The ETI 660 uses the CDP 1864 for sound output. For details on the CDP 1864 please see the data sheets for this chip. The ETI 660 controls the sound features of the CDP 1864 via OUT 4 to set the frequency and the Q flag to turn the defined tone on or off.
The frequency of the tone is depending on the CDP 1802 CPU speed as well as the OUT 4 value and is calculated as follows:
CPU frequency / 32
Tone frequency =
OUT 4 value + 1