Designing a better S-Video mod, also maybe YPbPr

[samiam]

Here's what I did: I was probing a wire connected to pin 40, and the particular point I was aiming for was right next to the 9V(!) power rail from the external power supply. Although I had the system switched off, the power supply was still on, and I was not being careful enough when approaching with the probe.

Stupid, stupid, stupid.

I've recapped a great many things over the years, but this is the first time I've done something that involves taking so many measurements on a live system. I see now that I didn't have a strict enough protocol in place. The way I'm going to prevent this from happening next time is to do the following.

1. Rig up a proper "master switch" using an extension cord and an on-off outlet switch with an indicator LED (like this). I want 100% no confusion about whether the system is on or not, and no inconvenience involved in switching it.

2. Add in a temporary wire to ground to use as a semi-permanent place to attach the probe's ground lead. This will help prevent accidents due to bad positioning.

3. Take probing any connection that leads directly to a chip a whole lot more seriously. When I was probing other places in the amp, it didn't actually matter if I shorted something because of all the resistors, and this lead to overconfidence and carelessness.


I'm too far along to give up on this now. I can probably get another white system for $20 or so here in Japan. In the meantime, I'll atone for my sins to the PCE gods by doing an RGB mod on this system and hopefully salvaging it. 

dshadoff , I'll PM ya.

keithcourage , for now, I've just got to get this finished. Maybe later, though.

[keithcourage]

Just thought I'd post an update about the common S-video mod. Had some free time today and found out that sure enough, if we cut the connection for Luma(pin 40) going to the regular composite output of the system, the s-video signal quality is much better. No more dithering effects on the screen whatsoever when things scroll now. I can see the difference in certain games. 

Like I mentioned before, the only downfall is then composite cannot be used doing this. As an alternative for composite video use still, one could mount a switch on the system to wire up the connection to be on or off. A more simple solution would be to just use one of these adapters instead. www.ebay.com/itm/S-Video-7-Pin-TV-to-RCA-AV-Adapter-Converter-Cable-R5G8/274126830125?epid=14020025305&hash=item3fd33b7a2d%3Ag%3A1VQAAOSwb5Zd55Q8&LH_BIN=1

or 

www.aliexpress.com/item/32998911671.html?spm=a2g0o.productlist.0.0.7246521aVfd5HU&algo_pvid=ce341bf3-d77c-425f-9e25-13c354d59a7e&algo_expid=ce341bf3-d77c-425f-9e25-13c354d59a7e-44&btsid=0ab6f82115832866378185047e31e2&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

If anyone wants to do this to their system, feel free to PM me and I'll talk you through the process. 

[sarge]

I'd be curious to see some video and see how much of an improvement it gives. I'm a bit reticent to jump into my system, although this certainly isn't something complicated to do. It's just that I've usually been working on much less expensive systems...

That cable is certainly an excellent solution, though, to get composite back!

[keithcourage]

Here are a couple games to look at. Both Keith courage and China Warrior make good examples of games to check for this. Almost as soon as each game starts as the game moves from left to right, you can see a little bit of shimmering / dithering on the red and orange backgrounds. Basically just look at all the buildings you pass.

Now just imagine none of that shimmering being there, the picture is 100% solid.

I opted to cut the trace on my duo-r system here instead of removing the resistor. This way if I did ever want to hook composite back up for whatever reason, I could simply solder a wire onto the board.

Cutting the trace for it is very simple to do on R,Rx, turbografx or PC Engine Systems. Black Duo systems will take a little bit more work depending on where the current transistor for the S-video mod is mounted to the board.

I'll probably put it in a different place for DUOs going forward so people have an easier option to do this for black Duo systems.

[sarge]

Alright, I'll be sure to give it a check when I get back from my work trip!

[samiam]

I thought so. Thanks for the report!

