HUSH stereo noise reduction chip SSM2000 application

    Abstract: SSM2000 is an advanced audio noise reduction system manufactured using patented HUSH circuit. It can be widely used in various sound sources, including sound and video tapes, radio and television broadcasts, or other noisy sound sources. This article introduces the main features, working principle and performance parameters of SSM2000, and introduces the application circuit of SSM200.

    Keywords: audio noise reduction; voltage controlled amplifier; filter; SSM2000

    1 Overview

    SSM2000 is an advanced audio noise reduction system produced using patented HUSH circuit. HUSH combines a dynamic filter with a down-expander, so it is very efficient and free of noise that is common in systems. Additionally, adaptive threshold circuitry detects nominal signal levels and dynamically adjusts both thresholds to provide optimal results regardless of the signal source. HUSH can be used with virtually any source, including sound and video tapes, radio and television broadcasts, and other noisy sources. The SSM2000 can be used with DolbtB encoded signal sources to produce good results. A key feature of this circuit is that it can be connected directly to the voltage-controlled amplifier port, allowing functions such as DC amplitude control, automatic level adjustment and compression to be implemented using minimal peripheral circuitry.

   The main features of SSM2000 are as follows:

·Noise suppression up to 25dB;

·Single-ended operation;

·Adaptive threshold can be dynamically adjusted;

·Can effectively decode DolbyB audio sources;

·Can be directly connected to the VCA control terminal;

·With logic controllable bypass and mute functions;

·100dB dynamic range (when noise suppression is turned off);

·Typical value of total harmonic noise to distortion ratio is 0.02%;

·The power supply voltage is 7~18V.

   SSM2000 adopts a 24-pin small package (SOIC), and its package appearance is shown in Figure 1. NC means not "connected".

   2. Main parameters

Table 1 lists the main electrical performance parameters of SSM2000.

    3. Working principle and pin functions

    3.1 Principle block diagram of SSM2000

    Figure 2 shows the functional block diagram of an audio channel of SSM2000. When the input audio signal is processed, the required frequency distribution and amplitude information of the signal and noise can be extracted. The audio signals of the left and right channels are filtered by a voltage-controlled low-pass filter, and then amplified by a voltage-controlled amplifier before being output. The filters and amplifiers are low-distortion and the noise caused by them is negligible. The cutoff frequency of the voltage-controlled filter (VCF) can be set by the user as needed between 1kHz and 35kHz. The VCA can be set to attenuate or amplify, which increases the flexibility of the SSM2000. Appropriate control signals can be obtained using appropriate algorithms and applied to VCF and VCA, thus achieving noise suppression of up to 25dB while causing minimal loss to the signal.

    3.2 Pin function description

    The pin functions of SSM2000 are as follows:

    LIN, RIN (pins 1, 2): left and right audio input. Both LIN and RIN use single-ended inputs with an input impedance of 10kΩ.

   VCF capacitor end (left channel pins 3 and 4, right channel pins 21 and 22): including LVCFC1, 2 and RVCFC1, 2. Capacitors connected to these ports control the range of the VCF low-pass filter.

    V+ (Pin 5): Positive power supply.

    ACOM (Pin 6): Internal reference ground for the audio circuit. In single-supply applications, this terminal is usually connected to low potential, and the potential should be half of the positive supply.

    VCA CONTROL PORT (pin 7): VCA control terminal. This end is connected to the voltage-controlled amplifier circuit. In addition to controlling the voltage-controlled amplifier detector, it can also externally control the voltage-controlled amplifier detector. It can also set and set the amplitude of the voltage-controlled oscillator externally. Control the volume. This port will produce an amplitude attenuation of 1dB for every 22mV applied to it, 0dB corresponding to 150mV.

    VCF DET IN (pin 8): This pin is the input terminal of the voltage-controlled filter control and noise threshold detection circuit. It is recommended to use 3-pole filters at SUMOUT and VCF IN.

    SUM OUT (Pin 9): This terminal is the combined output of the left and right inputs. The combined output can be used by filters and voltage-controlled amplifier detectors to determine the amplitude and frequency range of the audio signal.

    VCF DET TC (pin 10): Voltage-controlled amplifier level control input, the output voltage is used to control the voltage-controlled amplifier in the audio channel. It is recommended to use a single-pole filter at the SUM OUT terminal and VCA DETIN terminal.

