MegaDrive 1 CXA1645 mod

I believe they block out signal noise. The 75ohms are needed otherwise the picture is too bright.

 

Fudoh doesn't have an account here on Assembler, but he has been following this thread and asked me to pass this on:

 

With respect to Fudoh's expertise on such things, that's dealing with the problem rather than effecting a solution. If the jailbars aren't present then some kind of image processing is occurring, as Twimfy found out. Obviously filtering can ameliorate the problem but that's hardly the point.

Also, join up, Fudoh

 

It depends on your display whether you can omit them, consumer video is AC coupled which is what the capacitor does. http://en.wikipedia.org/wiki/Capacitive_coupling

>_< it's not the best circuit, I just can't verify anything better ATM. You can improve it immediately by putting the input capacitor first and resistor second in series, and replacing the 4.7k and 2.2k resistors back to a single 10k since it makes no difference, and remove the 10 ohm resistor and I think you can remove the 75 ohm resistor to ground too, or at least replace it with a larger one that wastes less current.

I wish, what they do is remove direct current (0 Hz component = the bias voltage) and only pass alternating current. Transistors are DC devices so to amplify AC like a video signal it has to be AC on top of DC.

What the output resistors do is divide the voltage in half, so it's 1.4V in, 1.4V out w/ 75 ohm output, which forms a voltage divider with a 75 ohm resistor inside the display = the proper 0.7V.

 

Well I know very little about electronics This works fine for me and until someone else tries this V2 and confirms it's much better, I probably won't change it again

 

Did you ever try using the C-sync circuit on the RGB lines? It might work too and would be even better quality than using an amplifier at all. The only reason for the amplifier is that it takes ~10mA to drive a RGB signal, so 3 of them + sync might be a little much for the chip to handle if it doesn't have enough GND pins. It also protects the chip against anything bad on the other end of the line.

 

Yes I have only used C-sync. It gives the most solid picture on the XRGB-3.

I'm not tapping RGB from the CXA1145, I'm tapping it from the 315-5313.

 

Fudoh has talked to a friend of his that knows a bit about electronics:

 

Nooo I mean use simply a capacitor and resistor from the VDP -> XRGB. Your entire circuit is doing the exact same work as these two components, but the amplifier is there to take the current requirements off the VDP. The VDP MAY be able to drive such current and this would make the amplifier unnecessary... Either doing this or if it doesn't work because the VDP can't supply enough current, replacing the NPN amp with a PNP amp would fix any contrast issues and simplify the design like I mentioned earlier. I got the thought because in your circuit the VDP is driving Csync which is ~1/3 the current.

:/ he shouldn't talk because these are the basics. I am not trying to be a dick or pretend to be an analog expert, which I'm not, this is just the truth.

1) this "jailbar" effect has nothing to do with sync, RGB lines carry picture information you can see, sync carries digital pulses which align the information on the screen.

2) the point isn't to "clean up sync", and especially irrelevant if you're already using Csync, but to remove interference into or after the blue amplifier.

3) disabling the oscillator doesn't turn composite into C-sync, it stops the I and Q components from being modulated, so chrominance becomes a DC component on composite which is then filtered out after being capacitively coupled to the TV. In other words it makes composite into luminance.

4) composite -> luminance is a side-effect of disabling the oscillator and also not the point, the point was just to stop the oscillator which is interfering with the signal one of the following ways:

-the power rail (the inverter switching current dropping the input bias voltage or something)
-via EMI from the inverter output -> blue input
-or through parasitic capacitive coupling of the composite output -> blue output

 

Yes I tried that. One of the first things I tried before thinking of building the amp Doesn't work. The colors are much too dark. Here is a photo I took of my TV. It's not very good and for some reason I didn't think of using the XRGB-3 to capture one at that time... :O



I think I have 3 BC558 transistors. Are those the PNP type?

 

Out of curiosity.

Why are you using an XRGB?

What size is that screen and how are you connecting to it?

Also, when you cut pin six were you still using the system in collaboration with the XRGB and or the amp that you built?

Only reason I ask is my original TV set is an 18" LCD connected via RGB scart. The picture is perfect, I mean really perfect after cutting pin 6. In fact it's too perfect. I can see the individual pixels Dynamite Headdy is made of and it's kinda distracting. There is not one jail bar and no sign of colour bleeding. This is not my way of saying fuck you mine is working yours is not; To test even more I've tried it on two other TV's one a 22" LCD and there other a 32" and the picture is still fantastic. A lot of people complain about older consoles looking bad on modern TV's but I find this only applies to 32bit onwards. I guess it's personal preference.

Anyway I was wondering if maybe this has gotten a bit overly complex. Do you have another MD you could try it on? Ditch the XRGB and just go straight RGB scart to TV without the amp and just the cut pin? Disable the TV's digital noise reduction and assign a totally factory default colour profile.

