TMS9918 Video Output Behavior?

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Por jdmcs

Rookie (22)

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06-06-2019, 17:28

I’m trying to figure out what the TMS9918 is doing differently when generating its composite video output. There’s something going on with the signal from my Yamaha CX5M that goes beyond “composite video is bad”.

If I plug my CX5M into a CRT TV, everything looks correct. The text screen is a solid blue background with white text by default.

If I plug the same CX5M into a video capture card or an LCD TV, I get one of two alternate behaviors:
1. The text is still white, but the blue background is a lighter blue, with a reddish tint, and jail bars.
1. The text is still white, but the background is comprised of alternating vertical lines of dark blue and light blue.

I looked over the TSM9918 data sheet, and didn’t see anything that explains this behavior. But it seems that the chip is doing something that analog TVs can see past but not modern ADCs... and unless I can identify the culprit, I can’t begin to find a solution (short of continuing to buy different upscalers until one works.)

And yes, I did consider locating a CX5M with SCART output, but I am in the US. That might solve some of the problem (it seems folks have better luck upscaling from RGB), but would introduce two others: PAL and 50Hz. (240V to a lesser extent, but a travel adapter would take care of that.)

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Por jdmcs

Rookie (22)

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07-06-2019, 04:52

Here are some examples of #2 as mentioned in my first post above, using my National CF-2000 (my CX5Ms were downstairs, and the results would look the same):

Using a Diamond Multimedia VC500 via its Composite input:

Using a Diamond Multimedia VC500 via its S-Video input*:

Using a Avermedia Easy DVD 7 (C039) via its Composite input:

Using a Avermedia Easy DVD 7 (C039) via its S-Video input*:

(OK, this one doesn't look too bad... the blue color is a bit off, but the vertical pattern is even.)

* S-Video inputs were using a StarTech Composite to S-Video adapter.

Por RetroTechie

Paragon (1563)

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07-06-2019, 20:07

Most important observation is right there:

jdmcs wrote:

If I plug my CX5M into a CRT TV, everything looks correct. The text screen is a solid blue background with white text by default.

If the video signal output by the CX5M had all these artefacts in it, then even the best monitor wouldn't be able to 'unsee' those, would it? In other words: looks like (no pun intended Wink ) the CX5M's composite output is okay.

Which leads to the logical conclusion (okay, assumption on my part):

  • $40..50 capture equipment simply isn't up to the job. At least not if you want decent quality
  • A composite -> S-Video cable doesn't help, because it's still composite you start with

And...

Quote:

If I plug the same CX5M into a video capture card or an LCD TV (..)

What brand / model / size? One LCD TV != another LCD TV. And in my experience, sometimes it relates to screen size. Quite a few small LCD screens come with crappy electronics inside. Most big screens (say 24" and up) are much more capable of producing something reasonable from what you throw at it. In between (say around 19, 20"), it's a crapshoot. With some model LCD you're in luck, some other model you're not. And as you noticed, most CRT's are pretty forgiving when it comes to signal quality.

Which leaves you with several options:

  • Use a CRT with the MSX
  • Invest in a better upscaler like XRGB or similar device
  • Use some other MSX machine than the CX5M. Specifically: one that has RGB output
  • Find an LCD screen that does like your CX5M's composite output

One more point: check your cable(s)! For composite you want a decent quality, 75 Ohms impedance cable of (preferably) no more than a few metres. 'Any' cheapo / thin coax with fitting plugs on either side, may be just enough to degrade a signal that receiver doesn't like it anymore. With RGB this matters also but much less so.

Por jdmcs

Rookie (22)

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20-06-2019, 06:01

I purchased a RetroTINK-2X, and that seems to have eliminated the vertical lines I am seeing in the video capture. That would seem to indicate that the vertical lines are an artifact of 240P output when the input does not support that resolution.

However, I am still seeing an unexpected color shift -- the MSX color Dark Red looks orange and MSX color Dark Blue looks muted. (The two yellow colors also take on a green hue.) I'm also suspecting the MSX video cable I purchased is not a 75 ohm cable.

Since I am trying to document the CX5M specifically (the longer-lived of the two MSX computers released in the US), I stand behind my decision to try to use the actual hardware. However, this exercise is giving me a headache in the limitations in the NTSC video standard verses improvements made to other standards that were devised afterwards...

Por sd_snatcher

Prophet (3092)

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20-06-2019, 14:46

S-video is not adequate for CVBS (composite video) input. It will show vertical lines as you have experienced, because the chroma filtering is too weak/inexistent.

The majority of the 80s computers/consoles, as a measure to save cost/energy, generated the CVBS signal using a trick called composite artifact colors. But, for that to work, it requires good filtering on the receptor side.