    VCF DET TC (pin 11): Voltage-controlled filter timing capacitor end. A 1μF capacitor is usually connected to this end. Its purpose is to control the change speed of the cut-off frequency of the voltage-controlled filter.

   VCA DET TC (pin 12): Voltage controlled amplifier timing capacitor, the function of this terminal is similar to the VCF DET TC terminal. A 33μF capacitor should be connected to this pin, which controls the rate of amplitude change of the voltage-controlled amplifier. A capacitance value that is too high will produce breathing noise, and a capacitance value that is too large will produce pumping noise.

    13 feet: No need.

    DEFAULT THRESHOLD (Pin 14): Default noise threshold pin. This pin can be used to set the maximum possible noise threshold. When using the maximum noise threshold level, HUSH will filter the audio signal to prevent very high noise; setting a lower threshold prevents the HUSH chip from attenuating already small noise, thereby reducing the sensitivity of the HUSH system.

    AUTO THRESHOLD CAP (pin 15): The capacitor connected to this terminal is used to control the rate of change of the adaptive noise threshold level. It is recommended to use a 0.22μF capacitor.

    DEFEAT (pin 16): "disable" terminal. Applying a 5V voltage signal to this terminal will "disable" the HUSH noise reduction system. The voltage applied to this terminal should be referenced to the GND terminal. It is recommended to use the standard TTL level for control when using the inhibit terminal. Activating the inhibit terminal will cause the voltage-controlled filter detector and voltage-controlled amplifier detector inside the chip to send the maximum control signal to the voltage-controlled filter and voltage-controlled amplifier respectively. If the inhibit terminal is controlled by a noisy digital logic line, good filtering measures should be adopted to prevent digital noise from interfering with the audio signal in the SSM2000.

    MUTE (pin 17): Mute terminal. Applying 5V voltage to this terminal will implement mute control on the SSM2000.

    Pin 18 (Pin 18): Unused end.

    GND: power ground terminal.

    V- (pin 20): the "negative" terminal of the power supply.

    ROUT and LOUT (pins 23, 24): left and right channel output. The output of the HUSH chip is buffered and therefore can drive a certain load. But for loads exceeding 2kΩ or 300pF (such as speakers or headphones), additional buffer drives should be used.

    4. Application circuit

    4.1 Typical applications

    Figure 3 shows a typical application circuit of SSM2000. Its pins 13 and 18 are generally left floating, while the default threshold terminal (pin 14) is generally connected to the analog common terminal.

    The SSM2000 is usually placed immediately in front of the volume control and power amplifier. And should be placed as far below the signal chain as possible. Since the SSM2000 has the best working effect at 300mV (root mean square value) level, it should be placed before the volume control and power amplification components. Figure 4 is a schematic diagram of the location in the system.

   The SSM2000 operates with a power supply voltage ranging from 7V to 18V, and can also use a stable ±9V power supply. In fact, the SSM2000 can operate with power supplies as low as 4.5V. However, to ensure good performance, the supply voltage should be within the given range.

    In dual-supply applications, the ACOM pin is connected to system ground.

    V+ and V- should be connected to the corresponding power lines respectively and must be well decoupled. It is recommended to use 1μF aluminum electrolytic capacitor and 0.1μF ceramic capacitor. Both power lines should be decoupled as such.

    When operating in single power supply mode, a virtual ground potential must be generated and connected to the ACOM terminal. As shown in Figure 5. The virtual ground is usually set at the midpoint of the power line. The virtual ground provided for the chip must be able to source or sink 10mA. This function can be achieved using the low-cost OP292.

    4.2 Automatic level adjustment

    In terms of design, SSM2000 has made many considerations to facilitate other applications. With direct access to the voltage-controlled amplifier's gain control and amplitude and frequency detectors, and with some external circuitry, the SSM2000 can control two audio channels with a single DC level without routing the audio signal to a control panel.

    Automatic level adjustment is similar to compression, but it does not attempt to attenuate all large-amplitude audio signals. It only attenuates the audio signal after the entire signal amplitude increases for a certain period of time (such as 1 second). The application circuit of automatic level adjustment is shown in Figure 6.

    This circuit actually only operates on the signal amplitude, so the input terminal of the circuit is taken from pin 12, and its input signal is gain adjusted and level shifted. After the operational amplifier, the signal passes through a positive peak detector and is averaged over the time constant RC. After averaging, the signal is sent to the control end of the voltage-controlled amplifier.