Filtered noise generator

SyntherJack

DIY sound synthesizers and me.

Make some pink noise!

All the designs were used only for comparison and not for any commercial purposes.

At the beginning…

Around a week ago I’ve got a strong internal need to synthesize a sea waves sound (we don’t have ocean in Poland), as I try to design an updated version of my Binaural Beat Generator. Usually this kind of synthesis starts with pink noise, later shaped with VCF or VCA modulated with sine (or triangle) LFO. The problem was, I had no pink noise generator. It seemed like a fun project, so I started to dig for some schematic. First of all I’ve chosen 3 of most popular that looked as they have a chance to work:

  1. MFOS “Pink Enough for Me” noise source
  2. Elliot Sound Products “Pink Noise Generator for Sound Testing” (with basic filter)
  3. Noise source with passive filter, published among others in ” The Noise Generator Cookbook” by Thomas Henry

The circuits were so simple, I’ve decided to build all of them to check the differences in generated sound. I think my more practical article will be a good read along with the one from experimentalistsanonymous , as it covers simulation and theoretical filters characteristics of pink noise generators).

Idea and prototyping

All popular analog pink noise generators share the same topology: white noise source, then -3 dB/oct lowpass filter. “White noise” part is an easy one, the problem lies in the tricky LP filter design. To get flat -3 dB/oct response filters are quite complex and ofter use uncommon part values (replaced by parallel common capacitor connection).

I’ve breadboarded (from top):

  • pink noise source with passive filter – 24 parts*,
  • Elliot Sound Products “Pink Noise Generator for Sound Testing” – 21 parts*,
  • MFOS “Pink Enough for Me” generator – 23 parts*.

(*) 1 opamp = 1 part, decoupling caps counted in

All schematics say “+/- 12 V supply”, but I used 6F22 battery pair (+/- 9 V) to power the breadboarded generators [I had problems with oscillations/ground loop (?) in MFOS design as I used lab PSU].

Three different pink nose generators on one breadboard

The output from each generator was later fed to a mixer to tune volume levels, then recorded via PC soundcard.

MFOS “Pink Enough For Me” noise generator

It is a very common circuit with white noise source (transistor based) and active lowpass filter. All resistor and capacitors are common and easy to get.

After I first prototyped this generator I thought I made an error in component placement or switched its values. There was almost no low end in the noise. I checked everything over 5 times, but after looking for other people experiences with this circuit – yes, thats how it works. I wouldn’t call the genererated noise pink :/ Hear by yourself! (wav, mono, 16 bit)

“Pink enough for me” noise generator schematic – Music From Outer Space design

Don’t forget to check the original MFOS design page. Maybe you will have a better luck

Noise source with passive filter

This filter design can be traced down to “The Audio Handbook” by National Semiconductors from the ’70, and with some small modifications can be found in “Noise Generator Cookbook” by Thomas Henry. It is told to have very flat +/- 1 dB response. Sounds really pink and natural. You can almost feel waterdrops on your face.

Pink noise generator (with passive filter)

I’ve added a common white noise source (with a lot if amplification) and an active output stage – the signal drop on passive filter is great and there was need to compensate it somehow. The hard to find 270 nF and 94 nF capacitors were replaced by parallel connection of more common ones.

Elliot Sound Products “Pink Noise Generator for Sound Testing” (with basic filter)

This is the simplest pink noise source you can find on Elliot Sound Products page (an amazing source for general audio circuits). The more complex one uses advanced pink noise filter, and the super expanded one includes an IEC 60268-1 defined band limiting filter. I had problems with getting 5,6 nF capacitor, so I replaced it by 1 nF and 4,7 nF parallel connection (5,7 nF, but assuming 10% capacitors and 5% resistors it shouldn’t make any difference).

This one is a pinkest of all, an instant waterfall. Turn off your mind, relax and float downstream.

Pink noise generator (with basic filter) circuit diagram – Elliot Sound Products design

The official project page will very detailed description and you can find it here.

