GB3EY off - nearly ready for full repeat mode
Posted: Wed Sep 02, 2020 11:45 am
GB3EY's beacon mode is off air because the DTX1 DVB-S modulator it was using is needed for integration into the digital GB3EY repeater hardware.
This has shown up an issue on the repeater logic based on the BATC I2C project. This is the original analogue/FM repeater logic used by GB3EY that has been substantially modified to make it work in a digital system. It was found that the DTX1 was only producing a blue screen when fed from video derived from the logic. Any through video was repeated normally.
Eventually, this was traced to the sync pulse train coming from the I2C's VDU that is not interlaced, despite assumptions and assurances that it was interlaced. As the knowledge and facilities do not exist to modify the software in the EPROM that would possibly allow an interlaced output, it was decided to explore a hardware solution.
A separate SPG, built from the £1 goody bags from the last CAT containing 2 x ZNA134 and a 2.5625MHz crystal, has been built that produces broadcast standard syncs. That is then fed to the PAL coder, an AD724. The syncs are also fed to the video input of the VDU used for genlocking and the SAA5231 teletext data slicer chip fitted that does the genlock function.
Surprisingly, it works; the video from the PAL coder is excellent despite interlaced syncs being used with non-interlaced RGB video but it is good enough to fix the blue screen problem on the DTX1.
Without this problem the repeater would have been on for the second anniversary of its NoV being issued, which is what we were working to achieve but weather permitting it should be on by the end of the month.
Clive G3GJA
This has shown up an issue on the repeater logic based on the BATC I2C project. This is the original analogue/FM repeater logic used by GB3EY that has been substantially modified to make it work in a digital system. It was found that the DTX1 was only producing a blue screen when fed from video derived from the logic. Any through video was repeated normally.
Eventually, this was traced to the sync pulse train coming from the I2C's VDU that is not interlaced, despite assumptions and assurances that it was interlaced. As the knowledge and facilities do not exist to modify the software in the EPROM that would possibly allow an interlaced output, it was decided to explore a hardware solution.
A separate SPG, built from the £1 goody bags from the last CAT containing 2 x ZNA134 and a 2.5625MHz crystal, has been built that produces broadcast standard syncs. That is then fed to the PAL coder, an AD724. The syncs are also fed to the video input of the VDU used for genlocking and the SAA5231 teletext data slicer chip fitted that does the genlock function.
Surprisingly, it works; the video from the PAL coder is excellent despite interlaced syncs being used with non-interlaced RGB video but it is good enough to fix the blue screen problem on the DTX1.
Without this problem the repeater would have been on for the second anniversary of its NoV being issued, which is what we were working to achieve but weather permitting it should be on by the end of the month.
Clive G3GJA
You couldn't write this stuff..