An amp for luma that involves multiple transistors will probably be even better for signal integrity since the 6260 can't source much current. Maybe just using a single transistor with a higher beta would do the job. Anyway, there is still more work to do if we want the best S-video mod possible.

Maybe I'll be able to get back into this next month.

[keithcourage]

The video quality is ridiculously good as it stands right now but by all means feel free to keep trying out other things. I have compared the quality of the S-video to my wii with good quality S-video cable and running the same games(240P). Video quality looks exactly the same. The wii is originally what told me to give this idea a try since the wii did not display any shimmering/dithering when the screen would scroll from side to side while using S-video.

[soop]

Jan 17, 2020 6:33:43 GMT samiam said:

OK, this is what got the color image to appear on my screen. The "400kHz" circuit is luma, and the "3.6MHz" circuit is chroma. Note that this does not account for all the resistors that you need to have in place before the first transistor in both amps. You'll have to follow the TG16 schematic to see what goes there.

Let me explain a couple of things about how chroma is getting through, and how the mod could change or possibly be improved.




I needed to add an AC coupling capacitor like in the above schematic to remove the DC bias on chroma. In the beginning, I wasn't aware of two conditions: First, when your signal is coming from the emitter of a transistor (or a diode), you must have a resistor in the position of the leftmost one here or else literally no signal will get through. That's why it's impossible for me to add an AC coupling capacitor on luma with the current circuit. Second, the value of the leftmost resistor cannot be too high; it must fall as close to the combined resistance on the other side of the capacitor as possible. 

The rightmost resistor in this image corresponds to the 75 ohm terminator inside everyone's TV. We can't change that. If we have to make the leftmost resistor 75 ohms, then that's means that your transistor will have to source a heck of a lot of current. Fortunately, in this case we have the middle resistor in place (to reduce signal level), but its value is a mere 10-20 ohms. Even if we can raise the leftmost resistor to 95 ohms thanks to this, that's still a lot of current to provide. Heck, even if we can get away with raising it to 120 or 150, which we mostly can, that's still a lot.

The main problem, then, is that the more current the transistor has to supply from its emitter, the more it has to source from its base, and the NEC design doesn't give us a terribly great amount to work with at that base. I never measured it properly, but I did see that using 330 ohms for the leftmost resistor was enough to depress the whole signal - a clear indication that we were pulling too much.

The solution was to use a transistor with a really high beta. The 2SC1815-Y, the two transistors recommended in the old mod, and the 2SC945 that NEC used in the real system all have betas in the 180-220 range. I took a 2SC1815-BL, which has a beta of over 550, and put it in the position of the third resistor in the chroma circuit. Having a higher beta means that it needs proportionately less current to drive the emitter. Fortunately, it seems that this made all the difference, and I could get away with the configuration you see in the big schematic linked above.

I hesitated to use high-beta transistors before because they have worse parameters all around and may degrade or distort the signal. That's something I still have to check. If this 2SC1815-BL and all others like it cause problems, it's back to considering op-amps. If we're still in the safe range with them, however, it might open up some options. 



If we can get away with making the bottom-left resistor 75 ohms to match the one on the right, that not only improves our performance with the capacitor, but it also should give us matched impedance, at least as far as signal reflections are concerned. It also makes it possible to add an AC coupling capacitor to luma. In other words, it makes the circuit compliant enough to recommend it universally.

Basically, we're at the fine-tuning stage. 

Hey samiam , is there any material I can read more about this kind of stuff with relation to transistors?  My knowledge is pretty basic tbh, but this is fascinating

*edit* this was interesting:  www.electronics-tutorials.ws/transistor/tran_1.html

[turboxray]

Jan 4, 2020 5:43:42 GMT samiam said:

Overview:

As turboxray demonstrated in another thread, the PCE's analog RGB output and its composite video output are essentially based on two separate color palettes, with the latter being arguably truer to developers’ intentions. While composite video itself leaves a lot to be desired, modding for S-Video gives us a chance to see the “real” PCE palette in a relatively clean format.

