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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Stereo decoder with KSS input filter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications How to improve the quality of received stereophonic sound broadcasting programs by simple means is of interest to many radio amateurs. The author of the article analyzes the method of forming the frequency and phase characteristics of the radio path and determines the links in which it is possible to introduce corrective elements at low cost to improve signal extraction at the output of a stereo decoder. The quality of stereo radio reception depends not only on the actual signal-to-noise ratio at the antenna input of the receiver, but also on the operation of the decoding device. As is known, in a stereo decoder (SD) a complex stereo signal (CSS) is converted into polar modulated oscillations (PMC), and then into low-frequency output signals of the left and right channels. The transformations taking place in this case determine, in particular, such an important parameter as crosstalk attenuation between channels. The best channel separation allows to achieve a temporal decoding method that eliminates subcarrier recovery and associated non-linear and phase distortions. Most modern integrated LEDs work on this principle. The quality of decoding is also significantly affected by the spectrum of the input CSS. The upper modulated frequency required to transmit an audio frequency of 15 kHz is 31,25 kHz for a stereo broadcast system with polar modulation (PM) and a subcarrier of 46,25 kHz, and 38 for a system with a pilot tone (PT) and a subcarrier of 53 kHz. kHz. A prerequisite for the absence of distortion and good channel separation is a horizontal (without blockage) frequency response and linear phase response in the supratonal frequency range, up to the indicated ones. At the same time, the most typical is the radio receiving path, which has a drop in the frequency response at the upper frequencies of the CSS. This rolloff is due to the limited bandwidth of the IF and FM detector path. If the cutoff frequency of the CSS at the level is 3 dB, denote Fcp, and the subcarrier frequency is Fsub, then the crosstalk attenuation between the channels can be calculated using the approximate formula p = 20 log (2 Fcp / Fsub). It is easy to calculate that in order to obtain a stereo channel separation of 30 dB, a bandwidth of signals with PM up to 88 kHz is required, and signals with FET - up to 107 kHz. Of course, these data are approximate and do not take into account the features of a particular decoding method. To correct the frequency response of the CSS in decoder models, certain corrective circuits are used, as a rule, of the simplest RC type. On the other hand, excessive expansion of the spectrum of the CSS leads to a sharp increase in noise and interference from the conversion of out-of-band signals. If the bandwidth of the CSS is not limited in any way, then the degradation of the signal-to-noise ratio when receiving remote stations can be 20 dB or more compared to the mono mode. And vice versa: the narrowing of the KSS band is an effective noise reduction technique. The conflicting requirements for the KSS are best satisfied by the maximum flat frequency response up to a frequency of 70 ... 80 kHz, with a further sharp drop, organized by high-order filters. This characteristic makes it possible to approach the maximum achievable parameters of a particular LED in terms of noise and crosstalk between channels. These provisions were fully confirmed during tests of a two-standard stereo decoder based on the KR174XA51 chip. In a typical switching circuit [1], a simple first-order low-pass filter with a cutoff frequency of about 10 kHz is used at its input. The 6dB/oct rolloff above 10kHz provides acceptable noise performance, but reduces channel to channel crosstalk from 43dB (typical without input filter) to 24dB for PM signals and 20dB for DET signals. In addition, the filter cuts off the upper part of the tone signal in the region of 10 ... 15 kHz, which makes the sound "muffled". In general, despite the progressive design solutions - a temporary method of channel separation with double sampling, additional suppression of the pilot tone, etc. - the mentioned SD worked worse than the decoder on the outdated BA1320 chip. Another disadvantage of KR174XA51 is unpleasant clicks in the audio path when the stereo mode indicator is turned on. Replacing the microcircuit with another copy did not bring fundamental changes. To improve the quality of work, the proposed decoder is supplemented with an input filter of the KSS, which forms the necessary type of frequency response with the possibility of manual and automatic correction. The advantages of the new LED also include separate indication of the stereo broadcasting system, which operates silently. Main Specifications
Functionally, the device consists of three blocks (Fig. 1): the KCC input filter, the switch on the DD1 chip, and the actual decoder on the DA1 chip.
The KSS filter is a further upgrade of the device [2]. Its parameters have been improved by computer simulation - the frequency response unevenness in the tonal region has been reduced and the cutoff steepness in the overtone region has been increased. The filter consists of an adjustable link R1, R2, C1, C2 and a 3rd order low-pass filter C3, L1, C4 with a cutoff frequency that can be switched depending on the stereo broadcasting system. The frequency response of the filter - in the reception mode from the FET (shown in Fig. 2).
Link R1,R2, C1,C2 - high-frequency bridge regulator KSS. In it, using a variable resistor R2, you can increase or decrease the level of overtone (and partially tonal) components, which leads to a proportional expansion or narrowing of the stereo base by changing the crosstalk attenuation between channels [2]. In the middle position of the regulator R2, the frequency response of the filter is horizontal up to the cutoff frequency (see Fig. 2, curve 1), in the two extreme positions - its unevenness in the audio range does not exceed 2 dB. The adjustment captures only the upper part of the sound spectrum - above 10 kHz, which allows you to emphasize the higher frequencies with a confident reception and thereby improve the sound quality. At the same time, the noise level also changes, it is minimal in the lower position of the R2 resistor slider, when the overtone part of the KSS is actually cut off and the sound is close to monophonic. Thus, the adjustable filter element makes it possible to obtain an adaptive quality of the output signal depending on the input - from the extended "Stereo" for powerful RF signals to "Mono" - for noisy and distorted, in particular, multipath reception. The U-shaped low-pass filter of the 3rd order is assembled on the elements C3, L1, C4. This filter is designed to effectively suppress noise and interference from the conversion of signals lying beyond the main information band of the CSS. The low-pass filter was synthesized by the Design application of the MicroCap6.0 program. Its parameters are: cutoff frequency in a FET system - 75 kHz, in a PM system - 60 kHz, slope behind the transparency band - 15...17 dB/oct, characteristic impedance - 4,7 kOhm. The cutoff frequency is structurally changed by switching the number of turns of the coil L1 by the electronic switch DD1. Thanks to computer simulation, the filter has a smooth frequency response (see Fig. 2) and a fairly linear phase response (Fig. 3).
