Design of DSP Intelligent Active Noise Canceling Headphones

With the rapid development of modern science and technology, in life, people will encounter all kinds of noise. Noise can cause irritability, and if the volume is too strong, it will endanger human health. The most direct harm is hearing loss. . Therefore, for these unavoidable noises, how to effectively and actively reduce noise is particularly important. Due to its fast processing speed, high parallelism, flexibility, and high programmability, DSP is the most widely used solution for baseband signal processing in recent decades.

The high-speed interface of DSP is used to connect the devices inside and outside the board. It is mainly used for high-speed data communication. It has the advantages of low price, low energy consumption, good noise reduction effect, and low distortion. The active noise reduction technology starts from the noise itself , proactively eliminates distractions. Now we plan to design a DSP-based active noise canceling earphone. In terms of wearing comfort and portability, compared with the large closed earmuffs of the old generation of noise canceling earphones, it has been greatly improved, and the sound quality of the original earphones is retained. When the noise reduction function is not used, it also has a high level of sound quality. According to the DSP technology, the noise is converted into an electrical signal, and the noise signal is reversed by an arithmetic circuit to obtain a reverse signal (phase reversed 180°) to neutralize the noise. In this process, the noise signal is first collected, and the analysis of different frequency signals and the generation of synchronous anti-phase signals are completed at the same time to ensure its "real time". In addition, many current noise-cancelling headphones are improved on the basis of the original finished music headphones, which cannot retain the sound quality of the original headphones to a certain extent. When the noise-cancelling function is not used, the sound quality is mediocre. However, the DSP-based active noise reduction earphones still guarantee the sound quality on the basis of intelligent active noise reduction. Negative effects are minimized. Intelligent active noise reduction earphones are mainly aimed at noisy environments, such as airplanes, trains, subways, buses and other occasions with continuous noise.

1. Module design of intelligent active noise reduction headphones

The core circuit of the intelligent active noise canceling earphone is composed of a music signal input module, a microphone signal input module, a microphone sound selection module, and a microphone signal supply module, as shown in Figure 1.

According to the DSP technology, the noise is converted into an electrical signal, and the noise signal is reversed through the arithmetic circuit to obtain a reverse signal (phase reversed 180°). After collecting the noise signal, the analysis of different frequency signals and the generation of synchronous reverse phase signals are completed at the same time This step achieves the purpose of neutralizing the noise, reducing the noise and ensuring its "real time". In this solution, the adjustment of each gain is very precise, because it uses a microphone amplifier with excellent performance, and it uses a low-power operational amplifier inside. When used with an external filter, it can accurately restore and Compensates for some distortion caused by the external environment.

Figure 2 shows the noise spectrum of an aeroengine. The main component of the noise is its low-frequency noise. However, in the aviation industry, physical sound insulation is used to reduce the harm of engine noise by relying on sound-proof earmuffs with a certain thickness, but usually The effect is limited. For engine noise above 800Hz, the sound insulation earmuffs can show a good suppression effect, but for low-frequency noise below 800Hz, especially below 400Hz, the sound insulation effect is poor. Compared with the above data and results, when reducing the noise of low-frequency signals, it is the best method to use active noise reduction technology to eliminate the low-frequency noise of the engine. When a new noise pops up, the microphone placed on the speaker inside the sound-isolating earcup comes into play.

Figure 3 is a schematic diagram of the feedback active noise reduction, the microphone can sample the noise heard by the human ear. After being reversed and amplified, the speaker is driven to produce an "anti-noise" to counteract the noise introduced from the outside through the earmuffs. At the same time, the system must have a high signal-to-noise ratio to avoid self-oscillation and software calculation necessary delay. It has the advantages of low power consumption, high sound quality and high integration. As long as the gain of the amplifier is selected reasonably, good noise cancellation effect can be obtained.

As shown in Figure 4, the process from the start of the noise-canceling earphones to the hearing of the human ear can be divided into the following steps: First, after the calculation of the internal circuit, the earphones can be heard by the miniature microphone placed in the earphones. Medium/low frequency noise in the environment (such as 100Hz ~ 1000Hz); Next, the noise signal is transmitted to the noise reduction circuit, and the noise reduction circuit performs real-time calculation; after the processing of the noise reduction circuit is completed, the signal generated by the Hi-Fi speaker is transmitted Sound waves with the opposite phase and the same amplitude as the noise cancel out the noise, allowing the ear to hear the original music. After mastering the basic principle of active noise reduction, use CAD software to draw the internal structural circuit, design the PCB board, complete the welding of the circuit board, and realize the assembly of the active noise reduction ear.

2. Headphone noise reduction test results and analysis

By comparing the voice signal processed by physical sound insulation and the voice signal processed by active noise reduction technology and performance analysis, as well as data analysis, draw the corresponding curve and output the analysis results. Figure 5 is a grouping of a large amount of data, and the signal-to-noise ratio within a period is taken as the value of this sampling point. For a set of 200 data, calculate the signal-to-noise ratio for each set of data, and collect 50 points. The data obtained in this way can make the signal-to-noise ratio curve in the figure more stable.

In FIG. 5 , the overall signal-to-noise ratio of the signal processed by the active noise reduction technology is 15.5 dB and the overall signal-to-noise ratio of the signal processed by the physical sound insulation is 9.5 dB. It can be seen that the overall signal-to-noise ratio has been improved by 6dB after being processed by active noise reduction technology. It can also be seen from Figure 5 that the maximum increase in the signal-to-noise ratio is 15dB. The analysis of the signal-to-noise ratio performance index shows that it can make a smooth transition when the environment changes and improve the quality of the output signal.

In conclusion

The overall signal-to-noise ratio of the signal processed by the active noise reduction technology is obviously improved compared with the overall signal-to-noise ratio of the signal processed by physical sound insulation, and the maximum signal-to-noise ratio of the signal is significantly improved, which proves that the active noise reduction technology It can make a smooth transition when the environment changes, so that the quality of the output signal can be improved, which in turn shows that its noise cancellation performance is effective.

The above is the development method of noise-cancelling earphones introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. for you. If you have design requirements for Bluetooth noise-cancelling headphones, you can trust us. We represent a variety of single-chip microcomputers, voice chips, dual-mode Bluetooth ICs, and wifi chips. Brands include Songhan MCU, Yingguang MCU, Jerry Bluetooth, Ankai Bluetooth, Allwinner, and Realtek. Our technical services include: PCB design, microcontroller development, Bluetooth solutions, software and hardware custom development, APP development, small program development, WeChat official account development, etc. It can also undertake the design of intelligent electronic products, the development of living appliances, the research and development of beauty equipment, the application of Internet of things platform, the smart home control system, the development of TWS earphones, Bluetooth earphone speakers, the development of children's educational toys, the design of electronic education products, etc.