However, the S-Video mod that's been commonplace to date has multiple shortcomings. Let's see if we can come up with something better.


What S-Video looks like:

Y / luma signal: 


C / chroma signal:


Including the negative sync tip, luma should be 1Vp-p and chroma should be 0.7Vp-p.


The relevant section of the PCE schematic:


(Click for full size)

Luma comes out of pin 40 of the HuC6260. Chroma is a combination of the output of pins 33 and 26. The color burst from pin 20 is also necessary for the chroma line.

Note that PCE models with on-board composite video output will not be identical to this, but I'm not sure what the exact differences are. It might simply be this plus what's inside the Turbo Booster.


The current mod:

It’s very simple. Take a single NPN transistor, connect the collector to 5V and the base to pin 40 of the HuC6260. Put a 220 ohm resistor in series after the emitter and connect that to the TV as luma. Then, tap composite video somewhere on the board, put a ~1000pf ceramic capacitor in series with it and run that to the TV as chroma.


The current mod's problems:

1. Cross-contamination. Luma is polluted by some amount of chroma that leaks into it via the 4.7k resistor that sits between them. Meanwhile, chroma has to be obtained from composite video via high-pass filtering that removes luma. There is no need to settle for this when both signals are available in pure form.

2. Impedance mismatching. This applies mostly to luma, but could also be relevant for chroma depending on whether you tap composite video from a proper source. By not having the output resistor at 75 ohms to match the 75 ohm termination resistor at the signal’s destination, you create a situation where the longer your cable is, the more distortion in the signal you get.

3. Drawing too much current from the HuC6260. Pin 40, which I measured, is only able to source 30-35 microamps of current. That’s about what the PCE’s on-board amp draws. Once you go beyond that, the signal begins to distort in a non-uniform way, starting with an ugly collapse in the signal's highest range. The current mod’s single-transistor/220-ohm-resistor configuration draws about 95 microamps by itself at full brightness, and since it doesn’t disconnect Y from the PCE’s on-board amp, you’re looking at a total of roughly 125 microamps drawn from pin 40. On top of that, the chroma pins are also having to supply extra current to this luma amp and may be suffering from a degree of signal collapse as well.


The solutions:

1. Remove the 4.7k resistor after pin 40 to isolate it. With this, chroma and luma don’t contaminate each other, plus none of the currents from the HuC6260 will be cross-loaded. The on-board composite video output should now effectively become your chroma signal without need of any filtering.

I'd recommend doing this to someone who has the current mod and doesn't want to reverse it but would like a quality boost.

2. Design an amp for Y with 75 ohm output impedance. This could get complicated if we wanted to be OCD about achieving an absolutely perfect 75 ohms (there are more factors involved than you might expect), but we don’t need to do that. A basic solution will be 99% accurate and vastly less issue-prone than the current mod.

The on-board composite video output should work as a source for chroma once the 4.7k resistor is removed, but the white PCE and US TG16 will need a small amount of extra circuitry equivalent to what’s inside the Turbo Booster to bring it up to spec.

3. Use multiple transistors to buffer the luma signal and reduce the current draw from pin 40. I’m not sure if this can be accomplished with two transistors yet. 75 ohm output impedance needs so much current that a single buffer might not do it. We may need three transistors, in which case we might as well just completely copy the on-board amp. This is the next thing that I’m going to try.


About YPbPr, aka component video:

R-Y and B-Y come out of the HuC6260 already amplitude-modulated to 3.58MHz. If we want to get Pb and Pr from these signals for use as component video, we’ll need to build demodulation circuits. I admit, I don’t know jack about this, but at a glance it appears that it can be done using a handful of basic, off-the-shelf parts.

Question for those who know: Is the PCE’s chroma carrier frequency always the same regardless of the dot-clock being used? It seems like it should be for general NTSC compliance, but I thought I’d ask. It’ll probably simplify things if we only have to deal with one carrier frequency.

