Filter module to combat acoustic feedback. Scheme, description

Free technical library

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Free library / Schemes of radio-electronic and electrical devices

Filter module to combat acoustic feedback. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Speakers

Comments on the article

In the article, the author proposes a simple block of notch filters designed to work as part of a modular mixing console ("Radio", 2003, No. 2, 3) of a sound amplification system. This block allows you to suppress the spurious acoustic coupling between the microphones and the speaker system without any noticeable deterioration in quality. As already mentioned in the first article, a changeable set of modules significantly expands the scope of this compact console for sound recording and sound reinforcement.

The biggest problem in amplification is usually the occurrence of acoustic feedback (“feedback”), and a large number of publications are devoted to combating it. To suppress this effect, there are many devices that have their own advantages and disadvantages. But it would be strange to recommend installing expensive digital filters or a frequency spectrum shifter into a simple amateur console [1]. Therefore, we will focus on simpler options.

It is believed that most often the acoustic "tie" occurs in the frequency range of 125 Hz ... 4 kHz, primarily at frequencies of increased sensitivity of microphones and acoustic systems with their unfortunate location or the presence of sound reflections in the room. It is clear that in order to disrupt the generation in the sound amplification system, it is necessary either to reduce the gain or change the phase of the signal. With a decrease in the overall gain, the meaning of the sound amplification itself is lost. It is possible to apply a simple squelch and reduce the amplification of only low-level signals, but this method is useful if you need to amplify the voice of only one person. But sounds whose sources are farther from the microphone will be suppressed; this is not always acceptable. This means that it is necessary to reduce the signal gain only at the frequency at which the "tie" arose. This can be done with tunable notch filters. In order not to introduce noticeable frequency distortions into the signal, notch filters should be narrow-band.

It is believed that "cutting" a frequency band from the signal spectrum with a width of less than 0,1 octaves is completely imperceptible even when listening to symphonic music, and in speech it is quite acceptable to increase this band to 0,2 ... 0,3 octaves. In practice, one-third-octave graphic equalizers successfully cope with this task, but they are very cumbersome. It is much more convenient to use parametric equalizers or narrow-band tunable notch filters. Usually narrow-band filters have a very large attenuation at the tuning frequency, but from their amplitude-frequency characteristics (AFC) it follows (curve 1 in Fig. 1) that a decrease in gain by 4 ... 5 dB occurs with a fairly wide band, and this is already detrimental to sound quality.

Filter module to combat acoustic feedback

There is no need to introduce a large attenuation to eliminate the "string". Therefore, during operation, it would be possible not to use part of the frequency response of the notch filter: if 20 ... 30 dB is "cut off" in the upper part of the filter's frequency response, then the rejection band will become very narrow and the attenuation introduced by the filter will remain quite sufficient. The easiest way to solve this problem is to use a tunable narrow-band amplifier instead of a filter and mix (summ) the signal from the filter in antiphase with the main signal in the required proportion. This "proportion" will ensure the formation of the desired part of the frequency response. Curve 2 in fig. 1 shows the frequency response of a narrowband amplifier tuned to a frequency of 100 Hz, and curve 3 corresponds to the filter response taken at the output of the adder

The module uses two such filters (Fig. 2), assembled on DA2 and DA3 microcircuits.

(click to enlarge)

The filter circuits are exactly the same and differ only in the values of the capacitors C15, C17 and C20, C21. The first of them can be tuned in the frequency band 50...750 Hz, and the second - in the band 0,5...7 kHz. If desired, you can change the quality factor and gain of the bandpass amplifier [2]. These parameters determine the following ratios of the resistances of the resistors in the filters:

Q=R17/R18;

K = R17/R16.

It must be borne in mind that with a very narrow band of cut frequencies, it is difficult to tune in to the “tie” frequency and it is very easy to knock down the tuning. With a wide filter band, the frequency distortion introduced by it is noticeable by ear. Trimmer resistors R15 and R27 (SDR-19a) allow you to select the used part of the frequency response by setting the proportion of the summation of the signals. It should be remembered that when the filtering frequency is tuned, the gain of the bandpass amplifier changes somewhat. Each of the filters is turned on, if necessary, by toggle switches SA1, SA2. Sometimes it becomes necessary to enable both filters.

The module additionally provides a tunable phase filter with op-amp DA1.3. Its circuit is much simpler, but by changing the phase of the signal, it is highly likely that generation will occur at a different frequency. In practice, to eliminate the high-frequency "tie", it is better to choose a capacitor C6 with a capacity of 2200 pF, and for a low-frequency one, increase it to 0,01 μF. In addition, the op-amp DA1.4, included as a phase inverter, allows using the SA3 toggle switch to change the phase in the amplification path immediately by 180 °. The remaining op-amps (DA1.1, DA1.2) operate respectively in the input and output stages. The output stage op amp simultaneously performs the function of an adder.

When you turn on the filter unit in the console after the input line, you need to apply a signal directly to pin 13 of DA1. Instead of elements C1, R1, jumpers are installed, since a similar chain is already at the output of the input line.

