In order to have better sound effects to add in the future to Galaxians clone or other games, I made some additions.
In every channel I added the option to add a second frequency related to the playing one. This frequency can be up to an octave higher. It is controlled with a 4 bit parameter n.
An other change is that the noise generator is clocked by the frequency of a channel when noise is enabled in that channel and its output is less harsh.