Whether going through all this would even result in significantly better video quality than S-Video is somewhat doubtful. There is not a huge quality difference between S-Video and component in the first place, at least at low resolutions, and the modulation/demodulation process will take a toll on R-Y and B-Y. However, if anyone wants to get really nuts and build a converter to output RGB, perhaps in order to integrate into an RGB-only setup, then having YPbPr would simplify things a lot.

You can begin to imagine a single board with all of this crap on it: luma amp, final-stage chroma amp option, R-Y and B-Y demodulators and amps, and a YPbPr-to-RGB transcoder. Audio options should probably be included as well. However, I think you’d have a really hard time fitting a board with this much inside of a white PCE. 


Anyway...

S-Video alone should be a piece of cake. Let’s get a really good design pinned down. I’ll share whatever I come up with.

Any news on this???

[keithcourage]

We have already determined that S-video looks and functions much better by cutting the trace/or removing the 4.7K resistor for Luma to the composite circuit. So take the original S-video mod and then cut one trace on the board or remove the transistor that comes after pin 40 on the HU6260 which is luma. This also fixes the excessive Power draw from pin 40 Luma on the video chip.

This was solution option number 1 from the above post for S-video. 


Not sure if anymore work has been done on the other video out options.

[dshadoff]

This is definitely getting me more and more interested...

I started another thread on the Homebrew development forum here:
pcengine.proboards.com/thread/979/improper-colors

Basically, after discussions with turboxray, we've come to the conclusion that nothing that uses RGB is displaying properly: RGB mods, emulators, FPGA systems.

So, we can continue to use this thread for the electronics solution (RGB mods), where the homebrew thread will focus more on how the numerical values should be translated in order to get proper output from emulators and FPGA systems.

We can also agree on the fact that the 3/3/3 RGB -> 5/5/5 YUV lookup table inside the Hu6260 doesn't just map the colour space from RGB to YUV, but also adjusts gamma and we believe that it does some additional colour correction.

Right now, we're trying several different approaches to get the actual values from that mapping table.

[Gaijin D]

Just got a Duo from Keith using his new mod method. Since I had to do a recording test anyway, I thought I'd upload some footage to YouTube: 
It comes across better on my CRT, especially the color, but maybe this'll give folks some idea how this looks.

[turboxray]

Jun 23, 2020 4:03:02 GMT Gaijin D said:

Just got a Duo from Keith using his new mod method. Since I had to do a recording test anyway, I thought I'd upload some footage to YouTube: 
It comes across better on my CRT, especially the color, but maybe this'll give folks some idea how this looks.

 Nice! What about games that use higher-res? Specifically, forgotten worlds?

[turboxray]

Figured I would share this from Artemio. If you know how to read waveform and vector output of NTSC video, there are some interesting things about PCE composite video.




A few things definitely immediately standout

1) Cyan is not at the right brightness level (which the next group of colors following it have relatively correct stepping, except blue, means they're all less bright as well)
2) Blue is not at the right brightness level (more than just a shift down like the colors above it).
3) Not surprising that the video doesn't start at IRE 7.5 (seems no consoles of that era do), but the max luma never hits 100 IRE (it's 90 IRE).
4) Relative to max Luma, Yellow saturation upper band should be 1.2mv above it.. but it looks like about 1.18mv above it.

On the vector side..


None of the colors reach their target (that's very weird). And I don't mean just max saturation, but far from its mark. Also, Red and Cyan are noticeably off and not in the color space they're supposed to be in. Yellow and Blue are only slightly off.


Besides the custom YUV table in the VCE, these characteristics for the composite signal (also generated by the VCE internal) are also what gives the PCE its unique palette.

Attachments:

[turboxray]

for reference:
Waverform for reference:


Where the color points should be in the vector scope:



And finally, this is the color space that maps to the vector scope (so you can see what the off range means):

Attachments: