SCS PTC-IIex, Руководство пользователя

Страница: 125 / 220

8 FAX
During transmit, the modulator has priority with respect to the LED control. The tuning
indicator displays the transmitted data condition, although the PTC-IIex, operating as a
FULL-DUPLEX MODEM, continues to produce receive data.
8.10.2 Tuning-display in COMPARATOR mode
The tuning indicator covers the range 1900
±
800 Hz in both transmit and receive
operation. In addition, during transmit, the Send-LED on the PTC-IIex lights green.
The LED´s are always switched to middle brightness during COMPARATOR operation,
again due to the computer time needed. The
BRightn
parameter has no thus effect
during the time the PTC-IIex is switched to COMPARATOR.
8.11 Tips and Tricks
8.11.1 IF-SHIFT
With the normal SSB speech reception, the speech frequencies stretch from 300 to 2700
Hz. Steep sided SSB filters usually have a 6 dB bandwidth of around 2.4 kHz. The FM-
FAX standard sets a center frequency of 1900 Hz. With standard resolution FAX and
SSTV pictures, the signal requires a bandwidth of approximately 2.5 kHz. The audio limit
frequencies that should be transmitted are 1900-1250 Hz and 1900+1250 Hz or 750 Hz
and 3150 Hz. The frequency band for FAX/SSTV appears to be shifted about 400 to 500
Hz higher in frequency. With normal SSB reception, the higher FAX/SSTV tones suffer
great attenuation and thereby cause asymmetrical reception.
In order to help this situation, it is recommended that the IF-SHIFT control be used. With
the TS-450 for example, it has been found that the best setting is around
3 o’clock . The
ideal setting is that at which the tuning indicator flickers symmetrically around the middle
point when only noise is present on the receive channel.
8.11.2 FIFO
We strongly recommend the use of a serial interface chip in the PC with a FIFO function.
Otherwise, especially at the higher baud rates for high resolution FAX/SSTV, there is a
great probability of data loss or mutilation at the serial interface.
8.11.3 TC-graph
We recommend the use of the so-called HiColor-Graphics in the JVFAX-configuration,
so that optimum SSTV display may be obtained.
8.11.4 New phasing in SSTV
In the SSTV modes, it is relatively common that the vertical (frame) synch pulse is either
not recognized, or that one will want to
start receiving in the middle of a picture
transmission. A manual picture start may be obtained with JVFAX by using <R> button,
this means
RUN .
111