Noise generators comparison

You can see a four spectrograms – outputs of four pink noise generators. From left:

  • MFOS “Pink enough for me”,
  • noise source with passive filter,
  • Elliot Sound Products “Pink Noise Generator for Sound Testing” (with basic filter),
  • reference – pink noise from Adobe Audition.

…or just listen to the recorded sample – 10 seconds of noise, then 1 for relax and so long.

Pink noise comparison spectrogram – from left: MFOS, “passive”, ESP and Adobe Audition generated

You can notice few things:

  • MFOS has problems in linearity of filters cutoff (not enough lows and augmented mids) and is furthest from the reference noise,
  • version with passive filter has low/mid frequencies (up to around 4 kHz) very similar to reference pink noise and is not as bad as it seems,
  • ESP noise generator is very close to reference with a slight difference over 10 kHz (reference noise is a bit brighter).

I also made a blind tests and some people said passive sounds a bit more natural (like ocean or waterfall), even if it is not perfect. ESP sounded too sharp to them and annoying, even if in theory more correct.

Conclusion

I would reccoment to listen to sample and choose between ESP designed or “passive” version. ESP version would be great for audio testing as it sounds more correct according to theory, but many will like “passive” for use in sound effects or noisemakers, as it sounds less harsh. Remember, the one from Elliot Sound Products can’t be used commercially without written authorisation.

And if you want know more, check the aricticles like:

  • “Pink Noise’ Is The Most Millennial Secret To Getting Those 8 Hours Of Sleep”
  • “Is Pink Noise The Secret To Sounder Newborn Sleep?”
  • “Why Pink Noise Is The New White Noise” (. ) .

Yeah, pink noise is THE THING.

Noise Generator Module

The White Noise Stage

The schematic at right, shows about the simplest white noise generator that can be devised. Q2 is used as a zener diode. It’s emitter base junction is reversed biased, which in the 2N3904 has a breakdown voltage near 6 volts. The zener action produces random noise, which is from a bandwidth point of view, considered to be “white noise”.

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Transistor Q1 amplifies this noise to a signal level in the neighbourhood of 2.5 volts peak to peak. This is a dead simple circuit to breadboard and the reader is encouraged to try it. Take the output and apply the signal to an amplifier. Turn the circuit’s power off and on to convince yourself that the noise is being generated by the circuit. If 15 volts is not convenient, the circuit will also operate at 12 volts.

While the schematic is drawn by LTspice here, it will not simulate the noise. This circuit must be tested in real hardware. The author’s breadboard circuit produced about 2.5 volts peak to peak noise. The transistors had a tested Beta of approx 408.

Spectral Response

Using DL4YHF’s Audio Spectrum Analyzer (“Spectrum Lab”) on a PC, I was able to get a crude test of the audio spectrum.

There are three plots shown in the graph.

From the red plot, it can be seen that the spectrum power starts at DC to be about -47 dB and then drops to about -55 dB at 20 kHz. A 6 dB drop over the audio spectrum.

Pink Noise

Unlike white noise, the pink noise spectrum drops at a rate 3 dB / octave with increasing frequency. Since RC filters operate at multiples of 6 dB / octave, building the required filter to generate pink noise from white noise, presents a bit of a challenge. So before I decided upon a circuit, I checked out some published circuits.

The MC4558 Pink Generator

For lack of a better title, I dubbed this one the “MC4558 Pink Generator”. This circuit is the “Pink Noise Generator for Audio Testing” by Rod Elliott. I have reproduced the white noise amplifying stage and the pink noise filter stage that follows in LTspice, shown at right.

While his circuit was designed to be used as an audio tester, I decided to bring out the white noise output to allow a comparison to the pink noise output. Also, his diagram shows potentiometer VR1 to allow adjusting of the pink noise output level. I’ve just shown this as R1, as if the pot was turned up for maximum output at “Pink”.

