SID chip "ring modulation"

One of the built-in features of the SID chip goes by the name "ring modulation". It is a feature that can be activated individually for each of the three voices but which only has an effect while the voice uses the "triangle" waveform generator. According to the sales brochure the feature allows to create "non-harmonic" effects.

What the feature actually does gets somewhat technical and since the whole effect is based on the "triangle" waveform I'll quicky sketch how that waveform normally works: As all the SID's waveforms do, the "triangle" waveform is generated based on the current value of a voice specific 24-bit counter register called "the accumulator". The respective register is incremented with each system clock cycle and the size of the increment depends on the pitch of the currently played note (the higher the pitch the bigger the increment). When the register value is at 0 then this marks the starting low point of the triangle's rising edge and with bigger register values the respective triangle edge rises linearly. The triangle reaches its highest point when the register value reaches the mid-point of its 24-bit range. From a technical perspective that point can be easily detected by looking at the MSB (most significant bit) of the accumulator: Once that bit is set this means that the triangle is in its descending edge. From that point on the waveform generator will not use the original register content to create the descending triangle edge but use an inverted version of the register instead (i.e. the rising register values are transformed into descending triangle values via a simple XOR operation).

The SID's "ring modulation" feature messes with the above mechanics by replacing the used MSB with something else: Rather than directly use the MSB from the voice's accumulator, that bit is first XORed with the respective bit of the "previous voice" (i.e. voice 3 will use the bit from voice 2, voice 2 the one from voice 1, and voice 1 the one from voice 3). This causes the triangle waveform to be distorted (i.e. inverted) by whatever is going on in the "previous voice". It doesn't matter that waveform that "previous voice" is using but the pitch of the "previous voice" makes all the difference.

However silly this design may sound, people have actually managed put it to good use and below are some examples.

SID chip voices (a red border signifies that this voice uses "ring modulation")

Here a selection of songs that use "ring modulation" (click a song name to play it on this page):

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