How to split and get both RGBHV and RGBS video output from your Amiga simultanously - jbilander/HowTos GitHub Wiki
Here's a little mod that I did to split the Amiga's 15 kHz video signal and have both RGBHV and RGBS output simultaneously (duplicate signals).
I use a simple RGB->VGA adapter to my Benq BL912 Monitor.
It uses 15kHz RGBHV from the Amiga and it works great but flickers a lot in PAL High Res laced mode. For simple wb stuff I sometimes want to run this 640x512 interlaced resolution to get more space on the desktop, so I got the cheap scart->hdmi converter that you all probably seen before.
It does the de-interlacing job well enough for me, no flicker, but it's a bit of a hassle having to plug/unplug the db23's every time you want to switch between wb and games, demos etc, so I decided to do some research on a video splitter solution, optimal would be to have both RGBHV (through d-sub hd15) and RGBS (through Scart) output at the same time, the Benq monitor has both vga and dvi inputs so both the vga-cable and the cheap scart hd converter can be plugged in at the same time and the input source can then be toggled through the monitor buttons.
I found this Ugreen VGA Male to 2 Female Video Splitter, on Amazon, that can be powered either from hd15 pin 9 or via micro-usb (typically through a +5V phone charger) and it supports DDC on vga out 1. I got curious and thought maybe I can pass csync in on pin 12 from the Amiga pin 10 and then power it with +5V from Amiga pin 23. If the Ugreen splitter would pass csync on then I can make a custom d-sub hd15 to Scart cable that carries RGBS.
I used a simple 22 awg wire here, yellow for csync and red for power, and took some inner cable shielding from a vga cable that i butchered and fitted some shrink-tube on the same length. Then soldered it in between pin 10 and 12. And the same for power between pin 23 and 9.
Plugged the video splitter and adapter into the Amiga and then started measuring output voltages...
Pin 14 Vsync, Pin 13 Hsync, Pin 12 Csync...
Vga out 1:
Vga out 2:
As you can see from the pictures above, csync only on vga port 1 which is a good thing not having to break pin 12 on the vga cable, just use port 2 for RGBHV, but remember to not plug that vga cable into port 1 by mistake, so maybe remove that pin anyway ;)
Now let's create a temporary d-sub hd15 to Scart cable so we can measure terminated csync. The +5V on pin 9 is not forwarded on the vga out ports of the splitter so I will use Vsync's ~4,5V together with a 100 Ohm resistor to lower the Voltage to more suitable 1-3V for Scart pin 16 RGB mode, and a 330 Ohms resistor to lower the TTL Csync voltage.
15-pin Vga 20-pin Scart ---------- ------------ R 1 -> 15 G 2 -> 11 B 3 -> 7 Csync 12 -> 20 (330 Ohm resistor) Hsync 13 Vsync 14 -> 16 (100 Ohm resistor) GND R 6 -> 13 GND G 7 -> 9 GND B 8 -> 5 GND HS 5 -> 17 GND VS 10 -> 18
Some soldering later...
And plug it into the Ugreen splitter and Scart->hdmi converter, Yay it works!!. Let's measure csync and RGB mode voltage input...
pin 16: Scart RGB mode, pin 20: Csync no resistor, Csync 100 Ohm, Csync 330 Ohm...
Let's make a real shielded hd15 to Scart cable:
And plug it in:
Now, Let's measure current...
Okay so it seems to hover just above the 100mA dependent of what you do in wb, open windows, start a game, etc. 100 mA is what you max should draw from your Amiga's +5V pin. If I plug in the external power source (the phone charger) the current drops to 9,5 mA...It's up to you how you want to do with external powering or not.
Now let's test running dual displays, Scart RGB to TV and d-sub vga to the Benq monitor...
And let's measure the current...
With the external power still the 9,5 mA as expected:
Yikes! With the external power disconnected it draws 155 mA, Well over 100mA...Quick, Let's re-plug that charger in again!!
And then finally a little video clip which shows switching from the flickery PAL High Res laced d-sub to the Scart->hdmi converter that de-interlaces the signal nicely...