After many years I bought myself a new old Amiga 500 (rev 5)
and wanted to hook it up to an LCD monitor (Samsung Syncmaster 152N) that has a VGA input.
I tried hooking it up directly by making an Amiga Video Connector to VGA plug (RGB, HSync and VSync connected directly to the respective pins on the VGA side), and using an 1:1 VGA cable to connect that to the monitor. That didn't work. Black screen output. The monitor did not display.
Then I tried a HD Video Converter (from China) that was supposed to work according to YouTube, and connected the Amiga to another monitor through it by using an original Amiga->Scart cable. White screen output. That didn't work.
Next I tried the GBS-8220 (also from China), that was supposed to work according to YouTube, hooked up the accompanying RGB-H/CSync-VSync cable to the Amiga connector and... a no "No signal" message on the screen. So the GBS-8220 did output to the LCD, but did not see or understand the input.
Then, in some corner of the net, I saw a comment about the Amiga not understanding H&VSync and you'd have to use CSync. So I disconnected H&VSync and connected CSync and I got an image!
Now, I've dabbled a bit in video electronics, and as far as I know, roughly speaking, HSync+VSync = CSync, and if CSync works so should its separate components on VGA.
- Top Graph; Amiga 500 CSync pin 10, 2 V/div. GND on the middle line.
- Bot.Graph; Amiga 500 HSync pin 11, 2 V/div. GND on the bottom line.
- Horizontally 10 uS/div
- Trigger on down slope of the CSync (scope filters VSync and triggers on that)
- I looked at VSync (pin 12) too and it looks like what you would expect - the low freq. part of the CSync.
- the Amiga video output has separate H,V and CSync
- unconnected to the GBS-8220, CSync tops about 4.5V. If you connect it to the GBS it drops to 2.5V but the GBS keeps working and the LCD shows the Amiga image.
- buffering H/V sync (with a 74LS07 buffer gate each) does not help making the GBS work with H/V Syncs. Neither does a buffer make CSync not work.
Now, if you look at the scope, you will notice that HSync (bottom Graph) has half the frequency of the HSync component in the CSync (top graph) signal! Both signals come from the same video connector on the Amiga and the HSync-part should be the same but are not.*
- (This likely explains why the HD video converter and the GBS did not work at first since I used H&V sync from the Amiga)
Any explanation appreciated. Feel free to go technical.