Page "MOS Technology SID" Paragraph 28

The noise generator is implemented as a 23-bit-length Fibonacci LFSR ( Feedback polynomial: x ^ 22 + x ^ 17 + 1 ).

When using noise waveform simultaneously with any other waveform, the pull-down via waveform selector tends to quickly reduce the XOR shift register to 0 for all bits that are connected to the output DAC.

As the zeroes shift in the register when the noise is clocked, and no 1-bits are produced to replace them, a situation can arise where the XOR shift register becomes fully zeroed.

Luckily, the situation can be remedied by using the waveform control test bit, which in that condition injects one 1-bit into the XOR shift register.

Some musicians are also known to use noise's combined waveforms and test bit to construct unusual sounds.