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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Sound equalizers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment Along with a generalization of information about various types of equalizers, the author gives several schemes of multiband devices based on new imported specialized microcircuits, gives recommendations on setting up the devices themselves and setting up audio systems with an equalizer unit. Modern audio equipment and speaker systems fully provide high-quality sound reproduction only in a specially equipped room designed for listening to music. Most residential premises, especially small ones, are unsuitable for this purpose. At any point in such premises, there is such a phenomenon as interference (addition with different phases) of sound waves that come directly from acoustic systems and are reflected from walls, ceilings, floors, and furniture. At the same time, at some frequencies, standing waves arise - antinodes and dips in sound intensity with an unevenness of up to 20 dB. which makes it necessary to adjust the frequency response of the audio system in certain frequency bands. Insufficient sound insulation of the room leads to the fact that you have to listen to sound programs with a level significantly reduced in relation to the one at which they are formed (about 90 phon). As a result, in order to maintain the timbre of the sound, it is necessary to increase the volume level at frequencies below 200 and above 5000 Hz. Appropriate compensation, which is entered into the volume controls, as a rule, is incomplete. Frequency response regulation is also necessary for solving other problems: correcting the sound of low-quality phonograms and equipment frequency response errors, compensating for age-related changes in hearing, selecting timbre sound to the taste of the listener. To adjust the frequency response, tone controls and equalizers are used. The tone controls - passive and active - are relatively simple. Detailed information about their schemes with calculation formulas is given in [1]. They allow you to smoothly change the frequency response at low and high frequencies by up to ±14 ... 18 dB at the edges of the range. The maximum frequency response slope in real regulators can reach 4...5 dB per octave. Is it a lot or a little? In small rooms, sound vibrations with frequencies below 40 Hz are greatly attenuated, and in the 80 ... 200 Hz band, on the contrary, they are somewhat amplified. The decline at lower frequencies can reach 10 dB or more. It would seem that the tone control has sufficient capabilities to compensate for this decline. But it is fundamentally impossible to equalize the frequency response in this area with a simple tone control. When the level of the lowest frequencies is raised, the already overestimated signals in the 80 ... 200 Hz band will be simultaneously amplified. The situation can be corrected only by an equalizer - a multi-band regulator that allows you to set the required transfer coefficient in a narrow frequency band. There are manual and electronic equalizers. The first, in turn, are divided into graphic, parametric and paragraphic. Until recently, graphic equalizers (GE) were the most widely used. In them, the audio frequency range is divided into bands (maximum - 24 bands in the Technics SH-8020 equalizer). The adjustment of the transmission coefficient in individual bands is carried out with variable resistors, for example, type SPZ-23. with a linear regulator, so that the position of their sliders on the adjustment panel clearly reflects the shape of the frequency response, this determines the name - graphic. In (2] several such devices for tube and transistor amplifiers are considered, having five or six adjustment bands with frequencies of 40, 90, 250, 2000, 6000, 15 Hz. Bandpass filters with LC circuits have not found wide application due to their bulkiness and the need for screening. Modern equalizers use only passive and active RC filters. The device of the HE with active band-pass filters is considered in detail in [3]. where a five-band equalizer is proposed. Having built such an equalizer, I was faced with the fact that the introduction of correction by the regulators causes a noticeable violation of the stereo balance of the sound picture. The reason for this phenomenon was named in [4]. To maintain stereo balance at any position of the controls, it is necessary that the values of the resonant frequency and filter quality factor in the left and right channels differ from each other by no more than 5%. Their difference from the calculated values is less significant. To do this, the device uses passive components with a tolerance of no more than ± 3%. It is difficult to do this in amateur conditions. The problem is solved by applying filters assembled according to the scheme shown in Fig. 1.
The frequency-setting circuit in each of them is formed by resistors R2 - R5 and capacitors C1, C2. Trimmer resistors R1 and R2 allow you to accurately set the required values of the quality factor (bandwidth) and the center frequency of the filter. In [4], a description is given of a ten-band octave equalizer assembled according to this scheme using parts with a tolerance of ± 10%. The filter settings in it are equal to 30, 60, 120, 240, 480, 960, 1920, 3840, 7080, 15360 Hz. The equalizer turns out to be cumbersome - in the stereo version it contains 11 op amps of the K157UD2 type, more than 250 resistors and capacitors. But this is a fee for the ability to build a device from parts without selection. The feasibility of using such bandpass filters was confirmed in the detailed work on filter design (5). It states that their sensitivity to the spread of component values is two to three times lower than in filters previously used. Graphic equalizers are widely used in audio equipment. The five-band equalizer of the Las-pi-005-stereo amplifier provides frequency response adjustment at frequencies of 40, 250, 100 * 0, 5000, 15 Hz. In the domestic tuner "Corvette-000-stereo" a six-band GE is used, and in a stationary equalizer with the same brand there are ten bands with frequencies of 004; 31,5; 63; 125; 250; 500; 1000; 2000; 4000; 8000 Hz. In modern radio receivers and audio frequency amplifiers, GEs are made on specialized microcircuits that simplify these products. The positive side of such GEs is also their low cost, which usually does not exceed $1. On fig. Figures 2 and 3 show the BA3822LS circuits from ROHM and LS2009 from SGS-Thomson, which form five- and ten-band equalizers. These diagrams are very simple and do not need explanations.
