MiSTer FPGA Forum

Can I use a passive VGA splitter with the MiSTer I/O board? I'd like to VGA output to go into two external scalers/processors.

If that doesn't work, can I use the HDMI Direct Video feature to create a second VGA output?

The goal of this is to add an external Extron VSC 500 480p to 480i processor. This would allow me to play the vertical arcade games on my 15 kHz monitor but using an interlaced resolution.

TL;DR: either use an active (powered) spliter or HDMI DV to VGA for best results, but a passive spliter will work with a dimmer image.

Because some components of VGA signals can be high frequency, for impedance matching on transmission lines reasons, a proper VGA connection is done with a series 75 Ohm resistor on the source, a 75 Ohm (coax) cable and the receiver/screen takes the voltage across a 75 Ohm resistors.

This setup means that the voltage as measured on the MiSTer VGA output (1.4V PtP) is double the voltage expected by the receiver/screen (which is 0.7V PtP). If you do use a basic passive spliter, plugged to two screens, the MiSTer VGA output will "see" a 75/2=37.5 Ohm resistor instead of a 75Ohm. The voltage will be reduced to 37.5/(37.5+75) = 1/3 of 1.4V PtP.

So you'll get a signal that is 0.45V PtP instad of 0.7V PtP, resulting in a dimmer picture overall. Some screens allow you to correct for this, some don't.

There is also the separate problem of the sync signal: that will depend on the devices that you plug into and what input impedance they have (many don't use 75 Ohm) and at what sync level they trigger. Most likely it'll all work fine.

Thanks so much for the detailed information!

The Extron VSC 500 actually has a "Buffered Loop-Through" VGA output. This might provide me with a secondary output, but the "buffered" wording makes me think it is introducing latency?

This setup would work perfect is there is no latency:
Also, is there a recommended HDMI to VGA (with audio) adapter on Amazon USA? I'm reading information about certain adapters crushing blacks, etc?

The buffer refers to something more like this: https://en.wikipedia.org/wiki/Buffer_amplifier

It doesn't introduce latency; it's not holding part of the frame in memory or anything like that.

Got it, thanks! I've heard the term "buffered" when taking about RGB amplifiers, etc. I wasn't sure if that affected latency though or what it meant.

It sounds like using that output port might work for me then?

I've also ordered a $10 HDMI-to-VGA adapter on Amazon as a backup.

Thanks so much for the help everybody!

Yes, an Extron buffered output will work and preserve the signal's integrity without introducing latency (it's essentially what a proper "active" splitter is). The buffer prevents the "loading" problem I mentioned above, i.e. the MiSTer "seeing" 37.5 Ohm instead of 75.

Awesome ... thanks so much for the information.