With the development of electronic communication technology and the improvement of people’s safety awareness, building intercom products have entered millions of households and have been widely used in intelligent residential communities, bringing people security, comfort, and convenient communication services. The building intercom hands-free system is composed of a doorway machine and several indoor units. The system is generally welcomed by users because of its convenient installation, simple operation, stylish design and close to life. As the hands-free intercom design is affected by the interference of environmental background noise, it is different from the audio signal received with the handset with the handle, according to the industry standard GA/T678-2007 "Networked Video Intercom System Technology The requirements for the audio indicators in the "Requirements" show that the overall loudness rating of the main call channel should reach 21±5dB, and the corresponding evaluation value of the response channel should be 18±5dB. As the most basic function of hands-free building intercom, intercom refers to the ability to make two-way conversations with occupants when visitors come in. The voice requires clear, coherent, unobtrusive and self-excited voices. This article proposes a quasi-duplex audio circuit design scheme between the full-duplex and half-duplex design for the intercom effect of the building intercom hands-free system, which can effectively eliminate the whistle during the full-duplex intercom process. Phenomenon, improve the intercom effect, and lower than the digital full-duplex audio processing cost.
Building audio intercom several audio processing methods Most building intercom manufacturers in the market use RS-485 bus to balance the communication transmission. The network topology adopts the terminal-matching bus structure. Its characteristic lies in that RS-485 can support multiple nodes. Distance transmission, high transmission rate, using a combination of balanced drivers and differential receivers, enhanced anti-common-mode interference capability, anti-noise and interference, and can achieve point-to-multi-way intercom functionality. At present, there are several audio processing methods used in residential building intercoms:
The transmission and reception of data in the full-duplex talkback mode are separately distributed by two different transmission lines, and both parties can perform transmission and reception operations at the same time. The full-duplex mode eliminates the need for directional switching, and there is no time delay caused by the switching operation. The door-to-door and indoor units of the building intercom system can implement two-way simultaneous talkback with clear voice. However, it has the disadvantage that the size of the sound is limited, and it is easy to generate a whistle due to the feedback of the sound, which affects the effect of the received sound.
Half-duplex intercom mode dedicated chip 34018 or 34118 is used to implement door-to-door and indoor unit intercom. That is, the same transmission line is used for both reception and transmission. Although data can be transmitted in both directions, the communication parties cannot transmit and receive data at the same time. In the half-duplex mode, the transmitters and receivers at each end of the communication system are switched to the communication line through the receiver/sender switch for direction switching, so the half-duplex intercom effect is loud, but the sound is transmitted. There is a significant delay phenomenon, and due to the high level of chip integration, sound effects are not as good as discrete devices, so there is distortion in the voice.
Using digital audio processing technology using digital audio processing technology, with noise cancellation and howling suppression function, can achieve high-definition video voice duplex talkback, door-door and indoor unit simultaneous talk and clear voice, and more for digital network transmission In the IP building intercom system, its advantages are clear and fluent audio, no external electromagnetic interference, unlimited transmission distance, and a network where the intercom can be realized; the disadvantage is that the intercom equipment has a high cost and is only suitable for network transmission. Connected and affordable high-end users.
Design Principles The quasi-duplex intercom method described in this article is different from the above three methods. It has the advantages of low cost, large volume, clear voice, etc. The actual audio indicator test results can reach 19.6 dB overall loudness rating, and overcome The traditional half-duplex intercom voice chip transmission delay phenomenon. The specific principle is described as follows:
In the original full-duplex intercom mode, the MIC and SPK of the door machine and the MIC and SPK of the indoor unit are turned on at the same time during the intercom, which results in feedback when the sensitivity of the electroacoustic device MIC and SPK is large. This phenomenon leads to howling, but at this time as long as one of the four electroacoustic devices does not participate in the work, the feedback is interrupted and the howling disappears. Therefore, the idea of ​​this paper is to increase the sensitivity of four electro-acoustic devices as much as possible in a full-duplex intercom circuit. At the same time, two different points are taken out in the audio transmission path of the door machine and the indoor unit (MIC, SPK). A) After impedance isolation, amplification, and shaping, send it to the CPU for sampling, digital filtering, and comparison to confirm whether the MIC or SPK is in the working state at this time, and the CPU automatically switches the control, and the effective party enters the working state. The invalid party Enter the closed state. Since only one of the four electro-acoustic devices does not work, the whistle phenomenon will not be caused. Therefore, in the actual intercom, the CPU only needs to control the electro-acoustic devices (MIC and SPK) on the door opening or indoor unit side to switch.
The quasi-duplex design system consists of a hands-free building intercom system including a doorway machine and a number of indoor units. The door machine includes a doorway MIC transmission path and a doorway SPK transmission path. Each indoor unit includes an indoor unit MIC transmission path and an indoor unit SPK. The transmission path, the door machine or all the indoor units also includes a CPU which is connected to the MIC transmission path, the SPK transmission path, and controls one of them to shut down. In the MIC transmission path, an impedance isolation circuit, an amplification circuit, and a shaping circuit are also connected in sequence between the SPK transmission path and the CPU.
