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TMS9918A Video Input


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Sidebar conversation - did the Video Controller do any video stuff with the transparent color? I was always so curious back in the day about why there was a transparent color. :)

It would be difficult to do so... at least on the NTSC 99/4A. The reset pin, needed for 9918A genlock, is soldered down and unusable for sync without cutting traces, and you can't externally detect transparent.


The datasheet shows that the component versions of the 9918 (such as used in the PAL machine) output specific levels to indicate transparent rather than mixing internally, but I don't know if that makes it out the rear of the console. :)


(editted to be less overgeneralizing ;) )

Edited by Tursi
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So are they using a sync'd clock? In my project what I'm trying to do is take raw composite video from a TRS-80 model 1 and merge it as an underlay for the VDP. In that case, I don't have a clock signal to work with. I have to treat the video signal like any other composite video signal and get it massaged so that it will work as an input to the VDP.


My current prototype is at github.com/calphool/TRS80GS

The two TMS9929A VDP share the same crystal and reset line. They can be addressed separately or together. The logic of the board mixes the R-Y, B-Y, Y signals, then a combinaison of diodes/resistors and caps give a RGB output.

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  • 3 years later...
On 1/29/2019 at 6:01 PM, TheBF said:

I spent 15 years in the Eng. Dept of a TV studio/transmitter complex (CFPL channel 10) and left the industry when everything was still analog. It's been over 24 years since I thought much about this but typically here is how this would work in the old days. ?

( I remember taking my TI-99 into the station to see if the Video outputs were RS170A compliant. (They were not quite correct as I recall, which is what we noticed about all the computer gear made for Video use in the old days by computer engineers. That eventually improved in the 90s. But Then they just digitized everything to solve that problem! ) :)

How it was done in a pro analog studio:

  • You would use an external video source that could take another NTSC signal as its sync reference. This input is typically called GENLOCK.
    * It used a PLL to lock to the horizontal sync pulses. With that is could find the sub-carrier burst (3.58Mhz) and phase lock its own color-oscillator to the external signal. (+/- 3 degrees of phase error was the max allowed for color accuracy as I recall)
  • So if you loop the TI-99 though the GENLOCK input/output (un-terminated) you can use the TI-99 Video output to also feed a monitor or a recorder.
    The final device must be terminated with 75 ohms to prevent reflections in the image.
  • The Genlocked device is now in sync so that video can be fed into the TMS9918 external video input with no problem, as long as the cables are not too long...
  • If the cables are long then you will need a vectorscope to check the phase alignment of color bars between the TI-99 and the now synchronized external source.
    Adjust the genlock phase on the external device to get that matched.

    The alternative to this involves use a digital time base corrector (TBC) which would be Genlocked to the TI-99 and then any video fed into it could be "forced" to sync to the TI-99 by removing the external sync signals, replacing them with new locked signals and dropping the external video on top of that new set of sync pulses.

    Chances are nobody has a TBC lying around these days. The TBC output could be put back into the external input and the picture edges and color could be tweaked to match the TI-99.


None of this may help but it gives you some idea of what a PITA analog video was. ;)



"And if you tell the kids that today, they won't believe it!"


Monty Python, Yorkshiremen routine

Exactly the purpose of the pin is to genlock. Like an Amiga without having to use an external genlocking box. The idea was so the TI-99/4A would have the ability to genlock with either a VCR or another TI-99/4A. In fact, they demonstrated it in some of the prototypes / pre-consumer release units. Some of the commercial / ads were essentially demonstrating it but before they started ramping up consumer production, they effectively disabled it (but with little hacking (modding)), with some modding, you can enable the function. This was because they didn't feel at that time, the TI-99/4A would need to be genlocking to anything in the regular home use. A couple mods here and there to connect it up, you would be able to genlock to a VHS with a relatively inexpensive interface what Amiga became super popular for. TI was kind of out of the box had the genlock circuitry that just need some lines and few modding to complete the circuit and then you can use it to do exactly what Amiga could do with subtitles, etc. Now, of course, we could conceivably make such an interface with a video card in the PEB that has an input line and then could genlock either to the onboard video chip in the TI-99/4A motherboard OR you could set it up with full genlock circuit as well and have 24 bpp composite or analog RGB (or fancy digital RGB circuit) video or something along that line and genlock to another video source on the 15 Khz HSync. and so forth.


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