How does this circuit perform. Is it a good candidate?

Simulation

Assuming that the random noise is 30mV as documented on Rod’s schematic, the output white noise level is only about 0.324 volts (at 1kHz). Clearly for a synth white noise output, this needs changing.

The pink noise output level on the other hand is an amplitude of 1 volt (at 1kHz).

The spectral plots immediately show the differences in relative output levels. Beyond that, it would appear that the white noise output is somewhat deviant from the ideal at the low end. It seems to flatten out starting near 100 Hz.

The pink noise slope is a rather good, with a small amount of wavyness.

Pink Enough for Me Generator

The next one I looked at was called Ray’s Pink Enough for Me Generator. Right away, from the name, you know that this circuit represents a compromise. But how much of a compromise is it?

Simulation

Ray’s circuit is first of all based upon +/- 9 volt supplies. This circuit should easily accommodate +/- 15 volt supplies. However, I’ll simulate it as it was documented to make sure we don’t arrive at any false conclusions.

The simulation originally assumed about a 30mV noise level going it. However, plugging in a 1kHz 30mV signal, it became evident that this level causes distortion in the output of pink opamp U1. Backing it off to about 17 mV or less corrected this.

In the white noise circuit at the top of the page, the 2N3904 transistor produced a signal with an amplitude of about 2.5 volts peak to peak (or 1.125 volts amplitude). If you assume a Beta near 100 (Ic is about 2.5 mA when Vcc=12V), then the white noise amplitude is likely around 25 mV. So the gain of U1 may need to be reduced.

The spectral response is shown below. It is rather disappointing. The drop below 10Hz is ok and desirable.

The 12 dB increase over the white response between 100Hz and 2kHz however, is very undesirable in terms of a level difference. A further disappointment is the sought after filtering slope from about about 2kHz to 20kHz is steep.

Frequencies lower than 2kHz is flat until you drop to about 100Hz, and then you have a negative response slope as frequency heads for DC.

The response curve doesn’t look very pink to me. The conclusion of this test seems to be that this is not the most suitable circuit inspiration for pink noise filter.

TLC2272 Noise Generator

The next circuit to be evaluated is the TLC2272 Noise Generator.

The schematic shows this one powered by a single 5V supply, but to simplify the simulation I have used a split +/- 2.5 volt supply instead. The other thing about this simulation was that the original schematic shows only 5uV going into the pink noise filter. For some reason LTspice shows the 5uV sine wave as a triangle wave, going into the opamp. So I increased the test input signal level to 3mV and tested the output (no distortion). So this should represent a valid filter test.

Also, since the circuit didn’t bring out a white noise signal (it was only 5uV to start with), we will skip that comparison and simply look at the pink noise filtered output.

Simulation

The AC analysis shows the plot shown at right. The slope is a little wavy but otherwise seems reasonable, right down to 1Hz.

The high frequency response seems to start to flatten out at 4-5 kHz, and remains at that level in the upper frequencies. This is consistent with the circuit description “will give a 1/f noise slope from below one hertz to over four kilohertz”.

In the original circuit, the noise is generated by the 150 kohm resistor and amplified to 5nV. Here, we’re just interested in the filter response.

The pink noise filter seems to accomplish what it claimed (up to 4kHz). The drop between 100 to 200 Hz (one octave) shows approximately 4dB, which is close to the targeted 3dB/octave slope.

Yusynth Noise Module

The next circuit is the Yusynth Noise Generator. This design is interesting because the filtering is partly performed by a single transistor stage and then augmented further by an op amp stage. We’ll only look at the pink noise portion of this module.

Simulation

The yellow plot below shows the Pink Noise output response. The red curve below it shows the response coming out of Q1 and into the op amp. Looking at the difference between the two curves, it can be seen that the op amp not only boosts the output level, but straightens out the pink curve somewhat between 6kHz and above.

There is probably some room for improvement between 10Hz and 200Hz. The response there should drop off a little more.

