The Hong Kong Famicom console is surrounded by mysteries regarding its internal workings. Is it PAL or NTSC? What does the Slow/Normal
switch do? How's the compatibility with JP Famicom software? Since HK Famicoms are not easy to come by these questions have been unanswered for a long time.
Today I was looking around on NESdev forums and thanks to its crap forum search function which finds everything except what you were originally searching for, I've arrived to this:nesdev.parodius.com :: Hong Kong version Famicom clock frequency is 21.3125MHz
A member of that forum got hold of a HK Famicom and was able to take it apart and see what made it tick and how it did it.
I finally got a Hong Kong version Famicom recently. That uses NTSC CPU and PPU chips, but there is an NTSC-to-PAL conversion chip in the power/modulator section. (Nintendo patented the conversion technique used by this, the patent is interesting reading.) That chip converts the colour encoding, but can also halt the PPU in order to get a 50Hz picture. There is a 50/60Hz switch on the back of the console. Strangely, on the old PAL TV I tested the console with, the 60Hz PAL picture is fine but the 50Hz picture appears in black and white.
Anyway. The crystal frequency in the Hong Kong Famicom is 21.3125MHz (vs 21.47727 for a normal NTSC Famicom). The reason for the difference is probably so the horizontal scan rate is closer to the 15.625kHz PAL standard. Maybe some PAL TVs were not tolerant of variations???
The result is that (in full-speed mode) the Hong Kong Famicom has a frame rate of about 59.5 Hz.
The console main board ID is HVC-CPU-NPC-26-01
A label on the power/modulator shielding reads HVC-HKG-26
The NTSC-to-PAL chip has 20 pins and is marked N NPC26
Thanks to his findings we can finally reveal the truth about the HK Famicom!
So, the HK Famicom is a standard Japanese NTSC Famicom sporting a 2A03 CPU and a 2C02 PPU (and thus it's fully compatible with JP Famicom software - in essence it's a NTSC model) but it has been fitted with a Nintendo made NTSC to PAL transcoder circuit to convert the NTSC color signal into a PAL one for HK TVs.
Additional to the NTSC to PAL transcoding, the circuit is able to produce a 50 Hz video signal by "freezing" the PPU chip during operation and thus reduce its effective frame rate. So this is what the Slow/Normal
switch does. It's a 50/60 Hz switch, albeit a very complicated one.
In the Normal
position, it allows the PPU to function as usual, making the system output PAL60 video (this means 60 Hz video with PAL color encoding, which violates the PAL standard but can be displayed by some TVs). In the Slow
position, the PPU gets halted at defined intervals to produce 50 Hz video, and thus a "true" PAL video output.
On the other hand, a PAL NES uses a completely different CPU and PPU (2A07 and 2C07) in addition to a different crystal oscillator. It runs at a lower CPU speed, generates a perfect PAL signal right from its PPU and thus needs modified software. The HK Famicom is a NTSC Famicom which has been kludged by Nintendo themselves to produce pseudo-PAL video output. It's compatible with NTSC games and incompatible with PAL games.
Further down the thread, another mystery is revealed. I remember seeing a Famicom like this one here in FW at some point in time:
We were all puzzled by the combination of an official Famicom motherboard and a seemingly unlicensed RF modulator board. This RF modulator board contains a chip in the middle labeled MK5060 and there's a multitude of wires running from it into the Famicom motherboard. The PCB is labeled Makko Toys, Ltd. Nobody at the time had a clear answer for this.
It turns out that this unlicensed RF modulator board does the same thing as the HK Famicom's video transcoder. The MK5060 chip converts the NTSC video output of the original Famicom into PAL video and provides the same PPU freezing facilities to enable 50 Hz video output (this explains the multitude of wires running from the chip into the motherboard). In this case the TV/GAME switch would work as a 50/60 Hz switch using the same technique as the HK Famicom. The rest of it is a plain vanilla Power/RF Modulator board which is probably tuned into European/Asian channel frequencies.
What could have been the uses for this? I presume that this board was used by unlicensed outfits to import Famicom consoles into PAL territories, especially China, Hong Kong, Taiwan, etc. By fitting one of these boards into a NTSC Famicom, it effectively turns into a PAL one which is able to run Japanese games.
It opens up the question: Which method came first? Nintendo's or Makko Toys'? Did Nintendo create the HK Famicom as an answer to the proliferation of PAL-modded Famicom consoles in places like Hong Kong? Or was it the other way around? Both scenarios seem pretty plausible to me.