Speech coding is relatively mature, and there are many existing standards.
Speech synthesis has a more mature program, such as the domestic iFLYtek.
Voice noise reduction technology has been developed for many years, mainly divided into single microphone noise reduction to remove smooth noise, and dual microphone noise reduction to suppress directional noise. In general, it is difficult to improve voice quality fundamentally because most of them are in the form of feature. After all, no signal processing technology can match the processing power of the human auditory system.
Echo cancellation technically belongs to audio signal processing. However, the residual echo suppression is speech signal processing. It can be regarded as an extension of voice noise reduction technology, and there is a certain connection between single mike and double mike. This has been widely used in VOIP technology, and there is little room for improvement.
The current technical framework of speech recognition technology is mainly based on pattern recognition, which has high requirements for data matching, and there is still a big bottleneck in the processing ability of dialects, accents, and spoken languages. For standard accents, it can still be handled, but it also requires a high degree of cooperation from the user. Overall, in practical terms, the current technology is still a little weak.
All of these technologies currently have a number of open source projects with good performance. Can be used for reference. But the common problem is that there doesn't seem to be a very bright way forward.