Added writeup

OSDSwitch/OSDSwitch.kicad_pcb

@@ -78,7 +78,7 @@
 		(tenting front back)
 		(pcbplotparams
 			(layerselection 0x00000000_00000000_55555555_5755f5ff)
-			(plot_on_all_layers_selection 0x00000000_00000000_00000000_02000000)
+			(plot_on_all_layers_selection 0x00000000_00000000_00000000_00000000)
 			(disableapertmacros no)
 			(usegerberextensions no)
 			(usegerberattributes yes)

README.md

@@ -3,3 +3,58 @@ FF-OSD Video Switcher for clean video output
 
 ![Video Interposer Board](OSDSwitch/OSDSwitch.png)
 
+This is a small board with a high bandwidth video switching chip
+on it. The idea is that this sits in between the video generation 
+circuitry and the video output socket of your Amiga (or similar
+computer) and is controlled by the On Screen Display signal from
+a modified Gotek (see [keirf/flashfloppy-osd](https://github.com/keirf/flashfloppy-osd).
+
+Normally the output of the OSD mod is just blasted unceremoniously
+over the green (or channel of your choice) signal, overpowering the
+circuitry in the receiving display system to give a green tint where
+the text is.
+
+This is not great. It's crude, and doesn't always work right, and
+the results are often less than desirable. 
+
+This small board is the answer. It actively cuts out the normal
+video signal when the OSD text should be displayed, and replaces that
+video signal with a statically defined colour of your choice (configured
+by 3 jumpers) to give a crystal clear OSD that is embedded within the
+video stream, not overlayed on top of it.
+
+I developed this board because I use an Extron video scaler connected
+to my Amiga for both video capture while livestreaming and for display
+on a modern TFT screen. But the traditional overlay setup really didn't
+work with the Extron.  Everything went pink:
+
+![Pink Amiga](Images/Pink.png)
+
+And so this board was born. The ADG1633BCPZ looked to be the ideal
+chip, with more than enough bandwidth for a video signal (one of the
+intended applications for this chip), and three channels, the board
+was pretty simple to design. And I think you'll agree the results
+are much nicer:
+
+![Non-pink Amiga](Images/Better.png)
+
+(This image is from before I tweaked the resistor values a little,
+so the OSD is a bit darker than I would have liked).
+
+Installation is reasonably simple. For the Amiga 500 (R6):
+
+* Lift the side of the three ferrite beads by the video connector that is facing
+  towards Denise (where you would normally solder the OSD wire to.
+* Connect three wires to where they were soldered
+* Attach the three wires to the INPUT side of the board's R G and B pins.
+* Solder the ferrite beads you just lifted one side of to the output
+  R G and B pins
+* Connect a 5V and GND signal from somewhere suitable
+* Connect the signal from your OSD generator to the IN pin.
+
+Choose the colour you want with the jumpers (bonus hack: if you use resistors
+instead of jumpers you can get a wider range of colours to choose from).
+
+And here it is installed in my Amiga:
+
+![Installed](Images/Installed.png)