Chosen White/Pink Filter Design

The schematic shown at right is an adapted pink filter circuit. Originally I had chosen the pink filter used in the TLC2272 circuit. But after further reflection I decided I didn’t like the chained capacitor links, preferring to use independent RC pairs instead.

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At the low end, the response tapers off toward DC to keep the noise source from having too much bass content. The rest of the curve approximates a 3dB/octave slope, except that at 20kHz there is a slight bend and flattening of the curve.

Potentiometers R11 and R12 allow the output levels to be adjusted. The white noise maximum output level should be approximately 3 volts peak to peak. The pink noise is similar, except that this is frequency dependent.

Construction

The photo at left shows the white noise generator circuit wired up and tested.

Measured Spectrum

The following sections measure the frequency response of the white and pink noise outputs.

The output potentiometers of the noise module were adjusted to maximum for these tests. The levels plotted should be taken as relative readings, since these signals are brought into the PC’s sound card through a mixing console.

The tests were done using a PCI Audiophile 2496 sound card, operating at a sampling rate of 96 kHz, using 24 bit samples. The plots were made with the software Spectrum Lab version V2.77b22.

Measured White Noise

The wild yellow plot shows the instantaneous noise levels.

The red smooth plot shows the long term average level (one minute average).

While not perfectly flat, there is a dip showing around 10kHz and higher. Some of this may be due to the authors test equipment and PC’s audio card used.

Measured Pink Noise

Again, the yellow wild plot shows the instantaneous noise levels (I was unable to suppress this plot).

The smooth red plot shows the one minute average of the levels.

Looking at the red plot, you can see that the level is about -25 dB at 100 Hz, dropping to about -43 dB at 10 kHz. This is an overall drop of about 18 dB, over nearly 6.5 octaves (starting at 100 Hz). This shows about 2.7 dB per octave drop.

Photos

The photo at left shows the noise generator module installed in the synth rack.

Документация

(Чтобы быть удаленным), Генерируют распределенный шум Rician

Rician Noise Generator будет удален в будущем релизе. Используйте блок MATLAB Function и randn функцию вместо этого.

Библиотека

Шумовая подбиблиотека Generators Источников Коммуникации

Описание

Блок Rician Noise Generator генерирует распределенный шум Rician. Функцией плотности вероятности Rician дают

σ стандартное отклонение Распределения Гаусса, которое лежит в основе шума распределения Rician

m2 = mI2+mQ2 , где ми и mQ являются средними значениями двух независимых Гауссовых компонентов

I0 является модифицированным 0th-порядком Функция Бесселя первого вида, данного

I 0 ( y ) = 1 2 π ∫ − π π e y потому что t d t

Обратите внимание на то, что m и σ не являются средним значением и стандартным отклонением для шума Rician.

Необходимо задать Initial seed для генератора случайных чисел. Когда это – константа, получившийся шум повторяем. Длина вектора Начального параметра seed должна равняться количеству столбцов в основанном на системе координат выходе или числа элементов в основанном на выборке выходе. Набор числовых параметров выше параметра Initial seed в диалоговом окне может состоять из векторов, имеющих ту же длину как Initial seed или скаляры.

Начальный Seed

Скалярный параметр Initial seed инициализирует генератор случайных чисел что использование блока, чтобы сгенерировать его Rician-распределенный комплексный вероятностный процесс. Когда несколько блоков в модели имеют параметр Initial seed, можно выбрать различные начальные seed для каждого блока, чтобы гарантировать, что различные случайные потоки используются в каждом блоке. Установите Initial seed на целочисленное значение для повторяемых результатов или используйте randi функция, чтобы рандомизировать ваши результаты.

Атрибуты выходного сигнала

Выходной сигнал может быть основанной на системе координат матрицей, основанной на выборке строкой или вектор-столбцом или основанным на выборке одномерным массивом. Этими атрибутами управляют Frame-based outputs, Samples per frame и параметры Interpret vector parameters as 1-D. Смотрите Источники, и Впитывает Руководство пользователя Communications Toolbox™ для получения дополнительной информации.

