The Slow-scan television transmission use frequency of video signal from 1500 Hz to 2300 Hz. A line and vertical synchropulses use 1200 Hz. Thanks to short spectrum it is possible to transfer images throught common voice channels and record its sound.
In '70s some reel-to-reel tape decks were used for an SSTV recording. Hamradio operators could record received signal and used it for later reproduction or as signal source for a monitor setup. Some operators lack of camera or flying spot scanner used to transmit recorded images for own QSOs.
Many old records of live transmission were well-kept on reels. So I decided to digitize recorded images. Because every nowaday SSTV program includes option to use 8 sec. SSTV mode I thought that is should be quite easy. It turned out, that so it was not so easy.
The first difficulty I've found was an improper tuning to signals. In the ´70s there was no common tunning indicators so operators tuned to signal by ear and by trim pot on monitor adjusted monitor to start displaying. The second problem was a inaccurate scan timing which causes a image slant.
|Mains electricity frequency||50 Hz||60 Hz|
|Horizontal scan||16.6 Hz (60 ms)||15 Hz (67 ms)|
|Number of image lines||120 or 128||120 or 128|
|Frame timing||7.2 or 7.68 s||8.0 or 8.53 s|
|— horizontal||from left to right|
|— vertical||from top to bottom|
|— horizontal||5.0 ms||5.0 ms|
|— vertical||30.0 ms||30.0 ms|
|Black level||1500 Hz|
|White level||2300 Hz|
|Used band width||2800 to 3200 kHz|
For a proper digitalization was necessary solve problems described above. I found that modern software has problem with horizontal sync detection because it depends on accurate free-run timing. So I used old DOS programs JVFAX and GSHPC with Hamcomm modem. They support horizontal sync detection and both SSTV norms too. Mode Wraase SC-1 8 sec. is European 50 Hz norm and Robot 8 BW is American 60 Hz version. Better features shown JVFAX, because reacts to 30ms tone of vertical synchronization (in modern implementations it is used longer pulse and digital header VIS). The problem with number of lines 120 or 128 (used in digital converters) didn't appear.
The most problematic issue is accurate tuning of recorded signals. I was wonder about how to tune recorded signal properly and maybe some sound processing software should help but I did not find the useful function. My solution was following. The signal from tape I was transmitting again by a SSB transceiver and with second one by first I tuned right! Maybe it sounds crazy but it works.
The last problematic issue is some record quality. The durability of magnetic tape reel was good, but sometimes was signal distorted by receiver quality. All old reels I've got was received by homemade receivers.
Records for digitalization provided: Zdeněk OK2PAD, Béda OK2TB, Jirka OK2PDE.
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