8 FAX
101
Chapter 8
8
FAX
8.1
General Information
In addition to the normal teletype modes the PTC-IIIusb supports the following modes;
FM-FAX (Shortwave), AM-FAX (Satellites), SSTV (All present standards) and NFSK-
Demodulation for decoding various shortwave teleprinting methods.
The algorithms used here, profit from the relatively high computing power of the PTC-
IIIusb and allow the system to easily reach the theoretical limits regarding definition,
filter performance and resistance to interference in all picture operating modes.
Additionally, particular for STTV, a new concept for recognizing and filtering the
synchronization impulses has been incorporated. With the use of DSP techniques, the
system delivers excellent linearity, both in the receive and transmission paths. This means
very good color rendering and reproducibility. A possibility for manually setting the filter
bandwidth and maximum picture definition allow the user to adjust for actual signal
conditions.
Transmit operation is also supported for FM-FAX/FSK, AM-FAX and SSTV. The
FM/FSK modulator is phase-continuous and highly linear, producing an extremely clean
transmitted signal.
The FM/FSK demodulated signal is available as a 1 bit square wave at PIN 6 of the RS-
232 interface in addition to the usual 8-bit serial transfer. This allows full compatibility
with the widely used radio teleprinter programs such as Zorns Lemma.
The PTC-IIIusb operates in all special MODEM functions as a FULL-DUPLEX modem.
This means the appropriate demodulator generates the correct data even during the
transmit phase. This is in most cases however, only of interest for testing purposes.
8.2
Basic info concerning FAX and SSTV
8.2.1
AM-FAX
This FAX variant is found mainly in the typical VHF/UHF/SHF FM frequencies, despite
the fact that one tends to think of AM as a shortwave mode. In practice, AM-FAX is an
FM transmission, but is concerned with the transmission of an amplitude modulated low
frequency carrier. The frequency of the carrier tone is normally 2400 Hz. The
instantaneous amplitude relates to the brightness information. When the tone is at its
maximum amplitude, then the receive program must paint a white pixel. When the tone is
very soft, then a black pixel must be displayed. (NOTE: with FM, the loudness of the
transmitted signal has no relation to the strength of the received HF-signal).
The most interesting signal sources of AM-FAX are mainly the weather satellites
(NOAA-Satellites on 137 MHz or the geostationary Meteosat 5 or GOES (USA) on
approx. 1.7 GHz.
To receive these satellites, it is recommended that a special receiver is used with an IF
bandwidth of approx. 30-50 kHz. For the 1.7 GHz band, a small dish or Yagi-antenna
with a low noise LNA or LNC will also be required.
