APU Mixer Emulation

The NES APU Mixer can be efficiently emulated using a lookup table or a less-accurate linear approximation.

Lookup Table

The APU mixer formulas can be efficiently implemented using two lookup tables: a 31-entry table for the two pulse channels and a 203-entry table for the remaining channels (due to the approximation of tnd_out, the numerators are adjusted slightly to preserve the normalized output range).

    output = pulse_out + tnd_out

    pulse_table [n] = 95.52 / (8128.0 / n + 100)

    pulse_out = pulse_table [pulse1 + pulse2]

The tnd_out table is approximated (within 4%) by using a base unit close to the DMC's DAC.

    tnd_table [n] = 163.67 / (24329.0 / n + 100)

    tnd_out = tnd_table [3 * triangle + 2 * noise + dmc]

Linear Approximation

A linear approximation can also be used, which results in slightly louder DMC samples, but otherwise fairly accurate operation since the wave channels use a small portion of the transfer curve. The overall volume will be reduced due to the headroom required by the DMC approximation.

    output = pulse_out + tnd_out
    
    pulse_out = 0.00752 * (pulse1 + pulse2)
    
    tnd_out = 0.00851 * triangle + 0.00494 * noise + 0.00335 * dmc