RTC (real time communications), real-time communication, is a rising tuyere industry. In a short period of one year, many players have entered the industry. The most typical application is live video and audio-video communication. However, many developers still have some confusion about some concepts, such as RTC and webrtc, RTC and live broadcast, RTC and im.
1、 What is the difference between RTC and webrtc?
Real time communication (RTC) is most likely to be confused with webrtc. In fact, they cannot be marked with equal sign.
RTC includes many links such as collection, coding, pre-processing, transmission, decoding, buffering and rendering from the functional flow. The above figure shows a brief flow of RTC communication. Each segment, there are more subdivided technical modules. For example, the pre-processing links include beauty, filter, echo cancellation, noise suppression, etc., and microphone array is collected. VP8, VP9, H.264, h.265 and so on are used for coding and decoding.
The above figure shows the relationship between RTC and webrtc, which is part of RTC. Webrtc is a standard and open source project for real-time communication of web pages. Only the basic front-end function realization is provided, including coding decoding and jitter buffer. If the developer wants to use the project based on webrtc developer, he / she needs to do a series of specific work such as server implementation and deployment, signaling front and rear end selection and deployment, and mobile phone adaptation. Besides, a lot of improvement and polishing are made in terms of availability and high quality, and self-improvement and polishing are also required The threshold of personal development capability is very high. A professional RTC technology service system needs to include not only the above communication links, but also the special communication network to solve the Internet instability, as well as the high tolerance audio and video signal processing algorithm for Internet channel. Of course, the high availability of conventional cloud services, the guarantee of service quality and monitoring and maintenance tools can only be regarded as a basic module of a professional service provider. Therefore, webrtc is only a few small segmentation technology combinations in RTC technology stack, and is not a full stack solution.
2、 What is the difference between RTC and live broadcast?
The above figure shows the relationship between RTC and live broadcast. One of the specific applications of RTC is the live broadcast in live broadcast scenario, namely, low delay live broadcast. In general, the common live broadcast, generally using TCP protocol, using CDN for content distribution, will have a few seconds or even more seconds of delay, the interaction between the host and the audience can only be carried out by text short messages or gifts. While live connected to the wheat, using UDP protocol, content real-time transmission, the host and the audience can interact with audio and video, real-time communication, the delay is generally as low as hundreds of milliseconds.
So what technology does RTC stack contain, we will provide a series of articles to interpret RTC technology stack.
Video coding and decoding of RTC stack
Audio codec of RTC stack
Audio video pre processing of RTC stack
Real time transmission of RTC stack
QoE quality assurance of RTC technology stack
This paper is the first in a series of articles video codec
The current situation of video codec:
The function of video codec is to compress the image and encode the image after the camera collects the picture and preprocess, and uses it for transmission. The advantages and disadvantages of codec are: the compression efficiency, speed and power consumption.
At present, the mainstream video coders are divided into three series: VPX (VP8, VP9), H.26X (H.264, h.265), AVS (avs1.0, avs2.0)
VP8 is the eighth generation video codec standard of on2 technologies, a video compression solution manufacturer. After Google acquired on2, it opened up VP8 and applied it to webrtc. At present, Google is also pushing VP9, a new generation of codec standards.
H. 264, is a highly compressed digital video codec international standard proposed by the joint video group (JVT, joint video team) which is composed of ITU-T video coding expert group (VCEG) and iso/iec dynamic image expert group (MPEG). Webrtc also supports H.264.
VP8 and H.264 are the standards invented more than a decade ago, and belong to the same generation of technology. These two standards are in the mature stage of development, and the coding efficiency, operation complexity and power consumption have reached a better balance. The technology and application level are also slightly different. For example, hardware manufacturers support H.264 widely, while VP8 support is limited.
VP9, development started in 2011. One of the objectives of VP9 is to reduce the bit rate by about 50% compared with VP8 when the same quality is guaranteed. In other words, the same rate of code, VP9 can be significantly improved than VP8. One of the big advantages of VP9 is patent fees, which Google claims can be used for free. This is different from H.264 and h.265 (although Cisco opened open264 in 2013 and claimed that it can cover the relevant patent fees without modifying the open264 code).
H. 265 The purpose is to transmit higher-quality network video with limited bandwidth, and only half the original bandwidth is needed to play the same quality video. It has similar algorithm architecture with H.264, and improves some related technologies to greatly improve video quality. For example, H.264 encoder can achieve the standard clear digital video compression with 1Mbps code rate; and h.265 encoder can encode 720p or higher resolution HD video with the same code rate. This also means that, in the current home network, our mobile devices such as smartphones, tablet computers, etc. will be able to directly play 1080p Full HD video online. At the same time, h.265 standard also supports 4K and 8K Ultra HD video.
