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WHITE NOISE GENERATOR This white noise generator is very simple and cheap to build and produces white noise which does not repeat over any short time frame. It has a variety of uses, as explained below. A white noise signal has equal intensity at all frequencies in the band of interest; for example, 20Hz-20kHz for audio. It’s the hissing sound that you hear if you tune an analog FM radio to a frequency on which there is no transmission. There are many reasons why you may want a white noise signal. For example, you can use white noise to drown out external noises that may interfere with your sleep. If that dripping tap is keeping you awake, don’t count sheep; try a low-volume white noise source with a speaker close to your bed. We can attest: it works wonders! It’s especially effective at helping babies get to sleep since they are used to hearing somewhat similar sounds in the womb. It can also be used to help treat (or at least mask) tinnitus (a persistent ringing sound heard in the ears when there is no sound present). White noise sources can be used to measure the bandwidth or impulse response of a circuit and to check room acoustics or optimise a PA system. They are also used in analog audio synthesisers to help produce the “ssshhhh” sound of various percussion instruments such as hi-hats, snare drums and cymbals. Generating white noise There are several ways to generate white noise. For example, if you reverse-bias a zener diode or transistor emitter-base junction (ie, the baseemitter reversed) with a low DC current level, an AC voltage will appear across it and this will have a white noise characteristic. 32 Silicon Chip This IC is not available any more but with modern components, we can make an even better digital noise source. An even better white noise source Fig.1: the “circuit” could hardly be simpler because everything is done in software within the PIC12F617-I/P. Noise output is taken from pin 7, while a 100nF capacitor bypasses the supply (pins 1 and 8) – pins 2, 3, 5 and 6 are not connected. But the resulting AC voltage level is quite low and typically needs to be amplified by a factor of several hundred times to make it usable. Alternatively, white noise can be generated digitally with a pseudo-random number generator. This has the advantage that the signal level is already high, it is consistent and it is not dependent on a particular transistor or zener diode’s characteristics. National Semiconductor used to sell a digital noise source IC, the MM5837 (designed in the 1970s) that used an internal 17-bit pseudo-random sequence generator to produce white noise for audio applications. Supplied in an 8-pin DIL package, it was designed for musical instruments, synthesisers and for room acoustics testing. Its main disadvantage was a noticeable cyclic repetition. The repetition was due to the full random sequence being produced in less than one second and being continuously cycled. by John Clarke Australia’s electronics magazine Our design uses a low-cost 8-bit PIC12 microcontroller to produce a 31bit pseudo-random sequence, which only repeats after 231 or approximately two billion cycles. That works out to nearly eight hours so the repetition is definitely not discernible. The basic “circuit” for our white noise generator is shown in Fig.1. IC1 is a PIC12F617-I/P which has a 2-5V supply fed into pins 1 and 8. A 100nF bypass capacitor is connected directly between pins 1 and 8 to ensure that it has a stable operating voltage. The noise output appears at its GP0 digital output (pin 7). Pin 4 is the master clear/reset (MCLR) pin. This is held at VDD during normal operation by an internal pullup current. If it is externally pulled low, this will hold the microcontroller in a reset state and so the noise output at pin 7 will cease. When released, the internal pull-up will bring it high again, allowing the processor to run and resuming noise generation. You just need the two components, the programmed IC and a 100nF bypass capacitor, as shown in Fig.1. And it’s dead easy to wire up since only four pins are normally used – the bypass capacitor can be soldered right next to the IC (even right across pins 1 and 8 if you wish!). The other four pins (pins 2, 3, 5 and 6) are not used and should be left disconnected. siliconchip.com.au While we are The noise frequency showing this as distribution is therea mini “project” fore even up to about in its own right 76.923kHz, which is for those who the Nyquist limit for need a dead simthis signal; ie, half ple white noise the sampling rate. source, it will Because the outbe quite familiar put is a square wave, when you read Fig.2: here’s what happens inside the PIC – the 28th and 31st bits are XOR-ed it will have compoand fed back into the first bit while the other 31 are shifted to the right by one. the next project nents at higher frethis month, the quencies than this Steam Train Whistle project, because into Q1, so you can think of this as a but they will have a decreased amplithis is exactly what we used for the modified type of “bit rotate” operation. tude and power level. “steam” component. The measured spectrum from our The Q31 bit value also determines the level on pin 7 and thus becomes prototype is shown in Fig.3. It extends Pseudo-random the noise output; hence we do not want over the entire audio spectrum (20Hzsequence generation to retain its value in one of the other 20kHz) and well beyond at both the Fig.2 shows how the software genera- bits; if we did, this would quickly lead low-frequency and high-frequency tors a pseudo-random sequence genera- to repetition. ends (the measurement bandwidth is tor using three 8-bit shift registers and a This approach has two advantages. only 20kHz). 7-bit shift register. The bits within the One, the 31-bit length leads to a very While the IC generates white noise, four shift registers labelled Q1 to Q31. long time until the sequence repeats it could potentially be used to generate These bits are pre-loaded with a spe- and two, the simple XOR gate used to pink noise with an appropriate filter at cific value when the micro comes out provide the pseudo-random effect is its output. But such a filter is not simof reset, to provide a starting point for very easy to implement in software and ple to design; it is something that we the random sequence; as explained be- takes very little time to process, allow- will cover in a future article. low, this can be any state but all zeros. ing for a high clock rate and thus givPink noise has its own uses, such as Each time a clock pulse occurs, the ing the noise signal a wide bandwidth. for calibrating audio equipment, simuvalue of Q1 is moved into Q2, Q2 into The entire process to update the con- lating background noise and can also Q3 and so on, up to Q30 which is moved tents of the four registers, including help with sleeping. into Q31. This means that all the bit the XOR operation, takes 13 software Ensuring a long values are updated from their neigh- instructions. bour, except for Q1. The internal 8MHz oscillator of the repetition time It gets its value instead from the out- PIC12 gives a 2MHz instruction rate (it You may be wondering how we know put of a two-input exclusive-OR (XOR) takes four cycles to execute one instruc- that the sequence generated by this argate, with its inputs being the values of tion) and this results in a sampling rate rangement won’t repeat for 231 cycles. bits Q28 and Q31. That is guaranteed by using the corof 153.846kHz (2MHz÷13). Once this shifting is complete, the When divided into the cycle length, rect “taps” (in this case, bits 28 and 31) value of Q31 is effectively lost, al- this gives us the approximately eight- to be combined to generate the new though it does control the value loaded hour repeat rate mentioned earlier. value of Q1 for each cycle. The list of taps required for various length shift registers to ensure a maximum length repeat cycle is given on page 5 of the following application note: www.siliconchip.com.au/link/aakr Note that this document refers to the use of an XNOR (exclusive NOR) gate rather than XOR gate. The only difference is with the lock-up state. That is the initial state of the shift registers where the generator stops producing a varying output. The XNOR version has a lock-up state when all values in the shift registers are ones (high output), whereas the XOR has a lock-up state of all zeroes (all low). The lock-up situation is prevented from happening when using either XOR or XNOR gates by starting the Fig.3: the spectrum of the white noise generated by the PIC. The power level is noise sequence with a number other consistent across all frequencies up to about 20kHz. The drop in level discontinuity than all zeroes or all ones. at 20kHz is due to sound card and computer software limitations. SC siliconchip.com.au Australia’s electronics magazine September 2018  33