How can the SD channel and HD channel of the same TV station (such as Hunan Satellite TV) broadcast programs synchronously?

To be sure, it was taken with a high-definition camera (not two cameras). Compared with other TV stations' HD simulcast systems, Hunan Satellite TV is very strange. They use a whole set of high-definition broadcasting system (high-definition signal channel) (other stations use two systems, except Shi Jing, Hunan). The standard definition channel intercepts the middle picture, and the high definition channel plays it in full screen (one platform, two formats), and then adds LOGO and program logo (of course, the definition is different). However, this system also has some shortcomings, some TV series shooting did not consider 4; 3 composition, leading to face cutting, high-definition users are also surrounded by black when watching advertisements. However, for Hunan Satellite TV, this way saves manpower and time. Since 2009, Hunan Satellite TV has only bought the HD version of TV series, not the SD version. This high-definition simulcast system seems to be independently developed by Hunan.