Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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Improving the reception quality of VHF FM programs at low signal levels is not an easy task. Very often, devices no less complex than the receivers themselves are used to solve it. The article proposes a simple way to reduce the noise level when receiving signals from a remote radio station.

It is known that in the adopted systems of VHF stereo broadcasting, both domestic - with polar modulation (PM) and foreign - with a pilot tone (PT), the frequency band of the complex stereo signal (CSS) is much wider than the 3-hour bandwidth. As a result, the "overtone" noise accompanying the CSS is decoded along with the stereo signal and appears in the audible region of the audio spectrum, thereby worsening the signal-to-noise ratio by 10...18 dB compared to the mono mode.

In order to maintain an acceptable sound quality, it is necessary to force the stereo decoder to switch to the "Mono" mode already during the reception of signals, at which the initial signal-to-noise ratio of mono reception drops to 40...48 dB. The available automatic switch "Mono - Stereo" does not solve this problem, since it reacts only to the presence of subcarriers of the CSS, and not to the real signal-to-noise ratio.

To reduce the noise of stereo transmissions, radio amateurs have proposed several devices [1, 2] that operate on the principle of parallel connection of the left and right output channels of the decoder while reducing the received signal to a certain level. This uses the fact that the noise components in the channels are out of phase and, when converted to monophonic mode, they are mutually compensated. Unfortunately, the quality of operation of such noise suppressors in the entire dynamic range is low; they can rather be considered as threshold devices that reduce the visibility of noise in pauses, which is noted in [1].

I propose a fundamentally different way to solve the problem, the circuit solution of which does not require energy consumption (it can be used in portable equipment) and does not introduce additional non-linear distortions. It is based on the dependence of the noise level and the degree of separation of stereo channels on the nonlinearity of the frequency response of the CSS in the supratonal frequency band - from 20 to 50...70 kHz. Thus, a cutoff in this region of 6 dB per octave, obtained using the simplest single-link low-pass filter in a decoder based on the K174XA51 microcircuit, sharply reduces the noise level, but at the same time worsens the actual channel separation from 34 ... 43 dB to 24 dB for PM and 20 dB for PT [3]. The reverse is also true - the rise in the frequency response in the specified area contributes to a better separation of stereo channels with some deterioration in the signal-to-noise ratio. In addition, the frequency correction of the KSS is also necessary to compensate for losses in the FM detector. In receivers of the highest category of complexity, it is even recommended to include preliminary ultrasonic frequencies after it, which provide a rise in the frequency response at the upper frequencies of the CSS [4].

Thus, by adjusting the frequency response of the CSS in front of the stereo decoder, you can get different final quality of 3H signals at its outputs: from very good, with maximum crosstalk attenuation between channels - for powerful stations, to satisfactory, close to "Mono" - for remote ones. This principle underlies the proposed device (Fig. 1), consisting of an adjustable link R1R2R3C1 and an unregulated low-pass filter R4C2R5C3.

The operation of the adjustable link depends on the position of the variable resistor R3 slider. In its upper position according to the scheme (max), a rise is formed in the supratonal and, partially, in the tonal areas due to the action of the R2C1 HPF (Fig. 2). As the slider moves down, the inflection frequency increases, the rise in the specified area decreases, and, in addition, the R1C1 low-pass filter begins to operate. In the lowest position of the slider of the resistor R3 (min), the high-pass filter is excluded, and the current low-pass filter provides the necessary drop in the frequency response.

The second low-pass filter is designed to suppress noise and out-of-band signals outside the frequency spectrum of the KSS. It helps to reduce interference interference and noise at the output of the decoder, and without degrading the real quality of the separation of stereo channels, since the tonal region of the CSS is not affected.

As can be seen from the graphs in Fig. 2, in the upper position, a rise of about 5 dB is achieved in a fairly wide frequency band of 5 ... 70 kHz. In the middle position of the regulator, the frequency response is almost linear up to 60 kHz, with a further decline beyond the boundaries of the CSS. And finally, in the lower position, a roll-off appears at frequencies above 19 kHz, attenuating the supratonal part of the stereo signal.

The obtained characteristics are acceptable for both PT and PM stereo decoders. In both cases, the necessary range of regulation of the parameters of the 3H output signals in terms of noise and crosstalk attenuation between the channels is provided.

Due to its simplicity, the filter can be easily applied in any ready-made device - a stereo or radio receiver, with the exception of, perhaps, the smallest ones. Of course, its use in new developments, for example, two-standard stereo decoders, is not excluded.

