixes the binary signal, and is a series of
the flags for about 1 sec. A "flag" comprises eight bits "01111110" (7E
when expressed in Hex), and the preamble comprises about 37 continuous
flags. Synchronization of the MODEM signals for procedure signals is
performed by prefixing the preamble with binary signals.
(2) Binary signal
Binary signals are signals by which facsimile procedures are handled, and
are obtained by frequency-modulating 1/0. Low-speed signals are used to
prevent transmission errors from occurring at a data transmission speed of
300 bps. The MODEM used at this time uses the modulation system rec-
ommended in
for checking for errors in the transmitted data.
(3) Image signal
On a G3 facsimile, image signals are coded by coding schemes such as
MH and MR. The image signal modulation system and the transmission
speed differ according to the MODEM in use. Image signals are transmit-
ted at data speeds within the range 2400 bps to 33600 bps. Before image
signals are transmitted, they are prefixed with a "training" signal.
(4) Tonal signal
The sine wave of a determined frequency, in other words a tone, is used as
the tonal signal. This means that the height of the tone has meaning. The
procedure for using this signal is called the "tonal procedure."
5–90
Type of Signal
Preamble signal
Binary signal
Image signal
Tonal signal
ITU-T
recommendation V.21. Binary signals have a function
Table 5-13 Signal Types
Meaning
Signal prefixed to binary signal
Signal expressed by 0 or 1 to communicate
meaning of procedure
Signal of coded image
Signal obtained by giving meaning to type of
tone (frequency)
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