The Sega Master System's SN76489 is a very basic sound chip. The pitched voices, of which the SN76489 has three, are set in terms of timbre (ie. the duty cycle cannot be changed).
By adding the three voices together at different octaves with different volumes, it's quite possible to generate various timbres that are usually impossible to produce using single voices. This is a basic form of additive synthesis, albeit with square waves.
In this example, all three voices are eventually played, but at different octaves. The upper two voices fade in and out over time.
This exploration of inter-oscillator timbre generation is related to previous examples of underclocking and phasing the voices of the SN76489, such as this timbral droner.