"Tint issue" on modified Toshiba HX-10 (LVA510 related ?)

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By Louthrax

Prophet (2076)

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10-10-2015, 23:15

Hi all,

I built myself recently a new Toshiba HX-10 from different broken models. I took the video output board from a UK model, so I had a black & white video output on my french CRT (but those are supporting NTSC composite output from Japanese MSX).

So I changed the LVA510-467 chip on this board for a LVA510-497 (the same chip as on french Toshiba HX-10). I also changhed the VDP for a 60Hz TMS9928ANH (easy to do as the VDP is socketed on the HX-10).

Everything was looking good at first sight (sharp display, white & blue were OK, "fullscreen" 60Hz), but when I first launched Nemesis 2, I noticed that the red & greens had almost the same tint. I tried to adjust the "tint" setting on my TV, but there's no way to fix that.

So it seems that my VDP (TMS9928ANH) & YUV to composite chip (LVA510-497) combination is not so good. It works fine if I replace the VDP by the HX-10 original one (can't tell which model it is as it's hidden by some kind of plate glued on it), but I'd really like to keep the 60Hz output.

Did you guys already experienced that ? Is there another LVA510 model that would work better ? Or maybe something like adding a resistor on the composite output to fix the tint ??

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By Louthrax

Prophet (2076)

Louthrax's picture

11-10-2015, 00:51

Mmmm... When reading this post, I have have the feeling I should try to put a resistor between the YUV output of the VDP and the little board that does the YUV->composite conversion (the one with the LVA510). If that's true, there would be a "voltage difference" between the TMS9928AN VDP I installed and the original one on the HX-10 ??

By tvalenca

Paladin (728)

tvalenca's picture

27-10-2015, 19:20

@louthrax, ask @l_oliveira. He had similar issues on a VG-8020/00 that he transcodified from PAL@50hz to NTSC@60hz:
http://www.msx.org/forum/msx-talk/hardware/can-be-switched-6...

By l_oliveira

Hero (520)

l_oliveira's picture

28-10-2015, 01:17

The numbers after "LVA510" are not part of the chip code. so the " LVA510-467" and "LVA510-497" chips will be the exact same thing. The different numbers simply mean the date the actual chips were manufacturated.

The reason for the hue differences are explained on the TMS99xx VDP datasheet. The 50hz version of the VDP is optimized for generating PAL video then it will output the color values with slightly different voltages than what the 60Hz chip would output. The LVA510 has mode select pins which are toggled to make the chip adapt to that variation. You're feeding it a NTSC-ish signal while the chip is set up for PALand that's why the hues are wrong.

So, to make it work correctly with the NTSC VDP, you connect pin 8 to GND (it's disconnected for PAL) and disconnect pin 10 (it's connected to pin 11 for PAL).

Let us know if it works for you.

By Louthrax

Prophet (2076)

Louthrax's picture

28-10-2015, 01:29

Hi tvalenca & l_oliveira,

Thanks a lot for your explanations,I was convinced the voltages were the same. That's great Smile will try your suggestion tomorrow and let you know of the result !

By Louthrax

Prophet (2076)

Louthrax's picture

28-10-2015, 12:47

"GREAT SUCCESS Running Naked in a Field of Flowers "

The composite output is perfect now, thanks a lot again guys !

By tvalenca

Paladin (728)

tvalenca's picture

28-10-2015, 19:51

I'm happy it worked to you.

By sd_snatcher

Prophet (3032)

sd_snatcher's picture

29-10-2015, 01:26

@l_oliveira

Are the color values different, or just the color burst that is different between the TMS9x28 and TMS9x29? I ask because the table on page 2-70 of the TMS9918 datasheet shows only a difference in the color burst.

Por PAL-M machines this difference should also cause problems, shouldn't it? The Expert goes around by converting the YPrPb to RGB and then feed it to the awful MC1377, and this chip generates a new color burst. But how does the Hotbit solves this issue?

By Louthrax

Prophet (2076)

Louthrax's picture

29-10-2015, 02:15

Maybe there are some hints in the LVA510 datasheet/manual (as it seems to have some "preset configurations" for each modes) ? I googled for it but no way to find anything !

By l_oliveira

Hero (520)

l_oliveira's picture

29-10-2015, 02:42

sd_snatcher wrote:

@l_oliveira

Are the color values different, or just the color burst that is different between the TMS9x28 and TMS9x29? I ask because the table on page 2-70 of the TMS9918 datasheet shows only a difference in the color burst.

The way phase encoding works on PAL and NTSC are different. Because of that the encoding circuit has to be different too. You wouldn't need such difference at RGB because it is converted internally on the video encoder chip into Y/R-Y/B-Y which is then converted to Y/C to then finally become CVBS.

I'm not sure about the specifics but on the case of NTSC versus PAL the difference happens to be on how R-Y and B-Y are dealt with. On Expert, they ultimately want RGB to feed into the MC1377 (which isn't a bad encoder at all, just complicated to use with 12V voltage requirement and needing a lot of external parts) for then achieve the color video.

Because the TI VDP is made to output Y/R-Y/B-Y instead of RGB, it has some of the work of the video encoder already done (convert RGB into Y/R-Y/B-Y) and that's the reason why the chips are different. At the time these chips were designed RGB wasn't really the most common encoding. GI videochip from the INTL also use Y/R-Y/B-Y and I think National Semiconductor's Pong chipsets, again use that as video encoding.

Many years ago I converted a VG8235 to PAL-M and all I had to change was the clock crystal at the encoder board from PAL (4.3Mhz) to PAL-M (3.57Mhz) carrier frequency. Also patch the ROM so the VDP vertical frequency became 60Hz.
It only worked correctly because the encoder board has a circuit which converts RGB into Y/R-Y/B-Y from the RGB signal output by the V9938. That is fixed no matter if the VDP is working in 50 or 60Hz.

sd_snatcher wrote:

Por PAL-M machines this difference should also cause problems, shouldn't it? The Expert goes around by converting the YPrPb to RGB and then feed it to the awful MC1377, and this chip generates a new color burst. But how does the Hotbit solves this issue?

Hotbit has a LM1889 which is a combo of Y/R-Y/B-Y video encoder and RF modulator on a single chip.

By l_oliveira

Hero (520)

l_oliveira's picture

29-10-2015, 03:14

Louthrax wrote:

Maybe there are some hints in the LVA510 datasheet/manual (as it seems to have some "preset configurations" for each modes) ? I googled for it but no way to find anything !

Not even a clue. I had Werner Kai take pictures of a LVA510 encoder board on a NTSC Toshiba Pasopia MSX1 (probably the same model as yours but on Japan region) and traced the circuit from the pictures. The only significant differences were the pins 8 and 10. I tried that on my VG8020/00 and it worked.

And here's where we stand at. Smile

Before that I tried really hard to get the LVA510 chip datasheet. But had no luck.

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