The conclusion is that your capture cards/TVs that are showing vertical bars on the CVBS input just have crappy filters on their input, so the vertical bars end up showing.

Quote:

the MSX color Dark Red looks orange and MSX color Dark Blue looks muted

Y

You mean when compared to the same machine connected to a CRT TV, or an emulator?

One thing is, CRTs were very well calibrated using SMPTE patterns to produce the best possible quality with such limited technology. They had to be calibrated, otherwise the image would look crappy all the time. But these days, flatscreen TVs have CVBS input just as a legacy that's is a burden to the maker. So those inputs are just slapped together until it shows some image, and that's it. Even better if it looks like crap, because the owner of the set will want to upgrade his older devices to more modern ones that output HDMI.

As a last resort, you could try to fiddle with the tint and saturation controls of your video grabber card. To see if it helps.

Por TomH

Champion (327)

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20-06-2019, 15:42

The vertical lines do indeed betray a deliberate deviation from the NTSC standard: the TMS produces in-phase video.

NTSC adds colour to the existing black and white signal via a colour subcarrier — a high-frequency and low-intensity signal that is added atop the black and white. Both of those features are meant to make it hard to perceive on an older or cheaper black and white set that doesn't filter it out, but so is a third feature: relative phase.

The frequency of the colour subcarrier is selected so that 227.5 cycles of it fit on every line. So the colour content on each line is 0.5 cycles offset from the lines on either side. So in an area of consistent colour, the peaks and troughs in the colour signal are in between each other, line by line. Furthermore, because there are an odd number of lines in a field, they alternate from one field to the next.

All of that helps to make them much less obvious to the eye.

As per the exact verticals, the MSX doesn't do that. It deliberately outputs lines that are fractionally too long: long enough that they contain exactly 228 colour cycles. That's a great simplification for digital video generation as it means no state needs to survive from one line to the next, and parts of the signal are easier to generate — especially the colour burst (an invisible part of the signal in the horizontal border that provides colour phase information) is now identical on every line and therefore not a function of anything. Just a fixed bit of signal to insert.

Some other machines do the same thing: all the 8-bit Ataris and early C64s spring to mind.

This can cause visible artefacts because there's no such thing as a perfect chroma/luma filter — a perfect way to separate colour and brightness information that have been combined in a composite signal. So some decoders will hedge their bets by allowing small amounts of the colour signal to survive in the brightness part, rather than more aggressively filtering the brightness in a way that would reduce sharpness. The trace amounts would be hard to perceive anyway, were the signal on-spec and including the difference in phase per line.

Por mohai

Paladin (841)

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21-06-2019, 17:55

NTSC= Never The Same Color
Wink

Por jdmcs

Rookie (22)

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21-06-2019, 22:02

sd_snatcher wrote:
Quote:

the MSX color Dark Red looks orange and MSX color Dark Blue looks muted

You mean when compared to the same machine connected to a CRT TV, or an emulator?

The same machine connected to a CRT TV.

Por TomH

Champion (327)

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22-06-2019, 15:15

Oh, right, but NTSC TVs have a tint control, don't they? To compensate for phase errors induced by transmissions bouncing off things before they get to you — and unlike PAL TVs, which can correct for that issue automatically. So do you definitely have the same tint value on both devices?

Aside: I'm told that the Nintendo people still don't have a consensus on that machine's 'true' colour palette because of the tint-control issue.

Por sd_snatcher

Prophet (3092)

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22-06-2019, 16:12

Note: PAL does not really correct any errors. That was a misleading marketing claim by Telefunken that stuck without questioning.

What PAL really does is:

1) Reduce the vertical color resolution by half
2) Replace the hue errors by saturation errors, which is less noticeable.

But the trick had it restrictions. More severe errors would just translate into Hanover bars.

Also, in direct CVBS cable transmission from the computer/console/DVD to the TV, phase errors are negligible. In this case, all that PAL does is to worsen the image quality by halving the vertical color resolution.

I'm not criticising, just explaining. This was the era of nifty tricks with very restricted hw resources. They did the best they could with what they had at hand.

But what doesn't make any sense is people bashing NTSC nowadays for direct CVBS cable connection, when PAL performs worse in this application.

Por TomH

Champion (327)

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22-06-2019, 21:39

On that final point, I still prefer PAL for direct connections: the reduced colour resolution vertically — only slightly better than half (allowing for 625 versus 525) — isn't so bad when composite colour is already relatively low resolution horizontally; and the higher bandwidth allocated, which allows for a higher frequency colour subcarrier, improves the horizontal colour and brightness resolution.

Of course, the trade-off of fewer channels fitting in the available broadcast space was never really a problem in my country, being an island with a decent amount of government control of broadcasting. So that's lucky!

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