DIrect TMS9928 - GBS8200 YPbPr connection

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By tvalenca

Paladin (728)

Аватар пользователя tvalenca

24-08-2016, 18:28

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

Indeed!

But note, The image quality of a TMS9x28 + LM1889 (the TMS9x18 has internal CVBS encoder) is only that good in NTSC. The HACK Sharp used on Hotbit is even scarier!

By tvalenca

Paladin (728)

Аватар пользователя tvalenca

24-08-2016, 18:26

Louthrax wrote:
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 ?)

That would make sense if we Brazilians didn't had to deal with PAL-M! Either NTSC or european PAL would be OK, but no, they couldn't allow us to import TVs, so they mixed the USA TV broadcast System with the european color encoding system...

That's not half the trouble French had with SECAM (I suppose) but it's still trouble enough.

PAL-M could had been a good thing if it were supported by industry. It was better than NTSC when it first came, but the industry continued to develop NTSC (specially Japanese Laserdisk companies) and PAL-M continued to be what it was on early 70's.

By sd_snatcher

Prophet (3093)

Аватар пользователя sd_snatcher

25-08-2016, 05:01

tvalenca wrote:

The HACK Sharp used on Hotbit is even scarier!

What hack specifically do you mean?

By tvalenca

Paladin (728)

Аватар пользователя tvalenca

25-08-2016, 17:24

sd_snatcher wrote:
tvalenca wrote:

The HACK Sharp used on Hotbit is even scarier!

What hack specifically do you mean?

l_oliveira mentioned once, and I am almost sure you know what I am talking about because AFAIK the Brazilian SMS uses the same color encoder (LM1889) the Hotbit uses.

It's a hack to generate PAL colors on a 525-line interlaced frame (60Hz refresh rate), since the LM1889 ties the phase-alternating to the refresh rate.

By sd_snatcher

Prophet (3093)

Аватар пользователя sd_snatcher

26-08-2016, 01:36

Maybe there was some misunderstanding.

In fact, the Brazilian Master System (fat model) has the Motorola MC1377 TV-Encoder. It's the same crappy encoder used in the Gradiente Expert and nearly all Sanyo MSX2/2+ machines. This is the one that needs a hack to "convince" it to produce PAL-M encoding. The trick is to set the PAL/NTSC selector pin to NTSC on VBLANK, then back to PAL when the screen is being drawn. This forces the chip to reset the PAL flip-flop, otherwise the upper quarter of the next frame will have inverted phase and thus will show incorrect colors.

The Hotbit has the National Semiconductor LM1889, that is a totally different beast. The LM1889 has no phase alternation circuitry built-in. Thus, for any PAL/derivatives it must be implemented externally. On the PAL Sega SC-3000 and in the Hotbit, it was done with discrete components and an external flip-flop (CD4013). Other machines with a TMS9x2x VDP have similar circuits. AFAIK, in the ZX-Spectrum the phase alternation is done by the ULA itself.

The phase alternating circuit of the Hotbit is done by the pair CD4013 (CI-301, 1/2) and CD4066 (IC-302, 1/2): the flip-flip CD4013 selects which clock source (normal or inverted) the CD4066 will send to the LM1889. The SC-3000 implements this selector exactly the same way, but each machine generate the said inverted clock is very different ways.

By DamnedAngel

Master (144)

Аватар пользователя DamnedAngel

02-09-2016, 20:03

Hi tvalenca,

Thank you for your help!

Quote:

HEY YOU HAVE AN EXPERT! you already have a good (if not great) quality RGB output.

Okay, you convinced me. Already bought a 510N and now will assemble the adapter cable. But some day later I will still try connecting the TMS9928 to GBS. I was suggested that is it a problem with impedance matching. When I have time, will get back to this.

Quote:

you are plugging YPbPr on an RGB input

Apparently this is not the case. GBS has YPbPr input and I was using it.

Quote:

you may have switched Pb with Pr and vice-versa

Also not the case. Just to be sure, I tested both configurations.

Tks again for the tips. I will post here my results.

best,

By tvalenca

Paladin (728)

Аватар пользователя tvalenca

02-09-2016, 22:12

DamnedAngel wrote:

Hi tvalenca,

Thank you for your help!

You're very welcome!

DamnedAngel wrote:
Quote:

HEY YOU HAVE AN EXPERT! you already have a good (if not great) quality RGB output.

Okay, you convinced me. Already bought a 510N and now will assemble the adapter cable. But some day later I will still try connecting the TMS9928 to GBS. I was suggested that is it a problem with impedance matching. When I have time, will get back to this.

Feel free to keep in touch so we could debug this togheter.

By DamnedAngel

Master (144)

Аватар пользователя DamnedAngel

21-09-2016, 14:40

Hi all (and specially tvalenca),

Quote:

HEY YOU HAVE AN EXPERT! you already have a good (if not great) quality RGB output.

Well, I just build the LM1881 circuit and image came out instantly. Pretty misadjusted, but I managed to torture my monitor into stretching the image into and acceptable box within the screen:


My big problem, however, are vertical stripes all over the image, which appear in every color but black and white:

Please note that the upper stripes are artifacts of the photo I took, but the lower stripes are real ones on the screen.

What is the cause of such stripes? Is my Expert's RGB output faulty? What can I do get rid of them?

Note: Please let me know if it would be adequate to start another topic, since it has nothing to do with the original subject (GBS8200/YPbPr).

TIA,

By Alexey

Guardian (2459)

Аватар пользователя Alexey

21-09-2016, 16:27

Those vertical stripes are quite common for RGB to VGA converters. You may even have them with some RGB to S-Video devices. However I never noticed them with SCART or Composite video connections. The best result you can get when using SCART to HDMI converter.

By maxis

Champion (512)

Аватар пользователя maxis

21-09-2016, 19:00

@ DamnedAngel

I can give 3 good reasons why you see those artifacts:

1. What you see is a result of a bad aliasing of the sampled CVBS or component video signal. The bandwidth of the PAL/PAL-M/NTSC QAM modulated color-difference signal must be limited by the bandpass filter. On the luma side the notch filter must coincide with the chroma bandpass. Typically, nothing like that is done in the encoder signal chain for the cost reasons. Hence - aliasing, since there are higher frequency components beyond Nyquist (for the TV front-end ADC), which spectrum folds back very badly onto the useful signal.

2. Also, what you likely see is a "beating" between the subcarrier clock generated by the PAL/NTSC encoder chip locally and the clock source of the computer (pixel clock). Common practice - derive the subcarrier clock from the pixel clock. And/or the analog video component signals/RGB etc is polluted by the dot clock derivative (/2 or /4).

3. Finally, since the computer runs in the progressive mode instead of interlaced, the monitor can't properly recover and use the COMB filter to restore the video signal -> the result is the pattern on the screen. (instead of having 525 lines, it will only have 524 per frame)

Bottom line - try to analyze the encoder schematics, IC application notes and see if you can/have to introduce the filtering (you can scrap them from the old TV color decoder).

Also, you can build something like that in order to convert YPrPb to RGB.

However, effectively you will also need the clamping of each of video channels (AC coupled) via the "clamps" driven by the sync separator like EL4881 etc

@sd_snatcher
MC1377, BTW, is a broadcast quality NTSC/PAL encoder used in the professional Video broadcast equipment quite often (video cards, titling machines, black burst generators, etc). It is difficult to have a performance better that one offered by MC1377 for the full signal path including the analog matrixing (RGB->YCrCb) - more than 7 MHz of a video bandwidth. Also, I simply wonder who on the planet for exception of Daewoo CPC puts the delay line components in MSX when using MC1377P?

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