Programmable Sound Generator (PSG)
(This information from a Genesis manual, but it's a standard
chip, so the info is probably public knowledge)

The PSG contains four sound channels, consisting of three tone
generators and a noise generator. Each of the four channels has
an independent volume control (attenuator). The PSG is
controlled through output port $7F.

TONE GENERATOR FREQUENCY

The frequency (pitch) of a tone generator is set by a 10-bit
value. This value is counted down until it reaches zero, at
which time the tone output toggles and the 10-bit value is
reloaded into the counter. Thus, higher 10-bit numbers produce
lower frequencies.

To load a new frequency value into one of the tone generators,
you write a pair of bytes to I/O location $7F according to the
following format:


First byte:     1   R2  R1  R0  d3  d2  d1  d0
Second byte:    0   0   d9  d8  d7  d6  d5  d4


The R2:R1:R0 field selects the tone channel as follows:


R2 R1 R0    Tone Chan.
----------------------
0  0  0     #1
0  1  0     #2
1  0  0     #3


10-bit data is (msb)    d9 d8 d7 d6 d5 d4 d3 d2 d1 d0   (lsb)


NOISE GENERATOR CONTROL

The noise generator uses three control bits to select the
"character" of the noise sound. A bit called "FB" (Feedback)
produces periodic noise of "white" noise:


FB  Noise type

0   Periodic (like low-frequency tone)
1   White (hiss)


The frequency of the noise is selected by two bits NF1:NF0
according to the following table:


NF1 NF0 Noise Generator Clock Source

0   0   Clock/2 [Higher pitch, "less coarse"]
0   1   Clock/4
1   0   Clock/8 [Lower pitch, "more coarse"]
1   1   Tone Generator #3


NOTE:   "Clock" is fixed in frequency. It is a crystal controlled
oscillator signal connected to the PSG.

When NF1:NF0 is 11, Tone Generator #3 supplies the noise clock
source. This allows the noise to be "swept" in frequency. This
effect might be used for a jet engine runup, for example.

To load these noise generator control bits, write the following
byte to I/O port $7F:

Out($7F): 1  1  1  0  0  FB  NF1  NF0


ATTENUATORS

Four attenuators adjust the volume of the three tone generators
and the noise channel. Four bits A3:A2:A1:A0 control the
attenuation as follows:


A3 A2 A1 A0     Attenuation
0  0  0  0       0 db   (maximum volume)
0  0  0  1       2 db   NOTE: a higher attenuation results
.  .  .  .       . .          in a quieter sound.
1  1  1  0      28 db
1  1  1  1      -Off-

The attenuators are set for the four channels by writing the
following bytes to I/O location $7F:


Tone generator #1:    1  0  0  1  A3  A2  A1  A0
Tone generator #2:    1  0  1  1  A3  A2  A1  A0
Tone generator #3:    1  1  0  1  A3  A2  A1  A0
Noise generator:      1  1  1  1  A3  A2  A1  A0


EXAMPLE

When the MK3 is powered on, the following code is executed:

        LD      HL,CLRTB        ;clear table
        LD      C,PSG_PRT       ;PSG port is $7F
        LD      B,4             ;load 4 bytes
        OTIR
        (etc.)
    
CLRTB   defb    $9F, $BF, $DF, $FF

This code turns the four sound channels off. It's a good idea to
also execute this code when the PAUSE button is pressed, so that
the sound does not stay on continuously for the pause interval.