DIrect TMS9928 - GBS8200 YPbPr connection

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By DamnedAngel

Master (143)

Аватар пользователя DamnedAngel

21-08-2016, 17:04

Hi MSXers,

I am sorry if this was already discussed in another topic, but I could not find the exact thing I am looking for.

I have seen that a lot of you are using the GBS8200 to connect your MSXs to VGA monitors - mainly using the LM1881N as bridge.

I researched the TMS9928, however, and found that it has YPbPr output. As soon as I discovered that, the question "Why the heck using lossy MSX's composite eletronics, then LM1881N to finally get the signal to the GBS8200, if one could simply direct connect TMS9928's YPbPr output to GBS's YPbPr?" came immediatly to me.

I then opened my MSX (Gradiente Expert 1.1), disconnected the 9928 from the composite module and wired it directly to GBS8200 and then connected the GBS to a standard VGA monitor.

I instantly got clean image, BUT (there is always a BUT) the colors were all wrong.

So my questions to you guys are: Why are the colors wrong? Is there more than one type of YPbPr ? Am I doing something wrong? Is there a proper way of directly connecting TMS9928 to GBS8200 via YPbPr?

TIA,
DA

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By Manuel

Ascended (15750)

Аватар пользователя Manuel

21-08-2016, 20:14

Perhaps you swapped Pr and Pb? ☺

By RetroTechie

Paragon (1563)

Аватар пользователя RetroTechie

21-08-2016, 21:36

Welcome to this forum, DamnedAngel! Smile

TMS9928 video outputs aren't designed to drive a 75 Ohm impedance like you'll find on most equipment that takes video signal(s). TMS9918A datasheet (see section 5.3) specifies behaviour of these pins when driving a 470 Ohm load.
Loading with 75 Ohm could result in a much reduced voltage swing, and for example one would expect poorly saturated colors or similar.

So you may need a simple transistor buffer on each of those VDP outputs. Buuuuttt... good chance those are already on the board. I don't know if you can find a schematic of your MSX, but especially then I'd have a look if there's a better place to pick up those signals.

Btw: you could also have a closer look at what colors you'd expect, what you are getting, and see if you can draw some conclusion as to how exactly the colors differ from what's expected. Or post a screenshot comparison of the full palette using YPbPr vs. composite?

By DamnedAngel

Master (143)

Аватар пользователя DamnedAngel

22-08-2016, 13:32

Manuel wrote:

Perhaps you swapped Pr and Pb? ☺

Yes, that was my first thought. But I made sure the pins were correct. I also tried swapping them, just to be sure.

Thanks for the idea, though!

By DamnedAngel

Master (143)

Аватар пользователя DamnedAngel

22-08-2016, 13:37

RetroTechie wrote:

Welcome to this forum, DamnedAngel! Smile

Hail! Long live MSX!

RetroTechie wrote:

TMS9928 video outputs aren't designed to drive a 75 Ohm impedance/So you may need a simple transistor buffer on each of those VDP outputs/have a look if there's a better place to pick up those signals.

Nice Shot! As soon I have time to get back to this, I will give it a try and let you know the outcome!

Tks for the ideas.

best,
DA

By Louthrax

Prophet (2082)

Аватар пользователя Louthrax

22-08-2016, 16:10

I think those signals ends at the LVA510 chip that does the composite conversion... Maybe they are corrected at this level ?

By tvalenca

Paladin (728)

Аватар пользователя tvalenca

22-08-2016, 20:43

DamnedAngel wrote:

Hi MSXers,

Hi, fellow Brazilian! Welcome!

DamnedAngel wrote:

I am sorry if this was already discussed in another topic, but I could not find the exact thing I am looking for.

I have seen that a lot of you are using the GBS8200 to connect your MSXs to VGA monitors - mainly using the LM1881N as bridge.

I don't think this is the case, because European TVs and some monitors have SCART connections, which in most times means RGB @ 15kHz. And they had the chance to buy RGB monitors, which was not our reality back in the 80's and 90's, so we Brazilian are stuck with PAL-M and NTSC composite TVs and monitors.

DamnedAngel wrote:

I researched the TMS9928, however, and found that it has YPbPr output. As soon as I discovered that, the question "Why the heck using lossy MSX's composite eletronics, then LM1881N to finally get the signal to the GBS8200, if one could simply direct connect TMS9928's YPbPr output to GBS's YPbPr?" came immediatly to me.

Just for clarification, no video signal is passing through the LM1881N IC. The only reason it is there is because most monitors don't like the SYNC signal from MSX computers, so we use it only as a buffer. it could be a 74LS00 on it's place (on a different wiring of course)

DamnedAngel wrote:

I then opened my MSX (Gradiente Expert 1.1), disconnected the 9928 from the composite module and wired it directly to GBS8200 and then connected the GBS to a standard VGA monitor.

HEY YOU HAVE AN EXPERT! you already have a good (if not great) quality RGB output. Seriously. Composite and RF connections on Expert may not have a good quality (and it's even worse because they're PAL-M) but they still are pretty decent and you should use it instead. You are good to connect any SCART TV, or 15kHz capable monitor (LG M1721A/M1921A, Samsung 510N/710N/913T) just using the LM1881N/74LS00 Sync Buffer (I supose you already know that the EXPERT RGB connector has a different pinout from every other MSX). The GBS8200 isn't a bad solution at all, but its far from being the best image quality you can get, any and every other option I gave you above are way better than it.