Число элементов в Initial seed и параметрах Sigma становится количеством столбцов в основанном на системе координат выходе или числа элементов в основанном на выборке векторном выходе. Кроме того, форма (строка или столбец) Initial seed и параметров Sigma становится формой основанного на выборке двумерного выходного сигнала.

Параметры

Любой K-factor или Quadrature components .

K = m2 / (2σ2) , где m как в функции плотности вероятности Rician. Это поле появляется, только если Specification method является K-factor .

In-phase component (mean), Quadrature component (mean)

Ми средних значений и mQ , соответственно, Гауссовых компонентов. Эти поля появляются, только если Specification method является Quadrature components .

Переменная σ в функции плотности вероятности Rician.

Начальное значение seed для генератора случайных чисел.

Период каждого основанного на выборке вектора или каждой строки основанной на системе координат матрицы.

Определяет, основан ли выход на системе координат или основан на выборке. Это поле активно, только если Interpret vector parameters as 1-D неконтролируем.

Samples per frame

Количество выборок в каждом столбце основанного на системе координат выходного сигнала. Это поле активно, только если Frame-based outputs проверяется.

Interpret vector parameters as 1-D

Если этот флажок устанавливается, то выход является одномерным сигналом. В противном случае выход является двумерным сигналом. Это поле активно, только если Frame-based outputs неконтролируем.

Output data type

Выход может быть установлен в double или single типы данных.

Ссылки

[1] Proakis, Джон Г., Цифровая связь, Третий выпуск, Нью-Йорк, Макгроу Хилл, 1995.

Nissan Primera 2.0 L 6-мкп 黒夢 tekna フル♠ › Бортжурнал › Маленькая победа над электро наводками и шумами.

После установки всех дополнений по музыке, звуку и видео, оставалась одна из не решённых проблем.

Это фон и шумовые наводки в динамиках при высокой громкости и при заведённом автомобиле. Победить эти шумы не удавалось ни с помощью альтернативных блоков питания, ни правильно экранированных проводов, ни прокладкой кабелей по феншую, ни фильтрами питания, ни стабилизаторами.Об этом я описывал здесь:www.drive2.ru/cars/nissan…lko/journal/1225515/#post

Тема будет интересна и полезна всем кто обладает устройством “ЯТУР” или “ИКСКАРЛИНК”, “АУДИОЛИНК”.

Дело в том, что у этих устройств, помимо USB входа, есть ещё так называемый AUX вход через который можно подключать любые посторонние источники звука к штатному СД. Будь то айфон, айпод, мп3 плеер, телефон и тд .И можно слушать музыку, как через впаянный аудио вход в штатной магнитоле.Качество на уровне, но есть одно “НО”.
Стоит подключить любое из этих устройств в прикуриватель авто или просто к бортовой сети авто, как сразу появляется фон и помехи, которые слышны при высоком уровне громкости в динамиках и особенно при включённом зажигании и заведённом автомобиле.При езде с каждым нажатием педали газа отчётливо прослушивается свист от генератора в колонках и это никак не может не раздражать. То же самое бывает при установке доп усилителя и нештатной акустики.

При установке усилителя часто появляются помехи из-за паразитных контуров. Несколько точек заземления на корпус с разным сопротивлением вызывают блуждающие токи, которые служат причиной появления шума. Для решения этих проблем служит линейный шумоподавитель NF 100.

Да если громкость не более половины, то всего этого почти не слышно, но меня это не устроило. А стоит вынуть питание любого постороннего девайса (источника звука) из прикуривателя и он начинает работать на встроенном аккумуляторе, а в салоне в акустике наступает полная тишина без шумов и помех, абсолютно чистый звук.
Боролся я с ним всеми вышеописанными методами и уже почти отчаялся, интернет и поиски помощников на драйве к сожалению тоже не дали результата. И вот наткнулся на просторах на :
АКУСТИЧЕСКИЙ ЛИНЕЙНЫЙ ФИЛЬТР

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Не ожидал я, что именно он решит проблему.Он подключается посредством тюльпанов в разрыв акустических проводов от источника к усилителю и не требует отдельного подключения к питанию.