Conclusion Since the intercom system of the quasi-duplex design is based on the original full-duplex system, after entering the intercom state, the electro-acoustic device of one side of the call is fully working, and the electro-acoustic device of the other side enters the switching state. In this way, the feedback whistle phenomenon is avoided, and the advantages of the original full-duplex intercom voice are clear and there is no delay, and the voice size is increased to a height that the half-duplex dedicated chip is difficult to reach. In the case of both parties talking at the same time, there will be no phenomenon of half-duplex chip sound blocking, but which side is louder in the call process. The design scheme has cost advantages and will be more widely used in building intercom systems.
Building audio intercom several audio processing methods Most building intercom manufacturers in the market use RS-485 bus to balance the communication transmission. The network topology adopts the terminal-matching bus structure. Its characteristic lies in that RS-485 can support multiple nodes. Distance transmission, high transmission rate, using a combination of balanced drivers and differential receivers, enhanced anti-common-mode interference capability, anti-noise and interference, and can achieve point-to-multi-way intercom functionality. At present, there are several audio processing methods used in residential building intercoms:
The transmission and reception of data in the full-duplex talkback mode are separately distributed by two different transmission lines, and both parties can perform transmission and reception operations at the same time. The full-duplex mode eliminates the need for directional switching, and there is no time delay caused by the switching operation. The door-to-door and indoor units of the building intercom system can implement two-way simultaneous talkback with clear voice. However, it has the disadvantage that the size of the sound is limited, and it is easy to generate a whistle due to the feedback of the sound, which affects the effect of the received sound.
Half-duplex intercom mode dedicated chip 34018 or 34118 is used to implement door-to-door and indoor unit intercom. That is, the same transmission line is used for both reception and transmission. Although data can be transmitted in both directions, the communication parties cannot transmit and receive data at the same time. In the half-duplex mode, the transmitters and receivers at each end of the communication system are switched to the communication line through the receiver/sender switch for direction switching, so the half-duplex intercom effect is loud, but the sound is transmitted. There is a significant delay phenomenon, and due to the high level of chip integration, sound effects are not as good as discrete devices, so there is distortion in the voice.
Using digital audio processing technology using digital audio processing technology, with noise cancellation and howling suppression function, can achieve high-definition video voice duplex talkback, door-door and indoor unit simultaneous talk and clear voice, and more for digital network transmission In the IP building intercom system, its advantages are clear and fluent audio, no external electromagnetic interference, unlimited transmission distance, and a network where the intercom can be realized; the disadvantage is that the intercom equipment has a high cost and is only suitable for network transmission. Connected and affordable high-end users.
Design Principles The quasi-duplex intercom method described in this article is different from the above three methods. It has the advantages of low cost, large volume, clear voice, etc. The actual audio indicator test results can reach 19.6 dB overall loudness rating, and overcome The traditional half-duplex intercom voice chip transmission delay phenomenon. The specific principle is described as follows:
In the original full-duplex intercom mode, the MIC and SPK of the door machine and the MIC and SPK of the indoor unit are turned on at the same time during the intercom, which results in feedback when the sensitivity of the electroacoustic device MIC and SPK is large. This phenomenon leads to howling, but at this time as long as one of the four electroacoustic devices does not participate in the work, the feedback is interrupted and the howling disappears. Therefore, the idea of ​​this paper is to increase the sensitivity of four electro-acoustic devices as much as possible in a full-duplex intercom circuit. At the same time, two different points are taken out in the audio transmission path of the door machine and the indoor unit (MIC, SPK). A) After impedance isolation, amplification, and shaping, send it to the CPU for sampling, digital filtering, and comparison to confirm whether the MIC or SPK is in the working state at this time, and the CPU automatically switches the control, and the effective party enters the working state. The invalid party Enter the closed state. Since only one of the four electro-acoustic devices does not work, the whistle phenomenon will not be caused. Therefore, in the actual intercom, the CPU only needs to control the electro-acoustic devices (MIC and SPK) on the door opening or indoor unit side to switch.
The quasi-duplex design system consists of a hands-free building intercom system including a doorway machine and a number of indoor units. The door machine includes a doorway MIC transmission path and a doorway SPK transmission path. Each indoor unit includes an indoor unit MIC transmission path and an indoor unit SPK. The transmission path, the door machine or all the indoor units also includes a CPU which is connected to the MIC transmission path, the SPK transmission path, and controls one of them to shut down. In the MIC transmission path, an impedance isolation circuit, an amplification circuit, and a shaping circuit are also connected in sequence between the SPK transmission path and the CPU.
Conclusion Since the intercom system of the quasi-duplex design is based on the original full-duplex system, after entering the intercom state, the electro-acoustic device of one side of the call is fully working, and the electro-acoustic device of the other side enters the switching state. In this way, the feedback whistle phenomenon is avoided, and the advantages of the original full-duplex intercom voice are clear and there is no delay, and the voice size is increased to a height that the half-duplex dedicated chip is difficult to reach. In the case of both parties talking at the same time, there will be no phenomenon of half-duplex chip sound blocking, but which side is louder in the call process. The design scheme has cost advantages and will be more widely used in building intercom systems.