Understanding the Micomsoft XAV-2s (WIP)

Started by NFG, October 24, 2016, 01:05:13 PM

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Micomsoft made a lot of useful tools for the gaming and video enthusiast.  One of which was the XAV-2s, the last of their chroma encoders.  They differed somewhat internally, using different chroma encoder ICs, so what I'm detailing below may not be entirely useful if yours is different.  Most of it, I suspect, will be the same.

RGB Input Circuit

This is how I read the RGB input circuit.  I'm not 100% confident about it, but I'm like 98% sure.  Anyone with a better handle on this stuff than me is welcome to point out where I've gone horribly wrong.  =)

Input on the left, output on the right.

S-Video Output Circuit

Interestingly the XAV-2s deviates from Sony's reference circuit when it comes to the S-Video output.  The Luma (Y) output follows the spec, but the Chroma (C) output is more complicated than normal (the spec calls for the same circuit as the Y output).

FM Towns Mode

The first DIP switch enables FM Towns mode.  In this case, that simply means dis/connecting the pin 7 (composite sync in) from the RGB-15.  If it's ON, the DB15 composite sync is fed into the CXA2075 circuit, at the same point (and connected directly to) pin 9 (composite video) on the RGB-21. 

If OFF, C-sync is created elsewhere from the RGB-15 horizontal and vertical sync inputs.


RGB Output

Normally the XAV-2s will accept either a 15-pin RGB input, or a 21-pin RGB input, and output s-video and composite video, as well as stereo AV.  I've hacked mine up to spit use only the 21-pin input, and output RGB through the 15-pin connector, so I can run multiple displays, and utilize the encoder IC (in this case the great Sony CXA2075) as an RGB amp.

Here's the theory: the RGB outputs from the CXA2075 are not used, so you can use them without modification.  Run each output through a 75 Ohm resistor and 220uf Capacitor.  The red marks on the photo indicate the places I cut the original RGB input lines on the PCB. 

Forgive the muddy photos, this board's been through a flood.  The grey wires are the new output signals.  In this case, RGB and H/V Sync.

There's a ton of empty space on the top side of the PCB, so I just left the caps and resistors there.

Chroma Circuit

It's getting deep into the circuits now.  This is the chroma and sync circuit.  Sync from RGB-21 pin 9 (and/or DB15 pin 7) comes in on the left,
(More to come)


Mod progress report:

The XAV-2s doesn't do sync separation.  I don't know what kind of sync input the CXA2075 is happy with, but it wasn't pure enough for any of my RGB devices, so I added an LM1881 sync seperator.  It takes sync input from the Luma (Y) line of the Svideo port and spits out proper composite sync, hardwired to H-sync on the DB15 (which is now an output).

I repurposed the first two DIP switches, since none of them ever really had an effect on how I used the XAV-2s.  The FM-Towns switch now connects Vcc to pin 7 (Ys) of the DB15 (or not), and the H-Sync STD switch now connects c-sync to the V-sync output on the DB15 - just in case I run into a device that wants it on both pins.

While testing, I discovered something interesting about the NEC PC-TV454 monitor I'm using:  If you don't give it good sync in the RGB port, it'll cheerfully pull sync from the Svideo port while displaying the RGB signals in RGB mode.  I didn't expect that.