This is an extension of a previous post. That is to say, it is involves modulation the clock line of an SN76489. Once again, the overall concept presented in this post are mostly someone else's (Mr. Christian Haines ), but I have taken care of the details and the practical realisation.
The setup I have been working with currently consists of two Arduino chips, a MIDI interface, a MCP42100 digital potentiometer IC, a 4093 quad, 2-input NAND gate and some resistors and capacitors. A simplification of the setup can be seen below.
The advantage of this setup in contrast with the earlier setup is that the modulation process is much more controllable in that the frequency of the modulation can be sequenced from a MIDI-based sequencer.
Additionally, the second modulation oscillator adds another dimension of control and variety over the output of the sound.
Here are some audio examples:
 Something with melody;
the modulation sequencing is subtle and is used as an effect
 Something with chords;
the modulation sequencing is more pronounced and is used as a more 'directly musical' elements.
 Haines, Christian. Conversation (in person). 14 May 2008.