Now - about parametric equalizers (PE). They differ from graphic ones in fewer stripes and themes. that the center frequencies of the bands and their width (filter quality factor) can be changed by the listener. Three to five-band versions of these devices allow for efficient frequency response adjustment with simple means, with results achievable in HEs with 8-11 bands. But in each PE strip, two regulatory elements and two controls are required. Methods for implementing parametric devices based on passive tone controls with variable frequencies of the inflection points of the frequency response are indicated in [1]. The first example of such a two-band controller was described in [6]. The scheme and detailed description with formulas for calculating a five-band PE are given in (7). According to the author, he has ample opportunities. Frequency tuning in bandpass filters is provided in the intervals 19...60...185; 62...190...600: 190...600...1850; 620...1900...6000; 1900...6000...18 Hz. those. two filters per frequency decade, with each filter also covering a decade. Fixed resistors and capacitors must have a tolerance of at least ± 500%. As a drawback of this circuit, it should be noted the need to use quad variable resistors to set the frequency of the filters. The quality factor of the filters varies within 5...0,5. Paragraphic equalizers do not differ from parametric equalizers in terms of circuit design. They only use the outer side of the GE - a visual representation of the frequency response. There is another class of equalizers - electronic, allowing you to set one of several pre-formed frequency responses. Forms of such characteristics for listening to sound programs of various nature and musical genres have already developed. Sony, for example, uses the following fixed frequency response settings in its products:
Practice shows that most owners of audio equipment choose just such settings for tone controls and graphic equalizers. To implement such a set of fixed frequency response settings, a number of specialized microcircuits have been released. In particular, these are M62412P from Mitsubishi and BA3842F SMD from ROHM. Switching of the frequency response options is carried out in them by applying voltages of different levels to the inputs of the microcircuit. In each of the M62412P stereo channels, the frequency response shapes can be adjusted separately. The BA3842F SMD chip (Fig. 4 shows a diagram of its inclusion) has a low-frequency boost unit. They are selected by a special filter and, after additional amplification, are added to the processed signal. There are three levels of this adjustment. Resistors R9, R10 are subject to selection in order to obtain the voltages indicated in the diagram.
More complex microcircuits are also being produced, allowing you to select up to 15 fixed frequency response settings, as in the Sony MHC RX110AV music center. Other types of such devices provide the listener with the opportunity to form three to five custom frequency response settings along with the fixed ones chosen during the design of the microcircuit. But all such devices are controlled by a special microcontroller. An example is the TEA6360 chip from Philips. The number of external components for it is minimal, there are no variable resistors. Strictly speaking, an equalizer with such a microcircuit can be called graphic only if, when it is adjusted, the shape of the frequency response is displayed on the display. Appropriate microcontroller software and new ergonomic indicators provide such opportunities. Due to the fact that the wiring diagram for the TEA6360 chip is shown with errors in some publications, in Fig. 5 shows its correct version.
