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Megadrive RF-Mod

 
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Mystic_Merlin



Joined: 15 Oct 2007
Posts: 496
Location: Bangkok

PostPosted: Sun Jun 07, 2009 6:41 pm    Post subject: Megadrive RF-Mod Reply with quote

Copiers are hard to find these days so this time I came back from the junk market with a Japanese Megadrive for half a $.
As usual the internals were such a mess: cockroaches, clay, toys...I can't believe I got that thing to work eventually.
A weird mod caught my attention though: a custom RF module.

Basically, the RF unit is connected to the 7805, the composite video output, the audio output...nothing special till I flip the board:
The mod is on the Sony RGB encoder
The trace is cut before pin 6 (crystal in) and a capacitor in serie with a 4.xx khz crystal is connected to pin 5 (crystal out).

Since nobody cares about RF, just out of curiosity I have a question: was this mod's purpose to assign a custom frequency to the RF unit?
There seems to be no channel selector so I guessed this is a manual way to create one, right?
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kyuusaku



Joined: 26 Jul 2003
Posts: 941
Location: .ma.us

PostPosted: Mon Jun 08, 2009 5:33 am    Post subject: Reply with quote

It sounds like they replaced the NTSC colorburst crystal with a PAL one (4.43 MHz), which makes sense since Thailand is PAL. Check pin 7 on the encoder, if tied low the output is PAL, if tied high the output is "NTSC 4.43" which is typically only viewable on multi-system monitors, so that's not likely. If it's modded to PAL and the MD is set to NTSC, you get "PAL60" which apparently was commonly supported even in the 90s.
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Mystic_Merlin



Joined: 15 Oct 2007
Posts: 496
Location: Bangkok

PostPosted: Mon Jun 08, 2009 5:54 am    Post subject: Reply with quote

kyuusaku wrote:
It sounds like they replaced the NTSC colorburst crystal with a PAL one (4.43 MHz), which makes sense since Thailand is PAL. Check pin 7 on the encoder, if tied low the output is PAL, if tied high the output is "NTSC 4.43" which is typically only viewable on multi-system monitors, so that's not likely. If it's modded to PAL and the MD is set to NTSC, you get "PAL60" which apparently was commonly supported even in the 90s.


Thanks kuusaku! Ok I was wrong again but I learnt something! Razz
Your explanation makes sense and obviously it has nothing to do with the RF.
Indeed I remember most TVs were not multi-system back then.
This MD also displays a different aliasing (barely noticeable unless you switch back and forth) to the other one I have which could be due to a slightly different rendering between PAL and NTSC on my TV.

Also I spotted a few official PAL Japanese MD to sale here. This fix must have been quite common on imported NTSC models

I'll run some more tests on a NTSC PSone screen as this one only takes NTSC 60.

EDIT: it just popped up in my head but using a similar mod, wouldn't it be possible to build a switch for the PSone screen to display PAL or NTSC?
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Mystic_Merlin



Joined: 15 Oct 2007
Posts: 496
Location: Bangkok

PostPosted: Wed Jun 10, 2009 7:17 am    Post subject: pal 60 MD-SNES mod + lockout Reply with quote

Ok I got a bit carried away but I found out some info that might be helpful as I never read it on any modding site...well let's see.

Opening all my SNES and MD bought here I found out indeed that they were all modified into PAL-60.

As far as I understand, the color subcarrier is some kind of "compressed data" embedded into the video signal. This method was used to keep retro compatibility with older B/W TVs using the same composite connection limited to 2 wires.
The crystal oscillators in the broadcaster/receiver act as some kind of compressor/decompressor using the "gaps" of the B/W signal to fit in the color subcarrier.
Due to a different refresh frequency of 50 Hz for PAL and 60 Hz for NTSC the color subcarrier is respectively of 4.43 Mhz and 3.58 Mhz...I know that might sound like a barbaric way to put it into words but so far that's what I understood. Rolling Eyes

This mod for the megadrive changing NTSC-60 into PAL-60 results in a B/W stable display on the NTSC PSOne screen via composite input.
So either I need to change the crystal in the screen or I need to change the md's output to NTSC-60 to display the signal properly.

On most modding websites I was reading that it's normal to have a B/W on the NTSC MD/genny when you add a PAL switch but never heard of the possibility of adding that "video mod" to correct the problem.



Also the SNES PAL-60 mod on a US model is interesting:

The oscillator is placed between R27 and pin 15 of the S-ENC/BA6594AF (I couldn't find any info about) and there's a jumper below to switch from SCLN (NTSC) to SCLP (PAL)

I also it seems I found out another CIC lockout mod on this SNES:
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kyuusaku



Joined: 26 Jul 2003
Posts: 941
Location: .ma.us

PostPosted: Wed Jun 10, 2009 6:17 pm    Post subject: Re: pal 60 MD-SNES mod + lockout Reply with quote

Mystic_Merlin wrote:
As far as I understand, the color subcarrier is some kind of "compressed data" embedded into the video signal.

When RGB is encoded into composite, the luminance (brightness - black & white signal) information is made by adding a bit of R, G and B together based on how well eyes can see R, G and B. The color information is made by adding/subtracting R, G and B together to form two more signals which are basically the Pb and Pr of component video. These two signals are then modulated (the high frequency signal is superimposed on the low frequency signal) on the luminance signal 90 degrees apart so that they don't interfere with each other when being decoded. The color subcarrier frequency is the "base" frequency where the TV tries to grab color information from.


Mystic_Merlin wrote:
This mod for the megadrive changing NTSC-60 into PAL-60 results in a B/W stable display on the NTSC PSOne screen via composite input.
So either I need to change the crystal in the screen or I need to change the md's output to NTSC-60 to display the signal properly.

There's more to changing NTSC to PAL than the frequency, the whole encoding process is a bit different. It's possible the screen doesn't actually support PAL decoding. Unless you can find information on a internal PAL/NTSC select pin, I wouldn't bother, it would be easier to switch it to NTSC. IIRC Those PSOne screens also have RGB input so that's another option.

Mystic_Merlin wrote:
On most modding websites I was reading that it's normal to have a B/W on the NTSC MD/genny when you add a PAL switch but never heard of the possibility of adding that "video mod" to correct the problem.

It turns the output to PAL without regard for the RGB input which can be formatted for NTSC or PAL, it goes the same way when you play a PAL60 game on NTSC display (stable image but B&W).
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