Five Key Points of Mobile Remote Monitoring System Technology

The mobile phone remote monitoring system mainly involves 5 major aspects, namely, the core video coding and decoding, network transmission, UI design, service end (mobile phone streaming media) and the combination with other systems.

The general process of browsing real-time video image on mobile phone is that the mobile client initiates a video preview request to the mobile streaming media, informing streaming media that the current client wants to browse all the way video, streaming media server to connect the front-end remote DVR/DVS to get its child The code stream data forwards and transmits QCIF picture quality video data to the mobile phone. The client software calls the decoding library to decode the received video data, and finally displays the interface through DirectShow.

Video codec

Considering what type of coded video stream is H.264 or MPEG4 or other formats of video data, the general video monitoring equipment transmits H.264 data with high compression ratio. Determine the video data encoding type is easy to handle, then go find its corresponding codec library, generally transplant open source ffmpeg to WM for optimization (has been done, we can directly Google find the corresponding source code), Transplanting its mpeg4 sp/h.264 decoder can support 32K, CIF, and 5-10fps without any optimization. It is sufficient for general streaming media applications. It will also be optimized through algorithms and compilations. After the decoding also need to go through yuv2rgb and scale, it should be noted that ffmpeg decoding has a blanking area, that is QCIF image linesize is not 176 but 192, if you find the decoded image is green, need to use img_convert () turn For a moment (the destination format is also PIX_FMT_YUV420P). Symbian uses DSA to write directly to the screen. Wndows Mobile can use sdl. Audio decoding mainly includes AAC, AMRNB, AMRWB. AAC and AMRNB are audio with gprs and edge bandwidth support (aac is better than amrnb), and AMRWB is a 3G audio format. Alphab/wb fixed point decoding has been supported in ffmpeg 0.5 release, which is very powerful. Or extract corresponding code from TCPMP player inside, TCPMP has N kinds of codecs available, among them there is H.264, the decoding efficiency is good, can learn.

network transmission

Whether the video data adopts TCP or UDP? Is the custom communication packet or RTSP/RTP/RTCP encapsulation? The streaming media network transmission must meet high bandwidth, low transmission delay, support multicast mode, and error recovery. The reliable guarantee and channel synchronization (especially the synchronization of video and audio streams). RTP/RTCP is a multicast-based application layer protocol and the most widely used protocol for streaming media transmission. The Realtime Transport Protocol (RTP) works in one-to-one or one-to-many transmission scenarios. Its purpose is to provide time information and achieve stream synchronization. A typical application of RTP is based on UDP, but it can also work on TCP or ATM protocols. RTP itself only guarantees the transmission of real-time data. It does not provide a reliable delivery mechanism for the sequential transmission of data packets, nor does it provide flow control or congestion control. It relies on RTCP to provide these services. Realtime Transport Control Protocol (RTP): Responsible for managing transmission quality and exchanging control information between current application processes. During the RTP session, each participant periodically transmits RTCP packets. The packets contain statistics such as the number of sent packets and the number of lost packets. Therefore, the server can use this information to dynamically change the transmission rate, even Change the payload type. RTP and RTCP can be used together to optimize transmission efficiency with effective feedback and minimal overhead. Therefore, it is particularly suitable for real-time data transmission on the Internet.

RTSP is currently the mainstream standard for streaming media transmission. Even Microsoft has abandoned MMS and turned to support RTSP. RTSP can support operations such as suspending playback of the client and stopping, basically eliminating audio and video synchronization issues (because audio and video are from different RTP PORTs. Read in buffer). It is worth noting that, after the success of RTSP, RTP transmission is started, divided into RTP OVER TCP and RTP OVER UDP. The former guarantees that each packet can be received. If it is not received, it is retransmitted, and it does not need to consider firewall NAT; Only guarantee the best transmission, will not retransmit dropped frames, real-time, to solve the firewall and NAT problems, because 60% of the world's GSM operators do so, so streaming media server can not connect to the phone. If mobile phone TVs with a relatively high frame rate are required, UDP transmission is recommended because delaying large retransmission data is meaningless to users and should be discarded. If you decide to use RTP/PTSP, the network part can use the powerful open source library live555 to implement the RTSP/RTP protocol, which is stable and supports the transmission of most audio and video formats. (Of course, ffmpeg also implements the network transmission part, and can also be used after modification) Live555 can be ported to symbian and windows mobile after being cut.

Now mobile Internet, the network transmission rate is generally not a problem, 2.75G EDGE network has a high speed (up to 236 kbps, enough for QCIF video picture quality transmission) and low energy consumption, it is understood that similar to GPRS. Current 3G modules consume more power. With the promotion of 3G in the future, as well as news that China Mobile’s TD-LTE (4G) will enter commercial use in 2010, downloading a 164 megabyte movie will take less than 2 minutes, while a typical 300 megabyte movie will only have It can be downloaded in 3 to 5 minutes. In this regard, the personage inside course of study introduces, 4G can reach the speed of more than 100 trillion, it is a qualitative leap for 3G again. If 3G is the national road, 4G is the highway. For the biggest difference between 4G and 2G and 3G, the technical personnel introduced that in addition to being faster than them, video surveillance and video call effects will be smoother and higher-definition. Watch high-definition video on the Internet, do not worry about the screen card or sound and images are not synchronized ... Compared with 3G, 4G bandwidth can reach more than 170M, its download speed is 80 times faster than 3G.

UI

The user is very concerned about the interface of the mobile phone software. If he does a good look, he will feel that he has a technical content. If he does a good job, he will like our product more. So a good set of UI is essential. Most of the mobile software development projects are doing UI systems. A good set of independent mobile phone software UI system is part of the core competitiveness of the product. In the Windows Mobile interface development, using the C + SDK to do a beautiful interface is not easy. Once there are many controls on the interface, the layout of the control is a headache. When screens are switched, they must be considered. The screen size of different mobile phones may not be the same. Under different fonts, the interface differences are also very large.

In fact, to make a good interface, it is still necessary to return to RECT. If you want to do very beautiful, it is still their own package of a set of interface controls, so easy to control. Horizontal and vertical screen problems, you should not write dead position coordinates, you should take the relative coordinates. In the horizontal and vertical screen switching will trigger WM_SIZE and other messages, which change the relative size of the horizontal and vertical screen is OK. Do interface recommended a MFC extension, Xtreme ToolkitPro, there are a large number of classes, you can refer to their class to write Your own control. This is the status quo, no way, it will be more difficult at the beginning. After the accumulation has its own set of control libraries, the development speed will increase.

Development and application of each method have their own advantages, not the best, only more suitable. For specific applications, select the technology that best suits you and your own familiar technology.

Server

As a service-side mobile streaming media, comprehensive considerations are taken from various performance indicators such as security, stability, and concurrency to provide a good video service access for the client; it mainly has the following functions: One of the basic functions is authentication, authentication The login request client has those permissions, you can browse the video, can PTZ control, on-demand playback of those files and other core functions is to effectively manage resources, such as video sources, the current login user, etc.; Applications communicate interactively, passing data; such as passing alarm events to the client.