Consider other technical characteristics of equalizers. bandwidth in them. as a rule, it is equal to 20 ... 20 Hz. This parameter for audio equipment is usually indicated simultaneously with the value of the unevenness of the transfer coefficient in a given band, but in relation to the equalizer, such a characteristic seems redundant. The input signal level is 000 ... 0,2 V and rarely exceeds the standard 0,5 V. The coefficient of non-linear distortion is less than 0,8%. The transmission coefficient at the middle position of the regulators is close to unity. The limits of its adjustment are usually ±11 dB. although in the literature there are descriptions of equalizer designs with adjustment limits of ±15 ... 24 dB (one of them indicated the possibility of expanding this range to ±40 dB). With any change in gain in any frequency band, the equalizer redistributes the energy of the input signal in a new way, amplifying or attenuating the intensity of its harmonics with frequencies within the adjustable band. This means that at its output there is a signal with a harmonic composition that differs from the input. Any deviation of the regulators from the middle position leads to the appearance of phase distortions, the visibility of which increases with the depth of the correction of the transmission coefficient and the increase in the frequency range of regulation. The appearance of such distortions in the signal that has passed the equalizer is a fee for the possibility of a deeper adjustment of the frequency response. It is widely believed that when the gain in the bands is changed by no more than ±4 ... 6 dB, the resulting distortions are quite acceptable and almost imperceptible even for a person with a musical ear. Significant changes in one or two bands of a 10 to 12 band EQ are also acceptable. But a wider use of its capabilities causes the appearance of noticeable rustles, ringing, jerky overtones. In portable and low-quality stationary equipment, passive two-way tone controls with fixed frequency response inflection points are now most common. described in [1], or three- and four-band HEs. The capabilities of these devices are close, but equalizers are preferable due to the greater visibility of the shape of the frequency response. The low characteristics of equipment of this class make it possible to use the entire range of adjustment of the transmission level in the bands without much damage to the quality of sound reproduction. In high-end equipment, equalizers with five or more bands of adjustment are used. In view of the foregoing, the operating range of regulation should not exceed ± 4 ... 6 dB. An indispensable addition to such an equalizer is an output signal spectrum analyzer. The expediency of using an equalizer on high-end systems (Hi-End) is questioned in many sources. Let us dwell on the methods of setting the frequency response, which are practically not covered in the literature, although they are relatively complex. An equalizer should correct two types of shortcomings: permanent ones (low volume level, defects in the frequency response of equipment, acoustic systems and rooms), as well as quickly arising ones - correction of low-quality phonograms, selection of the desired sound coloring. There are different ways to fix them. Frequency response error correction is rarely required - when installing an audio system in a room, after it has been repaired, or after changing the interior of the room. It is recommended to start it by clarifying the installation location of the acoustic system so that, without violating the size of the stereo effect manifestation zone, to minimize the intensity of standing waves in the room, which manifests itself in the unevenness of the sound field at low frequencies. To do this, you should try to move the speakers to another location or turn them in relation to the listener. Small changes in the arrangement of furniture, moving mirrors, paintings, photographs can make significant changes in the distribution of the sound field. The equalizer controls at this stage are in the middle position. The sound field (especially for the lowest frequencies) should be diffuse - s its intensity at all points in the room should be the same or evenly decrease with distance from the sound emitters. In practice, one can only approach this ideal within the area in which the stereo effect is observed and the listeners are located. It may be necessary to use additional sound emitters for this. Then, at frequencies where the uniformity of the sound field is not achieved, it is corrected by adjusting the equalizer. To perform this correction, measuring equipment is required: a noise generator, a microphone with a known frequency response for sound pressure, a spectrum analyzer. High-class equalizers manufactured by leading foreign companies are equipped with such equipment for their adjustment. If the specified set of equipment is not available, it will be replaced by a sound generator and a microphone with a millivoltmeter. Finally, you can use a cassette with a measuring magnetic tape, and determine the location of the points of maximum (minimum) sound pressure by ear. The resulting position of the equalizer controls (their sliders should be arranged in a smooth curve) should be considered as zero for this audio system in this room. Not bad if it is somehow marked on the adjustment panel. This will allow you to easily return to it after any changes in the frequency response. Operational adjustment should be made only for a while, to obtain any special sound of a particular piece of music, and at the end of it, return to the original position of the controls. In this regard, the best solution to the problem of adjusting the timbre of the sound is the situation when the audio system has two equalizers: one - to improve the acoustic properties of the equipment and the room, and the second - to quickly adjust the timbre of the music being listened to. As the first, it is rational to install a 10 - 12-band octave GE or a five-band PE. A large number of adjustment organs they have is a boon, because it allows you to fine-tune the frequency response. As a second, it is better to choose a simpler device: electronic with fixed settings or a five-band GE. A more elegant way to solve this problem would be to use an electronic equalizer that generates a variety of custom frequency response settings. Each of them should ralize the correction of the acoustic properties of the equipment and the room, and also impose one of the previously mentioned sound effects on this correction. It is generally accepted that in most cases the HE is adjusted blindly, by random enumeration of the regulator settings, and their best position is usually not achieved. This applies even more to PE. This work can be facilitated, made conscious only with the help of a built-in spectrum analyzer - a device for measuring and displaying signal levels in several frequency bands. Included in the audio path, it allows you to control the frequency distribution of the input signal energy and objectively control this process in order to achieve optimal sound. Literature
Author: V.Brylov, Moscow
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Comments on the article: Vladimir Thank you! A very detailed and visual aid, even for beginners more or less understanding in sound.
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