Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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When working on the air, the external acoustic background of the room (fan noise, the hum of a power transformer in the power supply, etc.), getting into the microphone, is amplified along with the operator’s speech signal and worsens its visibility for the correspondent. This is especially noticeable when conducting close connections. To suppress it, so-called noise identifiers are used, which, by analyzing the noise situation in the room, are able to distinguish the speech signal from the background noise, which has a relatively constant level.

Noise identifiers are becoming more and more common and are used, for example, in telephony to suppress external acoustic noise and electrical line noise.

A simplified functional diagram explaining the principle of operation of the acoustic noise suppressor using the background noise identifier is shown in Figure 1.

The signal from the microphone is amplified and fed to a controlled attenuator and a level detector. From the level detector, the signal goes to the background noise identifier, which reduces the attenuation level of the controlled attenuator in the presence of a speech signal and increases it when only background noise is received. From the output of the controlled attenuator, the speech signal is fed to the output amplifier.

It is this structure of the acoustic noise suppressor that is implemented in the Motorola MC34118 multifunctional linear microcircuit (domestic analogue 1436XA2), intended for use in high-quality loud-speaking telephones (speakerphone). A description of this microcircuit can be found in the reference sheet in "Radio", 2003, No. 10, p. 47-49.

In the design proposed below, only the transmit channel of the microcircuit is used, containing a microphone amplifier, a signal level detector, a background noise identifier, an attenuator control unit, a transmit attenuator, and one of the outputs of a pair-phase amplifier. In addition, the device can use cascades of filters for correcting the frequency response of the amplifier, which are also included in the microcircuit.

The electrical circuit diagram of the acoustic background suppressor in the transceiver is shown in fig. 2.

Consider the operation of the device. The signal from the microphone through the capacitor C5 and the resistor R4 is fed to the input of the microphone amplifier of the DA1 chip (pin 11), the gain of which is set by selecting the resistor R1. From the output of the microphone amplifier (pin 10) through the capacitor C3 and resistor R8, the amplified signal is fed to the input of the level detector (pin 17), and through the capacitor C6 the same signal is fed to the input of the controlled attenuator (pin 9).

The level detector comprises an operational amplifier having a large dynamic gain and a circuit having a short charge time and a long discharge time. The output of the level detector is internally connected to the background noise identifier, which, in turn, regulates the attenuation of the transmitting attenuator through the attenuator control unit depending on the type of the incoming signal spectrum. With a speech signal, the attenuator gain is +6 dB, with a background signal - -20 dB. When a signal identifier is received at the input, which is formed only by acoustic noise and does not have sharp changes in amplitude, a constant voltage accumulates on the R11C14 circuit with a significant rise time and a short fall time. Capacitor C12 of the level detector sets the rise time of the input signal, and the R11C14 circuit determines the response time of the identifier to a change in the background noise level (according to the scheme, it is 4,7 s).

The "noise" voltage applied to the non-inverting input of the identifier comparator is more positive than the inverting input, which is supplied with a reference threshold voltage to ensure that the comparator operates when the speech signal level exceeds the background noise level by 3 ... .4 dB. When a speech signal appears, due to sharp changes in its amplitude, the voltage at the non-inverting input will increase faster, which will cause the appearance of voltage at the output of the identifier, which reduces the attenuation of the attenuator.

From the output of the attenuator (pin 8 DA1), the speech signal through the resistor R5 and the coupling capacitor C1 goes to the output amplifier (pin 7), and from it through the capacitor C4 and the divider R2R6 - to the output of the device.

Switch SA1 is used to turn off the identifier by closing the output 16 of the microcircuit to the case. LED VD1 serves as an indicator of the inclusion of the squelch.

The device is powered by +5 V, which is available on the microphone connector of many foreign transceivers, or from an external battery. The current consumption of the circuit does not exceed 10 mA.

Installation is carried out on a printed circuit board made of double-sided foil textolite. Its topology is shown in Fig. 3 and 4. Any capacitors and resistors can be used. The electrolytic capacitor C12 should have as low a leakage current as possible, the ideal use of capacitors of the K53-4 or K52-1 type. The board is installed in a metal (metallized) case 55x80x25 mm in size. The common wire of the board must be connected to the case, next to the microphone jack.

If the output power of the transceiver is more than 100 W in the +5 V power circuit, it is additionally necessary to install a filter from a pass-through or reference capacitor with a capacity of 1000-4700 pF and a 100 μH choke.

Setting up and connecting to the transceiver

A millivoltmeter, an oscilloscope and, preferably, a non-linear distortion meter with a high-resistance input are connected to the output of the device, at the connection point of the capacitor C4 and the resistor R2. A voltage with a frequency of 1000 Hz at an amplitude of 1 mV is applied to the microphone input of the device from a sound generator. The amplitude of the signal at the output of the device should be about 300 mV, and the coefficient of non-linear distortion should not exceed 0,8%. Then the input voltage is increased until the beginning of the signal clipping is obtained. It should occur at an output voltage of 1,3 ... 1,5 V. All these measurements are carried out with the noise identifier turned off (pin 16 of the DA1 chip is closed to a common wire by the SA1 switch). After that, resistors R2 and R6 set the gain of the amplifying path as a whole. If the device will be connected between the microphone and the microphone input of the transceiver, it is recommended to set the overall voltage transfer coefficient to 1...1.5 (the values ​​of the resistors R2 and R6 are indicated for this option). In the case of using it as the main microphone amplifier, the output voltage is increased by reducing the value of the resistor R2.

After checking the amplifying path, the background noise suppression is checked with respect to the speech signal. This is best done with a special noise generator that has a calibrated acoustic emitter and a measuring microphone. However, the operation of the device can be estimated with sufficient accuracy as follows. An oscilloscope and a millivoltmeter are connected to the output of the device at the connection point of the capacitor C4 and the resistor R2. An electret microphone "Pine" or similar in sensitivity is connected to the microphone input of the device, after which some phrase is said in front of it.

Having noticed the amplitude of the output signal on the oscilloscope screen, bring the microphone to a source of uniform noise (for example, to a working transceiver fan or power transformer of the power supply) and achieve approximately the same amplitude of the noise signal. After that, the noise identifier is turned on (by opening the SA1 switch). The noise background should be suppressed by an average of 26 dB (20 times), and the sensitivity to the speech signal with the identifier on or off should remain unchanged.

Author: V.Khmartsev (RW3AIV), Moscow

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