8 FAX
107
8.6
MODEM commands in detail
Activating the MODEM commands is the task of the displaying programs on the PC,
e.g. JVCOMM32 or MSCAN. They are not intended for direct user access. If
somebody tries to call for example SSTV out of terminal programs like PlusTerm, he
will be punished with the output of a never ending data stream of hex characters
☺
8.6.1
Amfax
Starts the AM-FAX-MODEM. The measured amplitude of the 2400 Hz tone frequency is
given over the RS-232 interface at the rate set using the
MBaud
command.
The time-resolution required, and filter bandwidth may be pre-set using the
AResolut
parameter. The internal amplification may be pre-set using the
AGain
parameter. The
output values reach from 0 to 255. Measured values greater than 255 are limited to 255 by
the PTC-IIIusb. (The data width is limited to 8 bits by the serial format.) With an AF
input amplitude of 500 mV, and a standard setting of
AGain
(50), the PTC-IIIusb gives
an output value of 255. With an input of 250 mV the output value is appropriately 128.
For transmission use when using AM-FAX, refer to chapter
and chapter
.
8.6.2
Fmfax
Starts the FM-FAX-MODEM. The measured
instantaneous
signal frequency is sent via
the RS-232 serial interface, at the rate set by the
MBaud
command.
The time-resolution required, and filter bandwidth may be pre-set using the
FResolut
parameter. The
steepness
of the FM-detector may be pre-set using the
Deviation
parameter. The output values reach from 0 to 255. Measured values smaller than 0 are
output as 0. Values greater than 255 are limited to 255 by the PTC-IIIusb. The center
frequency is exactly 1900 Hz. With a frequency of 1900 Hz as AF input, the PTC-IIIusb
always gives out the appropriate value of 128. With the standard setting of 400 Hz shift
(Deviation = 400), then an output value of 255 is obtained with an input frequency of
2300 Hz, and the input frequency of 1500 Hz gives an output value of 0.
For transmission use when in FM-FAX operation, refer to chapter
, page
8.6.3
Sstv
Starts the SSTV-MODEM. This MODEM resembles very closely the FM-FAX-
MODEM (see Fmfax- MODEM command). It consists of an FM detector with a center
frequency of 1900 Hz and a pre-set shift of 400 Hz. (Independent from the Deviation-
Parameter). The Black and White limit frequencies are appropriately 1500 and 2300 Hz.
The maximum possible resolution and input filter bandwidth may be pre-set using the
SResolut
parameter.
Unlike the FM-FAX-Modem, values smaller than 0 (measured frequency lower than 1500
Hz) or greater than 255 (measured frequency higher than 2300 Hz) are not just limited to
0 or 255. In SSTV, It has been found advantageous to
fold-back
incorrect
values into the
correct area. If the system, for example, measures a frequency of 2700 Hz, then it does
not give out a value of 255. Instead, the output value is calculated using the formula