It can be seen from the diagram that the toggle switches allow you to turn on and off any link at will. Phase filters are turned off by the SA4 toggle switch (in the "OFF" position, the "bypass" mode), and the SA3 toggle switch switches the signal either to invert the signal phase, or to smoothly adjust the phase change.

The design of the filter module is similar to the design of the amateur mixing console modules [1].

Filter module to combat acoustic feedback

The printed circuit board drawing is shown in fig. 3, and the layout of the front panel of the module is shown in fig. 4.

Filter module to combat acoustic feedback

The design uses TL084 chips (TL074, K1401UD4). All filter capacitors must be film capacitors, for example, K73-17

In practice, such a block almost always makes it possible to stably eliminate acoustic feedback. Filter adjustment must be done during the system performance check before the event and after that, do not move the speakers and microphones. By the way, in equipment with much more complex digital filters, if you change the relative position of the microphone and speakers, the filters are automatically reconfigured again.

Literature

Kuznetsov E. Amateur modular mixing console. - Radio, 2003, No. 2, p. 12-15; No. 3, p. 10-12.
Horowitz P., Hill W. The art of circuitry. - M.: Mir, 1993.

Author: E.Kuznetsov, Moscow

See other articles Section Speakers.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Artificial leather for touch emulation 15.04.2024

In a modern technology world where distance is becoming increasingly commonplace, maintaining connection and a sense of closeness is important. Recent developments in artificial skin by German scientists from Saarland University represent a new era in virtual interactions. German researchers from Saarland University have developed ultra-thin films that can transmit the sensation of touch over a distance. This cutting-edge technology provides new opportunities for virtual communication, especially for those who find themselves far from their loved ones. The ultra-thin films developed by the researchers, just 50 micrometers thick, can be integrated into textiles and worn like a second skin. These films act as sensors that recognize tactile signals from mom or dad, and as actuators that transmit these movements to the baby. Parents' touch to the fabric activates sensors that react to pressure and deform the ultra-thin film. This ... >>

Petgugu Global cat litter 15.04.2024

Taking care of pets can often be a challenge, especially when it comes to keeping your home clean. A new interesting solution from the Petgugu Global startup has been presented, which will make life easier for cat owners and help them keep their home perfectly clean and tidy. Startup Petgugu Global has unveiled a unique cat toilet that can automatically flush feces, keeping your home clean and fresh. This innovative device is equipped with various smart sensors that monitor your pet's toilet activity and activate to automatically clean after use. The device connects to the sewer system and ensures efficient waste removal without the need for intervention from the owner. Additionally, the toilet has a large flushable storage capacity, making it ideal for multi-cat households. The Petgugu cat litter bowl is designed for use with water-soluble litters and offers a range of additional ... >>

The attractiveness of caring men 14.04.2024

The stereotype that women prefer "bad boys" has long been widespread. However, recent research conducted by British scientists from Monash University offers a new perspective on this issue. They looked at how women responded to men's emotional responsibility and willingness to help others. The study's findings could change our understanding of what makes men attractive to women. A study conducted by scientists from Monash University leads to new findings about men's attractiveness to women. In the experiment, women were shown photographs of men with brief stories about their behavior in various situations, including their reaction to an encounter with a homeless person. Some of the men ignored the homeless man, while others helped him, such as buying him food. A study found that men who showed empathy and kindness were more attractive to women compared to men who showed empathy and kindness. ... >>

Random news from the Archive

Lecture Schedule - on E Ink panels 18.05.2020

The Taiwanese company E Ink continues to offer the use of "digital" paper as information boards with low consumption and excellent readability in bright light environments. The idea of E Ink was met with understanding at the University of Tartu (Estonia), placing information "paper-like" panels near the lecture halls of the new center for scheduling lectures.

Monochrome E Ink information panels with 13,3-inch displays were developed by Artec Design from Tallinn. Power and data are transmitted to the panel via a twisted-pair Ethernet cable (Power over Ethernet, PoE). Power for E Ink screens is needed only when the information on the display is updated, then it is not consumed. A static picture can last for months without loss of contrast, if not longer.

Optional Artec Design dashboards can support touch control and work with RFID/NFC. This makes it possible to display additional information on the panel upon request. As the developers suggest, the information panel with an E Ink display will be a good help in the design of museums and other exhibitions where a multilingual description is needed. At the same time, the technology for displaying information on "paper-like" screens is such that it eliminates light pollution and makes it easy to distinguish even small details in bright ambient light. It does not irritate the eyes and improves the comfort of information perception.

Other interesting news:

▪ Complete absorption of light

▪ ferromagnetic semiconductor

▪ Automatic speed limit for electric scooters on sidewalks

▪ Danger of anticonvulsants

▪ TRENDnet USB 3.0 Keyfob with IEEE 802.11ac Support

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ site section Regulators of current, voltage, power. Article selection

▪ article to the left of common sense. Popular expression

▪ article Can you see a rainbow at night? Detailed answer

▪ article A simple bayonet with a hose. Tourist tips

▪ article Hectographic ink. Simple recipes and tips

▪ article Overhead transmission lines with voltage up to 1 kV. Grounding. Surge protection. Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

All languages of this page

Home page | Library | Articles | Website map | Site Reviews