How to see if a chip is for PAL or NTSC? VDP TMS9929AN / TMS9918AN NTSC/PAL 50Hz/60Hz ?

[How to see if a chip is for PAL or NTSC? VDP TMS9929AN / TMS9918AN NTSC/PAL 50Hz/60Hz ?]

Please take the following with a grain of salt; I did not check the TI's schematic; I'm talking video in general, as a TV repairman in the '80s would.

 

Actually, there's more to video than just 50/60 and RGB - Composite - YPbPr...

In the race for Colour TV, several standards were developed (we're talking post WWII here). We'll focus on NTSC (USA, Japan, etc) and PAL (most of Europe, France being an exception with SECAM).

 

To encode chrominance (chroma, or just colour) on the luminance (luma, luma+sync, brightness, or just Y) signal, a subcarrier is added and the chroma signal is sent as a phase-modulation of this subcarrier "on top". The luma (Y) signal alone comprises "ye olde" brightness and horizontal/vertical sync signals originally sent to B/W only TVs. Incidentally, this is why most of the time something is not right, you just get B/W: Luma+Sync (Y) is the foundation of TV broadcast. Colour comes... "afterwards".

 

NTSC (strictly speaking NTSC-N) uses a subcarrier of 3.579545 MHz (3.58 for short) while European PAL (PAL-B) uses 4.43361875 MHz (4.43 for short). This is a most important bit to know.

 

The various TMS99x8 chips encode the video differently, and the composite (NTSC, TMS9918) should be sent modulating a 3.59MHz, while the luma and chroma-difference signals (PAL, TMS9929) should modulate 4.43MHz. Any mixup will result in B/W display because the TV cannot lock its internal chroma oscillator to the input signal. And some noise may be seen in the B/W picture as the chroma subcarrier would not be filtered (dot-crawl is an example).

Also, PAL will phase-invert the RED component every scan line to fix -and make redundant- NTSC's hue shift (and its knob).

Again, this is for the dated CRT TVs available back in the day - With modern equipment, it is actually up to the TV model to say if the signal is "acceptable" or not. That is also why "some" modern TVs work and "some others" do not.

 

To add problems to trouble, the NTSC chip TMS9918 uses a single pin to spit out just composite, while the PAL devices TMS9928/9 use three pins, one for luma (Y) and two for chroma-difference (Pb, Pr) signals.

Regarding TI-99/4A modulators, the NTSC ones would just mix audio and composite, and amplitude-modulate the TV channel RF signal. An easy task.

While PAL modulators had to work harder by creating composite out of Y-Pb-Pr, and mounting it on the TV channel RF signal. That's where the LM1889N entered the stage to do the heavy-lifting.

 

All in all, I guess that if you just swap chips you will not have a proper picture. At a minimum, I'd say the chroma crystal may need to be changed. Also if you're going NTSC->PAL you need to route the difference signals Pb and Pr to the output.

 

Hope that sheds some light,

zxMarce.

[New User - What about N-PAL modulators?]

For the ones after it, this is the same KiCAD 5 schematic (corrected an omission of two caps' references and values), but including the italian-like video out modification.

 

Hope it serves someone.

 

Regards,

zxMarce.

Modulator N-PAL - Video Out.sch

[New User - What about N-PAL modulators?]

Here are some pics I took today of my modded N-PAL modulator, PHA-2031.

The last pic is the image as I see it on a 32" Philips TV via A/V Input. Sometimes better, sometimes worse.

 

The modification I made -I realized later- is one from the TI99IUC. Only changes I made was to replace the 68 Ohm resistor with a 75 Ohm one by connecting two 150 Ohm resistors in parallel (did not have any 68 Ohm at hand), and I used a SS9018 transistor (from a disassembled Timex/Sinclair 1000 RF modulator) instead of a BC547. I also connected my transistor between LM1889N's pins 13 and 14, given that pins 14 and 16 are both positive supplies.

 

I suspect something in my chroma subcircuit is dodgy because whenever I touch some component leads (mostly anything on LM1889N's pin 15, audio subcarrier input) I can make the circuit output color (noisy of course, but color), while lately I mostly get noise-free B/W. Maybe my 3.582056MHz crystal' s gone; I played a bit with its tuning trimmer without result.

 

For the eagle-eyed viewing my pictures, I also changed the non-shielded cable to shielded in the video mod; this I did after taking the photos.

Hope this helps someone; as I said, I'll do a second schematic with the modifications.
 

The modulator front:

 

The back:

 

The side with the A/V RCAs:

 

The guts:

 

Detail of the TV/Computer switching relay:

 

The horrible mess - I added the multi way connector to be able to separate the board from the input cable (and forgot to include my own outputs - oh well):

 

Detail of the multi way connector:

 

Detail of the video modification:

 

Same, but from the other side:

 

The... er... "picture" I get from my TV:

 

Regards,

zxMarce.

[New User - What about N-PAL modulators?]

Well... Got most of the original circuit for the PHA-2031 N-PAL modulator on a KiCAD 5 schematic. Attached.

 

The circuit does not include the LM1889's RF parts (for the TV channels), as I only wanted to get composite out of the device.
Basically, these RF parts are a L-C tank for the desired TV channel and that's pretty much it. Maybe an extra decoupling cap.

 

I'll make a second schematic with my modifications.

 

Regards,

zxMarce

Modulator N-PAL Schematic (KiCAD 5).7z

[New User - What about N-PAL modulators?]

Hi there, I just signed up for some TI goodness.

A brief history, off the top of my head - Actual facts may slightly differ.

 

The first home computer I saw was a TI-99/4A with its PEB at a computer dealer in the early '80s.

It was being used by a young person, and before my eyes a Parsec-style game slowly started to appear - the person was coding the game right there, live!

At the time, Texas Instruments did have some HQs here in Argentina (guess it still does); they not only had representation but they actually did manufacture transistors here (sadly, this is no more so). It appears that "our" TIs had a national modulator (PHA-2031) designed and manufactured locally for our own PAL-N systems (50Hz vertical, 15.625kHz horizontal, 3.582056MHz chroma burst, 4.5MHz audio subcarrier). Not the same as the European (PAL-B, as we know them here) ones.

Googled hi and lo, but could not get a schematic of it, so I got my hands dirty. I have a very rough and somewhat incomplete schematic of the beast (pencil!). I did not get to the RF part, basically because I just wanted to get to the A/V signal, and the RF part is just an L-C tank for the LM1889N it uses plus some support passives.

 

I did get some A/V out of the modulator, albeit video is slightly noisy (maybe a cap or two need replacing, be it in the modulator and/or the TI power supply itself).

 

I have the modulator board scan, a Pinta project with board's track and pad layers (flipped, so I could just pencil-draw over it the components by copying their positions from the board), and the handmade -partial- schematic of the modulator.

I did not make a second schematic with my modifications yet; I will do it and upload the whole bunch later to this thread.

My hopes are that someone sees what I did and yells me something along the lines of "Hey, if you get video this way [.....] it will be clean" or even "change caps Cxx, Cyy [...] and video will work".

 

Regards,

zxMarce.