DamnedAngel wrote:

I instantly got clean image, BUT (there is always a BUT) the colors were all wrong.

So my questions to you guys are: Why are the colors wrong? Is there more than one type of YPbPr ? Am I doing something wrong? Is there a proper way of directly connecting TMS9928 to GBS8200 via YPbPr?

Well, I don't know if you knew it, but YPbPr is different from RGB, even if the cable we use for the component connection have red, green and blue RCA plugs.

So, if you're getting a green-ish image, (every color is a shade of green) you are plugging YPbPr on an RGB input. (and some of them will work because of the RGB with Sync-on-green standard, which is common on some image projectors and the GBS8200 can understand it)

If you're getting a red-ish or blue-ish image, you could still be plugging YPbPr on the RGB input and you switched the connections on the RGB port.

If you're getting a image that isn't green/red/blue-ish but you still have wrong colors you may have switched Pb with Pr and vice-versa. or, you could short circuited both signals, so please stop reading and go check the connections you made. If they're correct and you still have bad colors, check what @RetroTechie pointed out. He knows a lot about what he is talking and we all should pay attention Wink

RetroTechie wrote:

So you may need a simple transistor buffer on each of those VDP outputs. Buuuuttt... good chance those are already on the board. I don't know if you can find a schematic of your MSX, but especially then I'd have a look if there's a better place to pick up those signals.

You're probably right, but I have to say... Gradiente did a really bad job on Expert's analog board design. (all Gradiente Expert models have a separate board that contains PSU, YPbPr to RGB matrix converter, Monochrome video output buffer, Audio output buffer and internal audio amplifier, Cassette input/output buffer and relay, and from the second version they had a third board which contained a MC1377 PAL-M color encoder and RF modulator. PSU is CRAPPY, the entire color encoder/RF modulator board is SUPER CRAPPY, but the RGB matrix is good and internal amplifier is pretty decent) I wouldn't dare modding the analog board on a Expert computer, because it is a miracle it is working and have such a good RGB output in the first place. I have some info on this matter, but the answer got too big and too offtopic that I just deleted it.

Louthrax wrote:

I think those signals ends at the LVA510 chip that does the composite conversion... Maybe they are corrected at this level ?

The Gradiente Expert uses the MC1377 for color encoding, but it converts YPbPr from the TMS9128 to RGB before the encoder. The LVA510 is pretty decent, but it can't encode video in PAL-M. (neither the MC1377, Gradiente engeneers made a pretty ugly hack so it would generate a M-System frame with a 180 degrees phase alternating per line color encoding. This wouldn't be possible on LVA510)

By RetroTechie

Paragon (1563)

Аватар пользователя RetroTechie

23-08-2016, 02:02

tvalenca wrote:

The Gradiente Expert uses the MC1377 for color encoding,

That sounds illogical.. an MC1377 datasheet I have here, says it takes RGB as input? For encoding Y/R-Y/B-Y to composite, LM1889 or LVA510 makes more sense.

Quote:

but it converts YPbPr from the TMS9128 to RGB before the encoder.

Strange way to do things imho. oO But if true, maybe RGB signals capable of driving (external) 75 Ohm inputs, can be found on the board? Question In terms of signal quality, RGB is as good as Y/R-Y/B-Y.

By tvalenca

Paladin (728)

Аватар пользователя tvalenca

23-08-2016, 19:20

@RetroTechie let's see:

Here is the schematic for the Gradiente GPC-1 (a.k.a. Expert 1.1) Analog Board: (PSU, Sound out, Sound internal amplifier & speaker, Monochrome out, and RGB out) http://msxpro.com/hardware/esquemas/Expert_Fonte.pdf

And here is the schematic for its color encoder/RF modulator board: (actually, this is the schematic for the external TV modulator for the Expert 1.0) http://msxpro.com/hardware/esquemas/Expert_TA1.pdf

The first schematic is clear about converting YPbPr to RGB, look at the bottom of the page. But, I don't think it's calibrated properly to a 75ohm load. Since Gradiente tried to make several things proprietary on its MSX, this is probably to force people use Gradiente

The second schematic is clear about using the MC1377P as video encoder, receiving RGB on pins 3, 4 and 5 as video source and outputting composite on pin 9. But I made a mistake about it modulating RF, Gradiente engeneers used the MC1374P for this.

By sd_snatcher

Prophet (3068)

Аватар пользователя sd_snatcher

23-08-2016, 22:45

An YPbPr->RGB converter plus 2 encoding chips to achieve a much worse image quality than with a single LM1889 (recommended in the TMS99x8 datasheet)? What a fine piece of engineering! Smile

By Louthrax

Prophet (2082)

Аватар пользователя Louthrax

23-08-2016, 22:50

sd_snatcher wrote:

An YPbPr->RGB converter plus 2 encoding chips to achieve a much worse image quality than with a single LM1889 (recommended in the TMS99x8 datasheet)? What a fine piece of engineering! Smile

Why making it simple when you can make it complex & tricky Wink ? (cost maybe ?)

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