Sync Fruv

ÍC4 )F CONVERTER

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Fig. 2. Required additions to the monitoi circuit to incorporate the APT mode.

unit. The 4Hz line rate in the satellite is derived by counting down from the 2400Hz subcarrier (2400/600= 4), We will derive the proper timing for the crt video display in precisely ihe same way. Although it is possible to couni down directly from the satellite subcarrier, a slightly more reliable procedure was chosen. A sample of the 2400Hz satellite subcarrier is fed to the input of an NE565 phase lock loop (PLL). Only a very small audio voltage is required for the operation of the device and diodes at the input limit the applied signal to a safe value, he PLL contains an internal voltage controlled oscillator (vco) which, with proper selection of ihe external components, will lock onto the audio frequency present at the input. The PLL vco tracks any variations in the frequency of the subcarrier signal ¡it the input and thus compensates for any frequency variations introduced by the recording process. The vco output is essentially noise free even if high noise levels are present at the input of the PLL. The output of the vco serves as the frequency standard for the sync system and is divided by 600 by three 7490 decade counters to produce a 4Hz square 'wave with the proper sync relationship to the satellite subcarrier signal. The 4Hz signal is fed to the horizontal monostable in the monitor where it triggers the horizontal deflection system 4 times per second, In order to achieve proper sweep size and linearity, an additional 1.5juF capacitor must be paralleled across the existing 0.47/uF horizontal discharge capacitor in the monitor circuit. The required 200 second vertical sweep is initiated manually using the vertical reset switch on the monitor. An additional IOOOjuF capacitor across the normal 50(jF vertical discharge capacitor provides the proper vertical sweep parameters. I ig. 1, shows the schematic of the video converter itself and Fig, 2, indicates the required additions to the monitor circuit to incorporate the APT mode. The additional capacitors in the discharge circuit can be switched in with a DPDT toggle switch, providing a convenient way to switch between the SSTV and APT displays. The video and sync connections have no effect on the monitor in the SSTV mode when the video converter is not in use.

The only problem that now remains is that although the line display on the monitor is properly synced to the line scanning in ihe satellite, the display may not be properly phased. Improper phasing means that the start of the monitor scanning does not coincide with the start of scanning in the satellite and is indicated by the presence of a vertical white bar, actually the satellite sync pulse, somewhere in the picture. The free-running frequency of the PLL vco is set to 2350 Hz. When SI is thrown to the phasing position the vco is unlocked from the satellite subcarrier and runs free, putting the horizontal display out of sync. When this occurs the white sync bar will shift over to the margin of the picture since the satellite line rate and monitor line rate no longer coincide. When the bar reaches the picture margin SI is returned to the run position, the vco locks to the subcarrier, and a synced condition is re-esiablished. This process is summarized in a diagram in Fig. 3.

Converter Construction

The circuitry is essentially non-critical and any mechanical assembly which permits proper interconnection of the circuit elements may be used. My own unit is constructed in a Ten-Tec enclosure shown in the photo. The unit need not be large and could actually be built into the monitor if desired. The power, phasing, and vco frequency control (Rl) are located on the front panel. The power connector, input and sync and video output are on the rear apron. The sync and video conncctors added to the monitor can be placed on the rear apron. The DPD i toggle switch for changing between the SSTV and APT display is most conveniently located on the front panel of the monitor although this is certainty not required.

Alignment and Use

Place SI in the "phase11 position and adjust the vco frequency control for 2350Hz at TP!. This frequency is not too critical and initially setting up for a frequency just slightly higher than the 2300Hz SSTV white frequency will do for a start, tf the free running vco frequency is too close to 2400Hz phasing will take longer to accomplish while if the frequency is too far from 2400Hz phasing will be too rapid to be accomplished accurately. In practice, the final setting of R1 is adjusted to provide a comfortable phasing rate.

The monitor display should be viewed in a darkened room to visually evaluate the nature of the pictures. A darkened room is absolutely essential for photography since a 200 second time exposure is required. With no APT input adjust the monitor brightness for a barely visable trace. Advance the APT

recorder output for a good black to white video swing on the monitor display. If the vertical sync bar is present, place 51 in "phase" until the bar moves to the margin of the picture and then return the switch to run.

Operational Notes

ATS 3 transmits on a frequency of 1 35.6MHz from 0730-0815Z and 2045-2130Z. The first transmission of the day consists of APT orbital prediction data and NOAA 2 gridded pictures. The second transmission sequence consists of 6 spin scan photos and updated NOAA 2 photos. There is a white calibration signal transmitted between each picture and phasing is easily set up at the beginning of a picture sequence, As long as the recorder is not turned off, proper phasing will be maintained throughout an entire sequence of pictures. The start of each picture is clearly audible and the vertical reset switch can be pressed to initiate the vertical scan.

Photographing the display consists of making a 200 second time exposure as the display reads out. I have used Plus X and

Panatomic 35mm film. Polaroid also works well, The 73 SSTV Handbook contains information on making photographs from a monitor display and the same techniques may be used for the APT pictures.

There are a total of six spin scan photos transmitted each day by ATS 3. These photographs are pari of a single photographic mosaic. The photos fit together, in order of their transmission, as shown in Fig. 4.

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