Both Alchy and I have confirmed this works. Yeah both our MD's are PAL but I really think it makes very little difference. The boards are all really similar.

Although I can follow what Calpis has been saying and understand it I don't know that much about electronics myself. But I'm beginning to wonder if the XRGB3 might be the root of the problem, perhaps it is overly sensitive and the signal is being affected by something else which hasn't been accounted for.

Have you tried a different TV set. I know these are all really simple and stupid questions but it just seems like this is getting way out of hand for something that two of us have fixed with a pair of pliers.

 

Because most (all?) HDTV's handles the 240p resolution from retro consoles like shit. Most likely it sees it as 480i and starts deinterlacing when it should not. In my TV's case this adds extreme interpolation and lag.

I have yet to see any modern TV that doesn't butcher the image quality completely from retro consoles.

The XRGB's are the best way to display 240p. Everything looks pixelperfect and stays that way when there is movement. It also has the ability to add scanlines so you get that CRT look. I actually prefer the XRGB + HDTV over old fashioned CRT's for various reasons.
The XRGB-3 is also pretty much lagfree. A HDTV is not though...

42". MegaDrive > XRGB-3 > Gefen VGA to DVI Scaler plus > TV (my tv doesn't have a VGA input).

Obviously with the XRGB I removed the amp as cutting pin 6 should fix this

Why wouldn't you be able to see that? If you are used to the pixels all blending together there's either something wrong with your settings or you are using composhite video

Fudoh actually said this a few posts back Yes the XRGB's are most likely overly sensitive. I just got an older XRGB2+ yesterday and it's the same on that. Obviously the problem still lies somewhat with the MegaDrive because Saturn, Dreamcast and SNES all look PERFECT through the XRGB-3

EDIT: MegaDrive 2 also looks perfect with the XRGB.

Nope and that wouldn't make any difference. The picture is processed by the XRGB-3 and no matter what kind of screen I connect that to it will look the same.

If you are interested Fudoh has a very nice website on video processors for retro gaming use here:
http://retrogaming.hazard-city.de/

 

Ahh OK, so that's solved, it's dark because the chip can't supply enough current.

Yup, they appear to be PNP and suitable. I think the circuit I posted a page back might work but I'm not sure how the MD VDP drives the pin.

 

I don't know what that means. What's the worst that can happen?

 

You could try it but I'm not sure how well it'll work. I'm having a hard time simulating it and PNP amplifiers in general are just confusing because everything is backwards. What I meant is that there are different ways to construct a DAC and without knowing how the VDP implements it (it can sink or source current) I can only guess, and very poorly.

 

Also I wouldn't call XRGB "sensitive", they should have the same ADC performance as any TV these days, or possibly worse than the high-end TVs which now have 10 or 12-bit ADC. This interference problem isn't some small 1mV blip that's being amplified via the XRGB, it's far more significant and certainly the MD's fault.

Why don't you use the XRGB-3's DVI output right to your display? You know you lose signal integrity with each DA, AD and scaling right?

Your signal chain is digital (MD pixel)->analog (MD DAC)->digital (XRGB ADC)->XRGB scaling->analog (XRGB DAC)->digital(Gefen ADC)->Gefen scaling->possible TV scaling->digital display

Each DAC represents non-linearity (distorted colors), each ADC represents quantization error (distorted colors and noise) and possibly loss of dynamic range (loss of sample resolution) due to biasing. And that's before losses from resampling a video line since processors can't filter/recover the line back into pixels (so fractional horizontal interpolation @ the ADC sample rate = blurry pixel boundaries) followed by 2D interpolation on top of that to get the scaled output...

Error from each stage compounds, so it's a testament to your hardware that you get a reasonable picture at all.

Ideally it should be:

digital (MD pixel)->analog (MD DAC)->analog display

or since true analog displays aren't easy to find:

digital (MD pixel)->analog (MD DAC)->digital (signal processor ADC)->processing/scaling->digital display

The point of using RGB afterall is for it's simplicity, elegance and purity right?

 

Because the XRGB-3's DVI output doesn't support its B1 mode (720x480 output with scanlines). The DVI outpus is only ideal if you choose to use the XRGB-3 as an upscaler, which it does quite poorly.

The XRGB-3 doubles the resolution before outputting in 720x480, ie. MegaDrive 320x224 > 640x448 with borders. That looks pretty perfect
I only use the Gefen because my TV doesn't have a VGA input but the Gefen scales the XRGB picture very well to 1080p. Each pixel are sharp and defined and all looks the same size. I don't think it can get much better

 

@Bearking.

Didn't mean to come across as a dick. Was just curious.

 

No problem. It's totally understandable

 

Hell I'd be glad that were the case here. Mine views 240p strictly over component (meaning composite and s-video are NOT affected whatsoever), as an unrecognized signal. :crying: