4-bit sample is pseudo randomly generated using the LSB of 8-bit LSFR (1,2,3,7) at 256Hz. The output is LPFed at nyquist frequency to remove the harmonics from the DAC output.
In this configuration there are 256 sample sequence which is spread into a second, which has a 128Hz worth of noise bandwidth for 1 second window.
You can increase the sample rate but it will narrow the noise bandwidth, so you also need to increase the number of generated random sample sequence.
