Features, Functions and Application of Hush Stereo Noise Reduction Chip Ssm2000
1. General
Ssm2000 is an advanced audio noise reduction system produced by patented hush circuit. Hush combines dynamic filter with downward expander, so it is very efficient and has no common noise in the system. In addition, the adaptive threshold circuit can detect the rated signal level and dynamically adjust the two thresholds, so it can provide the optimal result regardless of the signal source. Hush can be used for virtually any sound source, including sound and video tapes, radio and television broadcasts, and other noisy sound sources. Ssm2000 can be used with dolbtb coded signal source to produce good results. One of the main features of this circuit is that it can be directly connected to the port of voltage controlled amplifier, and the functions such as DC amplitude control, automatic level adjustment and compression can be realized by using the smallest peripheral circuit.
The main features of ssm2000 are as follows: noise suppression up to 25dB; single end operation;
adaptive threshold can be adjusted dynamically; it can effectively decode dolbyb sound source; VCA control terminal can be directly connected;
have logic controllable bypass and mute functions; 100dB dynamic range (when noise suppression is off); the typical value of total harmonic noise distortion ratio is 0.02%;
the power supply voltage is 7 18V.Ssm2000 adopts 24 pin small package (SOIC), and its package outline is shown in Figure 1. NC means no "connection".2. Main parameters
Table 1 lists the main electrical performance parameters of ssm2000.3. Working principle and pin function3.1 principle block diagram of ssm2000
Figure 2 shows the principle block diagram of an audio channel of ssm2000. After the input audio signal is processed, the frequency distribution and amplitude information of the required signal and noise can be extracted. The audio signals of the left and right channels are filtered by the voltage controlled low-pass filter and amplified by the voltage controlled amplifier. Filters and amplifiers are low distortion, and the noise caused by them can be ignored. The cut-off frequency of voltage controlled filter (VCF) can be set by the user between 1kHz and 35kHz as required. The VCA can be set to attenuate or amplify, which increases the flexibility of the ssm2000. Appropriate control signals can be obtained by using appropriate algorithms and applied to VCF and VCA. In this way, noise suppression up to 25dB can be achieved while minimizing signal loss.
3.2 pin function descriptionThe pin functions of ssm2000 are as follows:Lin, Rin (pins 1, 2): left and right audio inputs. Both Lin and Rin adopt single ended input, and the input impedance is 10K .
VCF capacitor terminal (left channel pins 3 and 4, right channel pins 21 and 22): including lvcfc1 and 2 and rvcfc1 and 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 of audio circuit. In single power supply applications, this terminal is usually connected to low potential, and the potential should be half of the positive power supply.
Vcacontrolport (pin 7): VCA control end. This end is connected with the voltage controlled amplifier circuit. In addition to controlling the voltage controlled amplifier detector, it can also control the voltage controlled amplifier detector externally, set the amplitude of the voltage controlled oscillator externally and control the volume. Every 22mv voltage applied to the port will produce 1dB amplitude attenuation, and 0dB corresponds to 150mV.Vcfdetin (pin 8): this pin is the input of voltage controlled filter control and noise threshold detection circuit. It is recommended to use 3-pole filter at sumout and vcfin.Sumout (pin 9): this end is the combined output of left and right inputs. The combined output can be used by a filter and a voltage controlled amplifier detector to determine the amplitude and frequency range of the audio signal.
Vcfdettc (pin 10): voltage control amplifier level control input terminal, and the output voltage is used to control the voltage control amplifier in the audio channel. It is recommended to use a single pole filter at sumout terminal and vcadetin terminal.Vcfdettc (pin 11): timing capacitor end of voltage controlled filter, which is usually connected with a 1 The purpose of F is to control the change speed of the cut-off frequency of the voltage controlled filter.Vcadettc (pin 12): timing capacitor of voltage controlled amplifier. The function of this end is similar to that of vcfdettc end. A 33 should be connected to this pin F, which controls the rate of amplitude change of the voltage controlled amplifier. Too high capacitance value will produce breathing noise, and too large capacitance value will produce pumping noise.
Foot 13: No.Defaultthreshold (pin 14): the 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 high noise; Setting a lower threshold can prevent the hush chip from attenuating the already small noise, so as not to reduce the sensitivity of the hush system.Autothresholdcap (pin 15): the capacitor connected at this end is used to control the change rate of adaptive noise threshold level. It is recommended to use 0.22 F capacitance.
Depeat (pin 16): "inhibit" end. Applying a 5V voltage signal at this end will "inhibit" hush noise reduction system. The voltage applied to this terminal shall take the GND terminal as the reference. When using the forbidden terminal, it is recommended to use the standard TTL level for control. Activating the inhibit terminal will cause the voltage control filter detector and voltage control amplifier detector inside the chip to send the maximum control signal to the voltage control filter and voltage control amplifier respectively. If the terminal is prohibited from being controlled by the digital logic line with noise, good filtering measures shall be taken to avoid the interference of digital noise on the audio signal in ssm2000.
Mute (pin 17): Mute end. Applying 5V voltage at this end will mute the ssm2000.Pin 18 (pin 18): unused end.GND: power ground terminal.
V - (pin 20): power supply "negative" end.Rout and lout (pins 23, 24): left and right channel output. The output of hush chip is buffered, so it can drive a certain load. However, for loads exceeding 2K or 300pf (such as speakers or headphones), additional buffer drive shall be used.4. Application circuit
4.1 typical applicationsFigure 3 shows a typical application circuit of ssm2000. Its 13 pins and 18 pins are generally suspended, while the default threshold end (14 pins) is generally connected to the analog common end.The ssm2000 is usually placed in front of the volume control and power amplifier. It should be placed below the signal link as far as possible. Since ssm2000 has the best working effect at 300mV (root mean square value), it should be placed before the volume control and power amplification elements. Figure 4 is a schematic diagram of the position in the system.
The power supply voltage of ssm2000 ranges from 7V to 18V, and a stable 9V power supply can also be used. In fact, the ssm2000 can work when the power supply is as low as 4.5V. However, to ensure good performance, the supply voltage should be within a given range.In dual power applications, the ACOM terminal voltage is connected to system ground.V and V - shall be connected to the corresponding power line respectively, and must be well decoupled. 1 is recommended F aluminum electrolytic capacitor and 0.1 F ceramic capacitor. Both power cords should be so decoupled.
When working in single power mode, a virtual ground potential must be generated and connected to ACOM terminal. As shown in Figure 5. Virtually, it is usually set at the midpoint of the power line. The virtual ground provided for the chip must be able to provide or absorb 10mA current. This function can be realized by using the low-cost op292.4.2 automatic level adjustmentIn the design, ssm2000 has made many considerations to facilitate other applications. Because it can directly access the gain control terminal and amplitude and frequency detector of the voltage controlled amplifier, with some external circuits, ssm2000 can control two audio channels with one DC level without introducing the audio signal to the control panel.
Automatic level adjustment is similar to compression, but it does not attempt to attenuate all large amplitude audio signals, but attenuates the audio signal after the whole signal amplitude increases for a certain time (e.g. 1 second). The application circuit of automatic level adjustment is shown in Figure 6.In fact, the circuit only operates on the signal amplitude, so the input 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 through the time constant RC. After averaging, the signal is sent to the control end of the voltage controlled amplifier.