Digital Noise Generator

Radio 1981, 07-08

The noise can be generated by using pseudo-random sequences, such as m-sequences. The difference between this sequence and a truly random is that the sequence is periodic from some point onwards and the state of this sequence changes by clock pulses. Inside a period of the sequence there is no difference between pseudo-random and truly random sequences. The spectral correlation characteristics of the noise are more suitable for a sound synthesizer than the noise generator based on a p-n-junction of transistor or zener.

The m-sequence generator is a digital shift register with feedback circuit that connects the output of the last stage to the input of the first stage. The feedback circuit includes modulo 2 adders with the following algorithm: if the input signals are different, then the output signal is high (or logic 1), and if the input signals are the same then the output signal is low (or logic 0). By combining the adders in the feedback loop, it is possible to obtain sequences with different period and composition.

The maximum period of the sequence generated by the shift register is 2n-1, where n - number of bits of the register. The all-zeros state is discarded as inoperable. To obtain a spectrum uniformly distributed over the whole audio band, the clock frequency is selected to be 35 kHz.

circuit diagram of the digital noise generator

Figure 1.
D1 - K176LA7 (CD4011); D2 - K176IR10 (CD4006); D3 - K176LP2 (CD4030); A1 - K153UD1B (mA709CH or 741 without the capacitor C6);
C1 - 100pF, C2 - 1μF x 10V, C3 - 2.2nF, C4 - 1nF, C5 - 15nF, C6 - 33 pF.

The schematic diagram of the noise generator is shown in Fig. 1. The clock generator circuit includes 4 CMOS inverters of the CD4011. The three logic gates D3.1-D3.3 forms a modulo 2 adder. At each clock pulse, the signals from the fifth, ninth and eighteenth bits of the register D2 are summed and written to the first bit of the register. The network R2C2 is intended to set the initial state the shift register after power on. The low pass filter (this filter is known as a filter Sallen–Key) is based on the op-amp A1. The cutoff frequency can be switched between 15 kHz and 500 Hz to produce "white" or "pink" noise.

V. Grigoryan, V. Martynovsky

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