User:Persune

Hello! I usually write music, or work on hardware/software projects for the Famicom. From time to time, I like to investigate certain aspects of the Famicom, be it audio, video, or otherwise.

Differential Phase Distortion

The output is subject to a differential phase distortion effect. This causes a rotation of the NTSC signal's effective hue, proportional to the voltage, causing more shift for brighter colors. Current estimates approximate about 2.5° (2C02E) or 5° (2C02G) of additional rotation for each row of the palette.

The reason for this distortion is that the output impedance of the PPU is dependent on the signal level. When combined with the board's capacitance, it slows level transitions, causes the edges at high signal levels to be less steep. The high frequency chroma signal is sensitive to this, and the delay to its phase causes the hue rotation.

A PAL NES is affected by the same differential phase distortion, but because of the alternating-line mechanism of PAL, the effect is mostly cancelled out on consecutive scanlines.

Simulating differential phase distortion

We can approximate this analog effect with a simple IIR RC low-pass filter whose time constant changes depending on the input voltage.

This is because the voltage levels more or less correspond to the resistance in the DAC of the PPU, and therefore the resulting impedance.

Assuming a raw square signal input, we can approximate the distorted output with the Python code below:

# Sampling rate, usually 2x master clock speed
PPU_Fs = 236.25e6/11 * 2
dt = 1/PPU_Fs

# 4 cycles of color $18
signal = [
    0.840, 0.840, 0.840, 0.840, 0.312, 0.312,
    0.312, 0.312, 0.312, 0.312, 0.840, 0.840,
    0.840, 0.840, 0.840, 0.840, 0.312, 0.312,
    0.312, 0.312, 0.312, 0.312, 0.840, 0.840,
    0.840, 0.840, 0.840, 0.840, 0.312, 0.312,
    0.312, 0.312, 0.312, 0.312, 0.840, 0.840,
    0.840, 0.840, 0.840, 0.840, 0.312, 0.312,
    0.312, 0.312, 0.312, 0.312, 0.840, 0.840,
]

# voltage level of $30.
composite_white = 1.100

# Output waveform after differential phase distortion.
# Must be same size as signal.
lpfilter = [0.0 for _ in signal]

# Phase distortion adjustment. Must not be zero!
amount = 3.0

# Initialize the IIR filter with the first sample.
v_prev = signal[0]

for i in range(len(signal)):
    voltage_ratio = signal[i] / composite_white

    # We adjust in 1e-8 increments for more controlled adjustments of the RC constant.
    alpha = dt / (voltage_ratio * amount * 1e-8 + dt)
    v_prev = alpha * signal[i] + (1-alpha) * v_prev
    lpfilter[i] = v_prev

This code approximates the impedance changes by using the raw signal's voltage ratio. The phase shift is more negative on higher levels. One can adjust this constant through the variable amount to control the amount of phase distortion. This variable adjusts the constant in 1e-8 increments for more fine control. The voltage ratio, multiplied by the RC lowpass factor amount*1e-8 = 3e-8 results in approximately a 14 degree delta between $0x and $3x colors. This more or less tries to match a 2C02G's estimate of differential phase distortion.

Famicom EPSM access

Due to the lack of EXP pins on the Famicom's cartridge pinout, an equivalent version does not have direct access to any of the methods above.

For the universal $4016 access, one may directly connect OUT1 from pin 11 of the Famicom's expansion port to the EPSM, or replicate OUT1 by latching $4016.D1 writes with external circuitry.

For the memory mapped access, the default circuit is the $401C-F addressing.

If another mode of mapper access is desired, a 2x15 pin header may be provided with the bare signals required for use with an external daughterboard.

   EPSM    | Daughterboard  |    EPSM
           +----------------\
    GND -- |01            30| -- +5V
CPU A11 -> |02            29| <- M2
CPU A10 -> |03            28| <- CPU A12
 CPU A9 -> |04            27| <- CPU A13
 CPU A8 -> |05            26| <- CPU A14
 CPU A7 -> |06            25| -- NC 
 CPU A6 -> |07            24| -> EPSM A1
 CPU A5 -> |08            23| -> EPSM A0
 CPU A4 -> |09            22| -> EPSM CE3
 CPU A3 -> |10            21| -> EPSM /CE2
 CPU A2 -> |11            20| -> EPSM /CE1
 CPU A1 -> |12            19| -- NC
 CPU A0 -> |13            18| -- NC
CPU R/W -> |14            17| <- /ROMSEL
     NC -- |15            16| -- NC
           +----------------/

For advanced data bus latching and controller access, an auxiliary pin header is available to be used, in conjunction with the one above

    EPSM     | Daughterboard  |     EPSM
             +----------------\
   CPU D7 -> |01            24| -> YMF288 D7
   CPU D6 -> |02            23| -> YMF288 D6
   CPU D5 -> |03            22| -> YMF288 D5
   CPU D4 -> |04            21| -> YMF288 D4
   CPU D3 -> |05            20| -> YMF288 D3
   CPU D2 -> |06            19| -> YMF288 D2
   CPU D1 -> |07            18| -> YMF288 D1
   CPU D0 -> |08            17| -> YMF288 D0
  EPSM A1 -> |09            16| -> YMF288 /CS
  EPSM A0 -> |10            15| -> YMF288 A1
 EPSM CE3 -> |11            14| -> YMF288 A0
EPSM /CE2 -> |12            13| <- EPSM /CE1
             +----------------/

Famicom / NES cartridge measurements

Label measurements

Note that these measure the dimensions of the label stickers themselves, not the allocated area on the plastic shell.

Front label measurements

Famicom 1st party short shells (SMB3 JP HVC-UM) - 90mm x 45.5mm

Famicom 1st party tall shells (Nobunaga no Yabou: Sengoku Gunyuuden KOE-NU) - 95mm x 53.5mm

NES shell (Romance of the Three Kingdoms II NES-XL-USA) - 50mm x 96.5mm

Back label measurements

Famicom 1st party short shells (SMB3 JP HVC-UM) - 99.5mm x 28.5mm

Famicom 1st party tall shells (Nobunaga no Yabou: Sengoku Gunyuuden KOE-NU) - 99.5mm x 28.5mm

NES shell (Romance of the Three Kingdoms II NES-XL-USA) - 78.5mm x 31mm

TODO

TODO:
  - learn wiki markup
  - write and publish personal research here:

      - Famicom cartridge PCB measurements, dimensions and footprints
        https://www.nesdev.org/wiki/Famicom_cartridge_dimensions
          Completed so far:
            - FC HVC-TGROM-01
            - FC HVC-CNROM-256K-01
            - TODO: publish HVC-ETROM-01 measurements

      - NES cartridge PCB measurements, dimensions and footprints
        https://www.nesdev.org/wiki/NES_cartridge_dimensions
          Completed so far:
            - NES-EWROM-01
            - TODO: measure unlicensed cartridge PCBs

      - NES/Famicom equivalent opamp amplifier gain
            - TODO: RF Famicom HVC-CPU-07, RF Famicom HVC-GPN rev, AV Famicom HVCN-CPU rev, Sharp Twin Famicom rev