Double 11 Cat Night is a well-known variety show, and this year, Alibaba Group provided a live visual feast for 25 million users. Netizens comment that this is a stable and smooth hd live broadcast, of course, behind this can not be separated from the technical support of Ali Cloud.
In this Tmall evening party, the video cloud adopted 4K and 50 frame technology for the first time, raising the overall picture quality close to the naked eye limit, while providing users with a silky live experience.
So how did the live broadcast of such a large event come into being? Aliyun video cloud technology expert Qiu Liangke took us from the stability, picture quality, fluency, monitoring four aspects began to interpret.
How to achieve 100% stability?
“The safest thing to do is to be 500 percent prepared,” qiu said.
The following figure shows the technical architecture of Double 11 live broadcast, which is divided into several parts: live broadcast source station, video live broadcast center, CDN distribution system and client. Simply look at this figure, all links are double – backup. In the source station of live broadcast, strategies such as multi-line stream collection, master and standby transcoder, and multi-line special line are adopted. The live broadcast center is connected to multiple computer rooms, and then multi-stream combination is adopted. When any problem occurs in any computer room, the output live stream will not be affected. After that, the live streaming center will transcode, record, intelligent processing, slice, time-shift playback and other processing of the live streaming. All modules in the middle are dedicated resource pools for large-scale activities, ensuring that they will not be affected by other activities. If any module is abnormal, the switch can be performed in seconds. The generated content is stored before distribution and is in primary/secondary mode. In the distribution process, the quality of the source station will be checked in real time and the switch will be carried out.
Under such architecture, the failure of any single point, single room, single line or single module will not lead to the unavailability of live broadcasting service, which is almost absolutely safe.
Of course, in addition to its own stability, security is also important. In terms of security, the video cloud adopts multiple authentication, IP blacklist and whitelist, playing format/region /IP restrictions, HTTPS, and anti-hijacking capabilities in the links of pushing, playing, and pulling streams to ensure full-link security.
How to let users enjoy the ultimate picture quality?
I. Real-time 4K live broadcasting
As an important indicator to measure user experience, video clarity is also the focus of video cloud technology team. This time, the video definition of The Cat night is upgraded again. Aliyun live service provides real-time 4K live broadcasting, and real-time processing of live 4K ultra HD and high frame rate video improves the picture quality. In 4K video processing, the live broadcast service uses GPU for video processing and transcoding on a large scale, which greatly improves the real-time video processing capacity and ensures the highest 4K HEVC real-time transcoding of live video. It is reported that 4K HD live streaming has been widely used in many game live streaming clients of Ali Cloud.
Two, 50 frame extremely clear
The 50-frame Ultra Clear technology jointly developed by Aliyun and Youku can predict the direction and trajectory of motion through artificial intelligence algorithms, and transform the original ordinary TV signal of 25 frames per second into high frame rate video content of 50 frames per second, providing users with a more fluid and immersive viewing experience.
At 50 FPS, it’s like going to the cinema to watch a big movie, but there’s no pause even when the action is very rich. This summer’s World Cup and this double 11 cat night both use 50 frame technology, visually very smooth.
Three, the best ratio of bit rate (bit rate)
In addition to 4K technology, content-based coding optimization is also an advantage of video cloud. “The essence of Ali’s narrow-band HD technology is to get every bit to where it is needed most,” he said.
Here are a few concepts:
- Resolution is the concept of image precision, representing the limit of quality, is not the bigger the better? Yes and no. Resolution is large, the point is more, the need for high bit rate, the need for bandwidth will become larger, transmission cost and network requirements will become larger.
- Bit rate, the ratio is the bit rate, it represents the number of bits of data transmitted per unit of time, video file size is determined by the bit rate, and is proportional.
- Frame rate, which represents visual fluency, is usually around 25 frames in China. At frame rates of 50-60, however, the gaps and differences are almost invisible to the naked eye.
So how do we achieve optimal balance between and within frames by allocating proper bit rates?
1. Reasonably allocate the bit rate between frames
Every frame needs bitrate to display the image, so how do we determine which frame needs more frame rate? Which frame needs to be less? If you think a frame is complex, like a Hollywood action movie, you allocate more frame rate, and if it’s simple, like a news broadcast, you allocate less frame rate. In order to achieve a reasonable inter-frame bit rate allocation.
2. Allocate frame internal code rate reasonably
In the whole picture, not everything needs to be very clear. For example, if you’re watching a party, you’re looking at the middle person and the guest, so identify the person and the face, where your eyes are focused, and assign more bit rates. At the same time, the pleated texture of the garment also allocates more bit rate, and the background, as the defocus area, allocates less bit rate. Through inter-frame and intra-frame bit rate allocation, so that the whole video quality is higher. At the same bit rate, obtain higher quality. For the same quality, you can save more bandwidth.
Then in the playback level, how to ensure smooth and slow?
Qiu liangke believes that CDN nodes with global coverage and precise scheduling system are indispensable to ensure the smoothness of live broadcast. CDN nodes adopt distributed architecture, with 1500 nodes all over the world and sufficient bandwidth reserve. The bandwidth of a single node is 40Gbps+, and the bandwidth output capacity of the whole network is 120 Tbps. At the same time, a four-layer intelligent scheduling architecture (as shown below) is adopted to ensure smooth distribution.
How to achieve accurate scheduling to ensure smooth peak of large events
However, in the face of large-scale activities such as evening parties, the burst peak is very high, so more accurate scheduling strategies are needed to achieve scheduling. Let’s say we have a glass with a lot of ice and water in it. If we want to use the small space in the glass, we have to put the ice in first and then the liquid water. DNS protocol restrictions are similar to ice. Other forms of scheduling, such as IP scheduling, can do request-level scheduling, that is, support any proportion of load balancing, just like liquid water. Therefore, in the scene of intelligent scheduling, the combination of “solid” and “liquid” can achieve accurate control of all nodes and water levels and achieve more accurate scheduling.
At the same time, in the case of a sudden surge in bit rate, the conventional traffic prediction algorithm was miscalculated, which would interfere with the flow control program. For this problem, Aliyun used AI traffic prediction for pre-scheduling, and the accuracy of prediction within 10 minutes reached 98%, and the accuracy of prediction within one hour was over 95%.
The surveillance system helps
After ensuring stability, picture quality and fluency, the live broadcast of a large event cannot be separated from the monitoring system. We definitely need to monitor the current state of the live stream to make sure we adjust our strategy in time.
Monitoring is carried out from the following four aspects:
1, flow monitoring: for each flow all the way to carry on the second level real-time monitoring, timely access to live streaming frame rate, bit rate, such as a timestamp state 2, broadcast quality monitoring: real-time analysis server slow speed ratio, client caton rate 3, availability monitoring: real-time return video 5 xx wrong data, timely orientation video failure reason 4, business monitoring: Obtain the current number of online users in real time
Youku, the only live streaming platform for the cat-night, attracted nearly 25 million viewers, double the number last year. This is also the fourth year that Aliyun video cloud supports double 11 cat night network broadcast. According to the monitoring data in the war room, various system data indicators run smoothly during cat night live broadcast, and a stable, high-definition and smooth large-scale event live broadcast is thus realized.
After the World Cup, Double 11 cat night and other times, the video cloud live streaming service has acquired a set of service experience in the live streaming of large-scale events/events/variety shows, and realized the empowering of ali Cloud customers in all industries, creating more value for the video industry.