VP9 and h.265 are the standards formulated in the last five years, and are the highest compression efficiency of the standards that have been completed. Similarly, h.265 is the international standard, and VP9 is the current standard of Google's main push. H. 265 is widely supported in hardware. Chips such as apple, Qualcomm and Intel all support h.265 hardware codec. VP9 hardware support is still very limited. Generally speaking, the coding efficiency of the new generation encoder can be improved by 30-50% compared with the previous generation, but the complexity and power consumption will be much larger than the previous generation. Therefore, the implementation of pure software coding has certain bottlenecks. Under the existing technology, it is still necessary to rely on hardware coding and decoding.
AVS is the second generation source coding standard with independent intellectual property rights in China. At present, avs1.0 has been widely used in the third world countries. Avs2.0, which is a new generation standard of the same level as h.265 and VP9.
Encoder is just standard and syntax, and does not limit application scenarios. Therefore, in practical application, we should combine the scene characteristics to improve and optimize the depth. Video encoder of sound network has made deep improvement for real-time audio and video communication, which is more suitable for the characteristics of public Internet, and has greatly improved its real-time and quality. Especially, it combines with the depth of the network, and at the same time, it also takes into account the fluctuation of packet loss and network bandwidth.
The exploration direction of video codec:
1. VR video standard
VR video standard is one of the hot research directions in both academic and commercial applications. At the RTC conference in 2016, we invited Professor Wang Ronggang to share VR video content generation technology and coding standards. Professor Wang Ronggang is currently the joint leader of the international MPEG Internet video compression standards group and the leader of IEEE virtual reality video content coding standard group.
According to Professor Wang Ronggang, the technical problems that VR video coding continues to solve are: image display quality, synthesis quality and transmission bandwidth.
VR video coding has been used in VR scene by applying the existing video compression standard. However, due to the particularity of VR video content and the limitation of network bandwidth, the current standard can not meet the compression requirements of VR video. The industry has a high voice for VR video compression standards. In the future, the advanced VR video form should be free immersion stereo video: provide anywhere + anytime + anyview + stereo immersion experience within a certain space.
2. high resolution requirements
In the H.264 era, encoder is mainly used in the medium and small resolution lower than HD, and it takes into account 1080p high resolution slightly.
But in the h.265 era, with the better hardware equipment and higher bandwidth, users began to require higher video resolution. People began to find that the requirements for video quality are endless. Therefore, the new generation encoder is more inclined to support high resolution, such as 4K high-resolution. The compression efficiency of the new generation encoder can be increased by more than 50%.
RTC 2017 At the third real-time Internet Conference, technical experts from Huawei, Google, AVS video group, AVS audio group, AVS test group, slack, houseparty and Atlas (JIRA) attended the meeting to share the exploration and application of the next generation coding standard. This is basically the most complete technology sharing party in China. Of course, there are other modules of RTC technology stack. They are committed to entering the market RTC developers can't miss it.
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How far(long) the transmitter cover?
The transmission range depends on many factors. The true distance is based on the antenna installing height , antenna gain, using environment like building and other obstructions , sensitivity of the receiver, antenna of the receiver . Installing antenna more high and using in the countryside , the distance will much more far.
EXAMPLE 5W FM Transmitter use in the city and hometown:
I have a USA customer use 5W fm transmitter with GP antenna in his hometown ,and he test it with a car, it cover 10km(6.21mile).
I test the 5W fm transmitter with GP antenna in my hometown ,it cover about 2km(1.24mile).
I test the 5W fm transmitter with GP antenna in Guangzhou city ,it cover about only 300meter(984ft).
Below are the approximate range of different power FM Transmitters. ( The range is diameter )
0.1W ~ 5W FM Transmitter ：100M ~1KM
5W ~15W FM Ttransmitter : 1KM ~ 3KM
15W ~ 80W FM Transmitter : 3KM ~10KM
80W ~500W FM Transmitter : 10KM ~30KM
500W ~1000W FM Transmitter : 30KM ~ 50KM
1KW ~ 2KW FM Transmitter : 50KM ~100KM
2KW ~5KW FM Transmitter : 100KM ~150KM
5KW ~10KW FM Transmitter : 150KM ~200KM
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1.How far you want to cover in diameter ?
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