Есть и масса аналогов таких например как: SUPRA, MISTERY и тд.

Как только я подключил звук из своего медиа плеера через этот фильтр — всё стало на свои места и звуковой мусор пропал даже на максимальной громкости, а просмотр видео в движении стал комфортным и чистым без звуков писка генератора в динамиках.

Если захотите что- либо подключить через ваш аудиолинк и подключить к бортовой сети авто, то вы непременно вспомните мои вопросы и ответы и сможете легко решить данную “не решаемую” проблему. ( я был и в радио мастерских, и на автозвуке и во всех магазинах радиоэлектроники, но все просто разводили руками — не знаем не сталкивались.А решение есть, оно простое и его стоимость не привышает 5-10 у.е

На очереди решение вопроса о сохранении изображения при заводке автомобиля(что бы видео продолжало работать без перезагрузки).
Спасибо за внимание, надеюсь будет полезным для вас.

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How to filter noise with a low pass filter — Python

Recently while I was working on processing a very high frequency signal of 12.5 Khz , i.e. 12500 samples per second or a sample every 80 microsecond. What was more interesting is that I had to derive various data points into this data set.

This is how my data in a single cycle looked like

You can see the noise when I zoom in the data

So now consider, if had to determine the point where the curve starts it rise.

With so much of noise there is a very high probability of getting false positive data point. Also imagine the performance of the algorithm with so much fluctuation in the data.

I would start with some signal processing basics , which are essential to understand before we jump into code.

Basics : Band Pass Filters

The four common filters.

  1. Low-pass filter, passes signals with a frequency lower than a certain cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency.
  2. High-pass filter, passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency.
  3. A band-pass filter can be formed by cascading a high-pass filter and a low-pass filter.
  4. A band-reject filter is a parallel combination of low-pass and high-pass filters.

Now lets see a sample data ,which would be ideal to work with

As you can see the distortion caused by a lot of noise has deformed actual data which is a sin wave data.

  • Sample Period — 5 sec (t)
  • Sampling Freq — 30 samples / s , i.e 30 Hz (fs)
  • Total Samples — (fs x t) = 150
  • Signal Freq = 6 signal / 5 sec = 1.2 Hz

This means we need a filter that would pass the signal with at most frequency of 1.2 Hz , However in real life the signal frequency may fluctuate , hence it would be good if we choose a slightly higher number than the ideally calculated frequency.

You can also try using FFT (Fast Fourier Transform) to find investigate the frequencies and amplitudes of the Signal vs the noise components, more details along with code can be found here

Butterworth Filter

The frequency response of the Butterworth filter is maximally flat (i.e. has no ripples) in the passband and rolls off towards zero in the stopband, hence its one of the most popular low pass filter.

Nyquist Frequency

The term Nyquist is often used to describe the Nyquist sampling rate or the Nyquist frequency.

The Nyquist rate or frequency is the minimum rate at which a finite bandw >dt, then the Nyquist rate is just 1/(2 dt ).

Lets jump into Code

Step 1 : Define the filter requirements

  • Sample Period — 5 sec (t)
  • Sampling Freq — 30 samples / s , i.e 30 Hz (fs)
  • Total Samples — (fs x t) = 150
  • Signal Freq = 6 signal / 5 sec = 1.2 Hz
  • Nyquist Frequency = 0.5 * fs
  • order = Polynomial order of the signal

Step 2 : Create some sample data with noise

Step 3 : Filter implementation using scipy

Step 4 : Filter and plot the data

It’s surprising how smoothly the filtered signal aligns to the data, feels like ‘butter’.

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