OPUS file format
The format has been created by IETF community for online broadcasting and is being used for lossy compression of audio data. It provides with excellent quality of sound and very good compression level due to the technology of dynamically adjusted streaming. Being a key audio codec of Skype, OPUS is based on the Skype message-transfer algorithms. As a container, they use .ogg extension.
The format uses CELT and SILK codecs, which allow easy change of bitrate within the range of 6-510 Kbit/s. That is why it has gained prevalence in video chats and video conferences, as well as IP translations.