8 FAX
106
The carrying out of a Modem command sets the PTC-IIIusb into the actual
FAX/SSTV/AUX modem function. It loads the appropriate new routines into the signal
processor, adjusts the tuning indicator to the new function and at once starts passing the
demodulated signal to the RS-232 interface - which may have had its baud rate changed
during the Modem command, refer to the
MBaud
-Parameter command.
The MODEM operation of the PTC-IIIusb can be ended at any time by inputting a byte
with the value 255 (dec). via the RS-232 port. It is essential to use the correct baud rate
(see
MBaud
-command!). After the ending of Modem operation, the PTC-IIIusb
announces itself with the normal
cmd:-
prompt, and is at once in the main menu. The
baud rate is automatically returned to the value it was before the Modem command was
called.
The PARAMETER commands are used for setting the various values needed for the
particular Modem mode chosen, e.g. the actual baud rate during Modem operation
(
MBaud
), the deviation for FM-FAX (
Deviation
), or the internal amplification for
AM-FAX (
AGain
parameter).
The Parameters must be set before the start of the chosen Modem function! During
MODEM operation, the Parameter commands are not available; only the change of
particular operating parameters in the operating mode
JVFAX
is possible using special
control-codes (refer to the
JVFAX
modem command).
Note concerning the data-rate during MODEM operation:
The received data is sent with the maximum possible speed to the serial interface in all
MODEM operating modes. There are no pauses between the individual characters, not
even through excessive use of the Packet-Radio function of the PTC-IIIusb multi-tasking.
With an
MBaud
-rate for example, of 57600 bits/sec, there is a new value available for the
connected PC approximately every 170
μ
sec (10 steps of 1/57600 sec per step). A
guarantee of modem data synchronization over a long period, cannot however be given.
Connects in Packet-Radio from outside are also possible during active MODEM
functions without restrictions. Receiving data will be buffered as far as buffer is
available.
8.5
The PTC-IIIusb as COMPARATOR-MODEM
The PTC-IIIusb is capable to be used as a simple COMPARATOR-Modem. This allows
compatibility to all at present available RTTY/FAX/SSTV programs that allow use of a
simple modem
(e.g. HAMCOMM modem). The operation as a COMPARATOR is
completely different to that of the other MODEM variants of the PTC-IIIusb. In the
normal MODEM operation (
Amfax
,
Fmfax
,
SSTV
commands), the DSP undertakes the
demodulation of the signal itself, and uses relative complex algorithms. In the
COMPARATOR mode, it is only used as an adjustable pre-filter (bandpass filter, Fresol
command). The pre-filtered signal is then only hard limited, and passed on as virtual
DCD signal in the USB data stream. The actual demodulation must be undertaken by the
PC program, so that considerable differences in quality may occur.
