Is there a such thing as a refresh doubler?

[Jack Burton]

Wouldn't it be nice if there were a device that would double whatever the refresh rate of the signal from a PCB was so that it would be playable on many CRT PC monitors?

Anyone ever heard of anything that can do this?  Could you set up a computer to do it?

[MonMotha]

It's certainly possible to build one (I've done it on an FPGA dev board as part of a larger project).  There are probably standalone boxes out there, but I'm guessing they command something of a price premium as they're probably targeted toward professional markets.  Line doublers are more common, but they won't do exactly what you're seeking.  They'll keep the refresh rate the same but duplicate every line to double the horizontal scanrate.

Using a PC to do this would be possible with the right equipment, I guess, but it seems non-ideal.  You'd need an RGB 4:4:4 video capture device capable of low resolutions (they exist but tend to be expensive as they are again targeted towards professionals), a video output device capable of the target output resolution (trivial, almost all PCs can do it), and some software to munch on the data in a timely fashion.  The amount of data isn't absurd, but it's non-trivial, so you'd need at least a decent PC to do it.  Conventional operating systems will also have trouble guaranteeing the timing you'd want to ensure low latency and smooth output with no tearing or skipping, but this isn't strictly a hardware limitation.  The software isn't too complicated (it's just a couple of framebuffers that swap every input frame and draw each output frame twice).

Get 10+ people together who want one of these and I'll build one in hardware, if you really want one.

[Numbski]

Couldn't you just take one of these Mini-ITX computer boards that have an SVideo input and encoder card built-in and flush the desired output to the VGA?

[Jack Burton]

@monmothma, thank you once again for great info, is there anything you don't know about this stuff?

A hardware solution would be ideal because as you mentioned any software solution would involve things like v-sync and input delay.  That would probably nullify a lot of the benefits of the idea in the first place. 

As long as the price wasn't too high, say less than $200 each, I think that 10 guys would be pretty easy to round up among here, SRK, Neo-geo.com, and shmups.  

I'll ask around and see how it goes.  Without making any promises of course.  

[MonMotha]

Couldn't you just take one of these Mini-ITX computer boards that have an SVideo input and encoder card built-in and flush the desired output to the VGA?

If your source is S-Video, sure.  I think it'll still be difficult to convince a typical PC OS (Windows, Linux, whatever) to get the video OUTPUT into the exact timings you want (exactly 2x the input refresh rate) and keep everything synced up, but there's generally no reason the hardware can't do it.

[MonMotha]

BTW, a device such as this will, out of physical necessity (unless you know how to get around that pesky causality thing) lag at least half an input frame (one output frame).  Practical implementations will generally lag one full input frame, crappy implementations maybe more.

$200 is a reasonable target, if aggressive, in qty 10.  I won't make much money, if any, at that price, so keep that in mind if you're looking for support.

[Gray_Area]

60hz is that bad, huh?

[MonMotha]

60hz is that bad, huh?

The intent is usually to double the horizontal scanrate to display standard and medium resolution games on a PC monitor or newer arcade that can't go below 30kHz horizontal but can go up to 120Hz vertical.  It does also reduce flicker, which can be nice.

The other option for displaying such signals on such monitors is a line doubler.  This doubles only the horizontal scanrate without altering the refresh rate, which can be nice since some monitors don't support 120Hz, but it messes up the scanline effects which some people really really like as it doubles the number if lines of video (by duplicating each one).  This can also be done with less latency (just one scanline) than a refresh doubler.

[Gray_Area]

Usually people just use scalers for PCBs to higher scan rate monitors. I wouldn't bother with either method.

[MonMotha]

I would tend to agree.  Find an outboard scaler that doesn't lag (!!) and use it.  But some people REALLY love their scanlines, which a scaler will destroy.  It will also tend to smear hard edges since most scalers want to assume photorealistic input images, and classic arcade games are pretty much not that.  Again, personal preference.  Many people are used to soft images, now, mostly due to the prevalence of scalers, but some people want things as "original" as they can get.

[Blanka]

You do need a line doubler instead of a refresh doubler. That would double the signal refresh as well (15->30Khz). 60Hz vertical is perfect for PC CRT's already. But 240 lines is not. VGA cards have a line doubler as well.

[MonMotha]

That's the idea.  Double the refresh (which will also double the horizontal scanrate since the number of lines is unaltered) without doing anything else.  This will preserve the actual image exactly as it was.  Each frame is simply displayed twice.  The only perceptible change would be less flicker and maybe a brighter image.

This works great...if your monitor can handle 120Hz refresh.  This pretty much limits you to PC monitors, but there were a few high res only (30kHz+) arcade monitors made that can do it, too, as well as some low res (15kHz capable) arcade monitors, but you could just display the original video on those.  On a PC monitor, this will look a little funny.  PC monitors weren't generally designed for such low resolutions, so the scanlines will be VERY pronounced.

A line doubler would double every line, messing up the scanlines some.  On a PC monitor, this may be preferable (see above), but on an arcade monitor with a coarser grain dot pitch, it might not look as good.  And yes, if you're generating the graphics with a PC, you don't need an outboard device to do this (or double the refresh, for that matter) - just enable doublescan.

Note that doubling both the number of lines AND the refresh rate will QUADRUPLE the horizontal scanrate.

A niche application device, to be certain, but if someone wants it, it is doable.