What's the difference between the old and new Yuefu poems?

Emperor Wu of the Han Dynasty set up a special organ to manage music, called Yuefu. Yuefu is responsible for the music used in court meetings, banquets, ceremonies and parades, and collects folk songs from all over the country. Later, the music collected and created by this institution was called Yuefu, so Yuefu became the name of an ancient poem. The songs written by later scholars with "the ancient theme of Yuefu" or imitating Yuefu style are famous works, also called Yuefu.

That is, what is ancient Yuefu (or old topic), what is new Yuefu (or new topic) and what is new Yuefu. The concepts of these three nouns are quite confusing, and their explanations are varied. "New Yuefu" refers to "New Songs of the Tang Dynasty" (Yuefu Poetry Volume 90), which naturally belongs to the category of "new topics". Yuefu poetry, a new work in the Tang Dynasty, also includes "modern lyrics", which Guo thinks is "from Sui and Tang Dynasties" (Yuefu Poetry Collection, volume 79). Accordingly, the new Yuefu and modern lyric poems in Yuefu poems can be classified as "new Yuefu". Because these Yuefu were born in Sui and Tang Dynasties, they are "new" than Yuefu in Han, Wei, Jin and Southern and Northern Dynasties, so they can be collectively called "new Yuefu". Before that, Yuefu should be collectively referred to as "ancient style Yuefu" or "old style Yuefu". As for how to distinguish new Yuefu from new Yuefu, this issue has always been very different. The ancients often called the new Yuefu directly, such as Guo Maoqian himself. There are also many people who refer to the new Yuefu as the new Yuefu, such as Feng Ban (in his Miscellanies of Daige, Du Zimei is the new Yuefu, full of Yuan Bai). Later, Zhao Zhixin, a poet in A Qing, said in his "Music Score" that "New Yuefu poems are all self-created, and they talk more about current affairs, but they contain beautiful thorns". Since then, people have gradually reached a consensus that new Yuefu poems refer to new works different from ancient Yuefu poems before Sui Dynasty. The fundamental difference between the new topic and the old topic lies in the difference of the topic. Even if there is only one word difference, the inheritance of content, the continuation of style and the related factors of genre are not considered. For example, before and after "Chusai", there was this topic before the Tang Dynasty. If the Tang people do this topic again, it will be called the old topic Yuefu; Song on Saipan and Song on Saipan were both created by the Tang people and should be included in the new Yuefu.

The concept of new Yuefu is different from that of new Yuefu, which is only a part of new Yuefu and belongs to its category. Its three elements are what Zhao said: using new topics, expressing current events, and including beauty. In this way, people call Du Fu's new Yuefu, which is mainly narrative and satirizes the shortcomings of the times, "New Yuefu" to distinguish it from other new Yuefu without these three elements. This distinction is scientific and basically conforms to Bai Juyi's new Yuefu theory. Although it is different from Guo Maoqian's statement, it was finally accepted by most people and has been used ever since.