MAMEWorld Forums - EmuChat - Re: Question about ALG driver status

> Could you expand on this please? As I understand it, part of the issue with laser
> disc support is the ongoing discussion of accuracy.
>
> Disclaimer: I'm not presuming to question MAME design decisions, I'm just wondering
> about a different path.
>
> What caught my attention is this: What you're proposing sounds to me like you're
> discussing the capture of the parts of the "frame" that are relevant to the game and
> disregarding the parts of the "frame" that are present only because of the
> requirements of the co-opted technology (Laserdisc).
>
> I'm wondering if that is an interesting discussion point as MAME doesn't attempt to
> emulate all of the intricacies of the underlying technology (Not counting discrete
> circuits). MAME isn't attempting to emulate any frequencies outside of the human
> range of hearing for example, so I'm wondering if not grabbing unused frame
> information might be similar?

I also dont want to question MAME design decisions, but in this case i would just like to know more about, how and why the tools for LD games work that way.

With my bad english, i will still try to describe my concerns with the current approach of MAME (and the LDverify tool in general), of capturing the whole frame (525 lines).

I attached screenshots from (LDverify tool approved) capturings that SailorSat did on some games. If you take a close look at the first 20 or so lines, you will see a lot color noise and distortion, but normally the VBI range has no color subcarrier, this only happened because the capture card was forced to grab the whole frame. In practice this is not how professional equipment is working. In practice the card would capture seperately the relevant VBI data and put that on top of the capture, resulting in a clean (from distortion and color) frame, at least for the VBI data.

So the downsides of the LDverify tool are:

- forcing/hacking a capture card to capture the whole frame.
- by this method also driving a cheap card to the absolute limits, resulting in more poor capturing.
- predetermined errors while in verify-process, because of distortion and colornoise and you will often need to capture a source like 3-4 times and make a single working capture of it, that is finally approved by LDverify tool.
- capturing the whole frame, introduce a high risk that you destroy/break the fields. Remember that just one shifted scanline can make the capture useless.

I dont write this to bitch on the MAMEdevs or the tools, i just want to show why the approach is not the best. VBI data was initially introduced for subtitles in broadcast stations, however LD games abused this technique for their own purpose. Only three lines are relevant to get this working (the 4th line is just a copy for safety). A good card just needs to know, which lines needs to be captured. Different manufacturers, used different techniques, that resulted in different lines that are needed.

So the initial approach of the LDverify tool is perfect, if you have no clue, which lines are important, to make the game work properly.

[ATTACHED IMAGE]

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