Timewave PK-900, Operating Manual

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9.2.1 Finding SSTV Frequencies
In the US, SSTV is presently most popular on the 20 meter HF amateur
band. Most evenings an active group of SSTV enthusiasts can be
found on 14.230 and 14.233 MHz. This is a good place to start
looking for signals. Once you recognize the different sounds of
common SSTV signals, you may find activity on other frequencies and
bands especially as propagation changes.
9.3 FAX and SSTV Analog Signal Operation
To accommodate the characteristics of gray-scale FAX and SSTV analog
signals, the PK-900 uses a special Analog mode. Software programs
to display images should automatically set the PK-900 Analog
parameters so most users need not concern themselves with the
following information. If you are curious how the PK-900 works in
this mode, or are interested in writing your own display software,
this information may be helpful.
Note: The Analog mode requires special display software that has been
written for each family of personal computer be used. Software
is necessary in order to handle the gray scale FAX and SSTV
images. This software is not provided with the PK-900
because the unit operates with many different types of computers.
Without display software, the Analog mode should not be used.
The Analog mode is designed to pass data for communications methods
that require "gray scales" or color such as FAX and SSTV. In HF FAX
and SSTV signals, the information is contained in an audio signal
that varies from 1,500-2,300 Hz for FAX and 1,100-2,300 Hz for SSTV.
The analog modem is designed to bandpass filter the analog signal.
The filtered signal is then passed to a limiter - zero crossing
detector. The detected signal is further filtered to reduce noise
then digitized. This signal is converted to RS-232 levels and
switched to pin 6 of the DB-25 serial connector. This pin is used for
data only when modem 8 is selected.
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