This article refers to the address: http://
The increasing popularity of mobile phones has become an indispensable communication tool in life. Moreover, the function of the mobile phone is not limited to the call. The mobile phone has already become a symbol of the fashion trend and a carrier of audio and video entertainment. MP3, FM radio, mobile TV, digital photography, etc. have gradually become an indispensable function of the high-end mobile phone. In particular, the launch of music phones puts higher demands on the sound quality of mobile phones. However, in the design of mobile audio, Pop noise has always been one of the problems that plague engineers. This article will separately explain the generation mechanism of Pop noise for mobile phone speakers and stereo headphones, and introduce the solutions proposed by STMicroelectronics for these problems.Pop noise
Most audio devices for mobile phones use a single power supply because it simplifies power supply design and extends standby time. At this time, the input signal and output signal of the audio amplifier are often superimposed with a DC voltage bias of 0.5 times the power supply voltage. Therefore, the input voltage (Cin) needs to be added to the audio input terminal to filter out the bias voltage (Vbias) of the sound source; when the single-ended connection is used at the output end, the output capacitor (Cout) is also required to filter out the bias voltage to avoid it. Additional power is generated across the speaker. In addition, a bias capacitor (Cbias) is needed to stabilize the bias voltage while increasing the voltage rejection ratio of the audio amplifier.
Pop noise refers to the blasting sound generated by the power supply charging the input capacitor, the bias capacitor or the output blocking capacitor at the moment of power-on, power-off, and standby switching. Instantaneous charging of the capacitor tends to produce a spike at the output that is applied across the speaker. As shown in Figure 1, the current is generated and the magnetic lines are cut to create a force that causes the speaker to emit Pop-click noise.
Pop noise solution for mobile phone speakers
The TS4990 is a mono Class AB amplifier with 1.2W output capability from STMicroelectronics. The typical application schematic is shown in Figure 2. Figure 2a and Figure 2b are single-ended and differential connections, respectively, and the outputs are BTL. Not only can the output power be increased, but also the output blocking capacitor can be eliminated. In the figure, the input capacitor Cin and the bias capacitor Cb can be seen. The reason for the Pop noise is the charging sequence of the two capacitors.
When the amplifier is started, the bypass capacitor Cb, which is the internal bias voltage filter of the audio amplifier, is linearly charged at a constant rate. As shown in Figure 3, the amplifier starts to work normally when the voltage across Cb is charged to 0.5 times the supply voltage. This charging time is the start time of the audio amplifier. There is no signal output at the amplifier output during the start-up time. At the same time, the input capacitor Cin is also charged at an exponential rate, but the gain resistor Rin is connected in series in the charging loop, and the gain resistor is often several tens of kΩ, which greatly prolongs the charging time. If Cin completes charging during the startup time, there will be no Pop noise problem, but if Cin does not complete charging during the startup time, Pop noise will be generated, as shown by the red line in Figure 3. This is because the bypass capacitor Cb has been fully charged, and the normal working audio amplifier amplifies the Cin charging signal to the amplifier output, producing a noise voltage as shown in FIG.
In order to eliminate the Pop noise, choose the appropriate Rin, Cin, Cb values, to ensure that Cin has completed charging before the Cb charging is completed, that is, the time constant tin of Cin should be much smaller than the startup time corresponding to Cb: < Figure 4 is the maximum startup time and Cin time constant corresponding to Cb, which can be used as a reference when selecting a capacitor. However, the input capacitor and the input resistor Rin form a high-pass filter. Usually this frequency is set at 100 Hz or higher. Some high-quality audio handsets will limit them to a few tens of hertz in order to couple lower frequency sounds. Therefore, the minimum input frequency of the gain and audio signals should be considered in the design. The cutoff frequency calculation method is as follows.
If you can follow the above rules when designing the circuit and selecting the device, the TS4990 can achieve close to zero Pop noise even with a large gain. The TS4990 exhibits excellent Pop noise rejection when compared to a typical audio amplifier. In addition to the design on the peripheral circuit, a software method can be used to eliminate the Pop noise. Audio amplifiers usually have a Standby pin. When playing music or voice, you can first put the audio amplifier in the Standby state, then input the audio signal, and then turn on the audio amplifier after tens of milliseconds, which can effectively eliminate Pop noise.