The KSS filter is connected to the stereo decoder (DA1 chip) instead of the remote input circuit R1C1 [1]. The attenuation introduced by it (12 dB) is compensated by the large gain margin of the DA1 chip (up to 14 dB). When receiving signals from the PM, pin 8 of the DA1 chip is set to a low logic level close to zero. The control inputs 5 and 6 of the switch DD1 has a high logic level supplied from the midpoint of the divider R4, R5. In this case, the key K2 is closed at pins 4 and 3, pin 3 of the coil L1 is connected to capacitor C4. The filter is set to a cutoff frequency of 60 kHz. At the same time, the short circuit key is open and through its pins 8 and 9, the indication voltage from pin 7 of the DA1 chip is supplied to the HL1 LED, which indicates the "PM" mode. When recognizing signals from the FET, the voltage level at pin 8 of the DA1 chip changes to high, which is actually equal to the supply voltage. This signal is fed to the control inputs 12 and 13 keys K1 and K4 switch DD1. Key K4, opening, reduces the voltage at the midpoint of the divider R4R5 to a low level. The keys K2 and short circuit at the same time switch to a non-conductive state, as a result of which the output 3 of the coil is disconnected from the capacitor C4, and the HL1 LED goes out. At the same time, key K1 opens, which connects output 2 of coil L1 to capacitor C4. The inductance of the coil decreases, which leads to the tuning of the cutoff frequency of the low-pass filter to 75 kHz. In addition, the cathode of the HL2 LED turns out to be connected to a common wire through the K11 key open at pins 10 and 4, and on its anode there is a voltage coming from pin 7 of the DA1 chip. In this case, the HL2 LED indicates the "PT" mode. Switch SA1 can be forced to turn on the "Mono" mode. In this case, both LEDs are off, since there will be no voltage at pin 7 of the DA1 chip. The allowable supply voltage of the KR174XA51 microcircuit is 2,7 ... 7 V. It has been experimentally established that characteristic clicks when the stereo mode indicator is turned on occur only when the supply voltage is over 4 V. In this case, the voltage at pin 15 of the DA1 microcircuit is limited by the VD1 zener diode at level 3,9, 1 V. At the same time, the indicators HL2, HLXNUMX turn on almost silently, the parameters of the microcircuit remain high. The stereo decoder uses fixed resistors MLT-0,125, ceramic capacitors - type KM, electrolytic - imported. Switch SA1 - P2K button. Variable resistor R2 - any small-sized, for example, SDR-4b, with type A characteristic. Due to the reduced supply voltage of the DA1 microcircuit, the emitters HL1, HL2 must have high light output at low current. KIPD05A LEDs satisfy this condition, but you can pick up others with the maximum brightness of the glow, including imported ones. Coil L1 is made on a ferrite ring K20x10x5 mm from ferrite grade 2000NM. Winding 1 - 2 contains 110 turns, winding 2-3 - 30 turns of PEV 2-0,2 wire. The quality factor of the coil is high, so the low-pass filter parameters practically do not worsen the resistance of the open channel of the DD1 microcircuit (about 270 ohms), connected in series with the L1 coil. Device nodes such as the KSS filter and the DD1 switch do not require configuration. In the stereo decoder DA1, only the trimmer resistor R8 should achieve stable recognition of the stereo "PM" or "PT" mode by turning on the corresponding LED HL1 or HL2. After that, the operation of the adjustable filter element is checked by rotating the handle of the resistor R2: the sound should change from the extended "Stereo" to "Mono". The subjective effect of this adjustment is well described in [2]. It is recommended to note the middle position of the R2 regulator, which corresponds to the horizontal frequency response of the KSS (see Fig. 2] and the normal "Stereo" mode. The effectiveness of the 3rd order low-pass filter is easy to check by temporarily turning on the P2K switch (fixed button) to switch it. When the button is pressed, one group of contacts P2K should short-circuit the terminals 1 - 3 of the coil L1, and the other - disconnect the terminals of the capacitors C3, C4 from the common wire. Turning off the filter by pressing a button is accompanied by a sharp increase in noise and interference, even when receiving not very weak signals. Reception of distant and weak signals in stereo becomes impossible at all. Turning on the low-pass filter, on the contrary, cleans the signal from noise, interference whistles, etc., while the channel separation remains high. In general, the quality of the proposed SD was significantly higher than the original one [1]. The CSS filter can, of course, be applied to other decoders as well. Due to the relatively low characteristic impedance of the low-pass filter, its output matches well with the input of almost any LED. For single-standard SDs, the switch DD1 is not needed and the circuit is greatly simplified (Fig. 4).
The number of turns of the coil L1 is chosen equal to 110 for a stereo broadcasting system with PT or 140 for PM. However, for a specific SD, it is better to refine it experimentally. In this case, the L1 coil is made with several taps (after 10-15 turns) and they are switched during tuning, achieving a minimum of noise and a good separation of the stereo channels. This work is best done when listening to sound with stereo phones. Literature
Author: A.Pakhomov, Zernograd, Rostov region
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