OPUS file format
The IETF community has created the format for online broadcasting and is being used for lossy compression of audio data. It provides excellent quality of sound and a perfect compression level due to dynamically adjusted streaming technology. Being an essential 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.