Another point to note is that differential input audio amplifiers have excellent noise immunity, such as ST's Class AB audio amplifier TS4994 and Class D TS4962. However, when selecting the input resistance and input capacitance of the differential circuit, pay attention to the problem of precision matching between the two resistors and the capacitor. Try to use high-precision resistors and capacitors when selecting. If the accuracy is not high, it is easy to bring Pop noise problems.
Pop noise solution for stereo headphones
Stereo headphones in mobile phones have a load impedance of about 32 Ω per channel and a small output power. Therefore, the audio amplifier output for headphones is a single-ended class AB amplifier, which is also a single-supply power supply. TS488/9 is a STMicroelectronic stereo headphone driver. It has a Pop noise suppression circuit. The input capacitor, bias capacitor and output capacitor are both present. Pop noise comes from two places, firstly due to the charging of the output capacitor, followed by The charging sequence of the input capacitor and the bias capacitor is very similar to the TS4990. The following focuses on the Pop noise problem caused by the output capacitance.
Unlike audio amplifiers that output as bridge-connected (BTL), single-ended connections require the addition of a large DC-blocking capacitor Cout to eliminate Vbias, otherwise Vbias can cause DC losses on the headphones, reducing efficiency and even damaging the headphones. During power-on, the output signal of the amplifier is instantaneously charged to Vbias for Cout; when the power is off, the voltage across the capacitor is discharged from Vbias to 0V. The waveform is shown in Figure 5. The instantaneous charge/discharge of the output capacitor will be Pop noise is generated on the headphones.
At the same time, since Cout and the load form a high-pass filter (the following formula), the size of Cout directly determines the cutoff frequency (fc). In order to achieve better bass performance, it is generally greater than 100mF. Such a large capacitance makes the Pop noise more and more obvious, and the size of the Pop noise becomes an important indicator for evaluating the sound quality of the mobile phone.
In order to eliminate the Pop noise caused by Cout, the precharge and discharge of Cout is generally adopted. The output capacitor is precharged in the initial stage by an ST single-pole double-throw analog switch STG3155, and the charging time is adjusted by setting the resistor R and the capacitor C. To avoid charging signals for Cout falling into the audio range (20kHz~20kHz). After charging is complete, turn this switch off. Experiments show that this method can effectively eliminate the Pop noise caused by the DC blocking capacitor.
In actual use, since the output capacitor Cout has a large capacitance value, the area and cost of the PCB board are increased, and the low frequency performance of the audio signal is reduced. STMicroelectronics continues to introduce new audio amplifiers to eliminate output capacitance. The new TS4909 stereo headphone amplifier adds "virtual ground" characteristics to the TS488. The so-called "virtual ground" means that the output intermediate end is not the real ground, but 0.5 times the power supply voltage, so that it is no longer needed. The Cout at the output filters out the bias voltage. The TS4909 eliminates two large-capacity output capacitors, completely eliminating the Pop noise problem caused by Cout, and reduces PCB area and cost.
to sum up
It can be seen that the Cin, Cb and Cout of the front end of the audio amplifier may become the cause of Pop noise. In the process of designing the mobile phone, the cause of Pop noise should be analyzed first, and then different methods are adopted. Whether it is a mobile phone speaker or a stereo earphone, Pop noise can be effectively suppressed by selecting the appropriate device and properly configuring the peripheral components.
The three defense lamps are: waterproof, dustproof and anticorrosion. The special anti oxidizing and anticorrosion materials and silica gel seal ring are used to realize the protection requirements of the lamps. The lamp on the circuit control plate corrosion, waterproof, anti oxidation treatment, the electrical box sealing characteristics of cooling ability is weak, the wisdom of three proofings lamp special temperature control circuit, reduces the power converter working temperature of electrical isolation protection, connectors for double insulation treatment, to ensure the safety and reliability of the line. According to the actual working environment of the three prevention lamps, the surface of the lamp protection box is treated with Nano Spray and moistureproof, and the entry of dust and water is eliminated.
Tri-Proof Led Light,Double Tube Tri-Proof Light,Led Tri-Proof Light,Waterproof Led Tri-Proof Light,IP65 Led Tri-Proof Light
Luxon Optronics Technology Jiansu Co.,Ltd , http://www.luxonim.com