In the author's version, the following parts were used: a variable resistor from TELPOD (analogous to SPZ-4M), constant ones - MLT 0,125, capacitors - type KM-4.

The variable resistor R3 (see Fig. 1) in this device performs the functions of a smooth switch "Mono - Stereo". Therefore, when refining a stereo receiver, the existing "Mono - Stereo" step switch becomes unnecessary. It can be removed, the circuit switched by it is fixed in the position corresponding to the "Stereo" mode. In place of the remote switch, a variable resistor R3 with a handle of a suitable design should be installed. Another option is also possible - to leave the existing "Mono - Stereo" switch, and place the R3 smooth control next to it.

Embedding the filter comes down to dismantling the existing input circuit of the stereo decoder (except for the transition capacitors) and installing the elements of the proposed filter in its place. Mounted, arbitrary, the only requirement for it is to connect the mounted part of the circuit with a variable resistor R3 with a shielded wire to avoid interference.

The attenuation introduced by the filter at a frequency of 1000 Hz is the sum of the losses in the regulated link (5 dB) and the signal reduction by the divider formed by the total resistance of the resistors R4, R5 and the input impedance of the stereo decoder chip. The latter may be different, which affects the overall transfer coefficient. Since the filter does not contain special tuning elements (their introduction would increase the attenuation to an unacceptable level), the following method of matching with a specific type of microcircuit is recommended.

Before alteration, a sinusoidal voltage of 50 mV (effective value) with a frequency of 1000 Hz is applied to the input of the standard circuit immediately after the FM detector and the output signal of any channel of the stereo decoder is measured after the corrective low-pass filter. The filter is then built in as described above and, with the same test signal applied to its input, the output voltage is again checked at the same point. If it has changed significantly, then the values ​​​​of the resistors R4, R5 are selected, bringing the output signal level closer to the original value, and then the capacitances of the capacitors C2 and C3 are proportionally changed according to the condition of the time constant τ = R4C2 = R5C3. A similar operation can be performed with the output low-pass filter for frequency pre-distortion correction, while maintaining their time constant of 50 ... 70 μs. The technique ensures that the receive volume does not change after the filter is set.

The filter was applied by the author in the radio "SHARP GF-6363" with a stereo decoder on the BA1320 chip. Subjectively, the quality of his work can be assessed as follows. When receiving powerful stations and setting the R3 control to the upper position according to the scheme, the stereo effect is significantly improved and the upper frequencies of the sound spectrum are emphasized, since the R2C1 high-pass filter already operates from a frequency of 4 kHz, i.e. it also works as a tone control. At the same time, the sound picture seems to become clearer (compared to the middle position of the regulator), the instruments are localized more clearly, and an excellent stereo panorama is created. The increase in noise depends on the initial quality of the received program, and for the most powerful stations (at the reception point - "Russian Radio") it is hardly noticeable, even during pauses. In the middle position of the control, the sound corresponds to the usual "Stereo" mode with a slightly reduced noise level. As the regulator slider approaches the lower position according to the scheme, the stereo effect worsens, the upper frequencies are muted, but at the same time the noise level decreases, which allows receiving distant stations in this mode. In the extreme position, the sound is almost monophonic, and for weak signals, the stereo decoder automatically switches to the "Mono" mode, as indicated by the extinction of the stereo indicator.

From the foregoing, it is clear that the proposed KSS filter is a very convenient and flexible adjustment element that makes it possible to obtain an adaptive signal quality in almost any reception conditions. The filter operates smoothly and throughout the entire dynamic range, which distinguishes it favorably from Mono-Stereo step switches and threshold noise suppressors. Its further automation, as well as sharing with the mentioned devices, is not excluded.

Literature

1. Bogdanov V. Reduction of noise in the pauses of stereo transmissions. - Radio, 1985, 3, p. 37.
2. Churin S. Automatic switching of a stereo decoder to monophonic mode. - Radio, 1991, 10, p. 72,73. 3. Alenin S. Microcircuits of the K174 series. Bi-standard stereo decoder KR174XA51.- Radio, 1999, 5, p. 43 - 45; 6, p. 47.
3. Reference book for a radio amateur designer. (A. Bogdanov, N. Borisov and others). Edited by N. Chistyakov. - M.: Radio and communication, 1990.

Author: D.Pakhomov

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