Ogg mapping for Opus
The IETF Opus codec is a low-latency audio codec optimized for both voice and general-purpose audio. See [tools.ietf.org/html/draft-ietf-codec-opus the spec] for technical details.
Almost everything about this codec is either fixed or dynamically switchable, so the usual id and setup header parameters in the header packets of an Ogg encapsulation aren't useful. In particular, bitrate, frame size, mono/stereo, and coding modes are all dynamically switchable from packet to packet. A one-byte header on each data packet defines the parameters for that particular packet.
Remaining parameters we need to signal are:
- magic number for stream identification
- comment/metadata tags
Additionally there's been a desire to support some kind of channel bonding for surround, and some kind of option signalling for "Opus Custom", in particular the granulerate.
Granulepos is the count of decodeable samples at a fixed rate of 48 kHz.
Two headers: id, comment
- 8 byte magic signature 'OpusHead' (64 bits) - 4 byte Input sample rate (32 bits, max 192 kHz) - 1 byte channel mapping flags (bool in byte) - 1 byte channel count (8 bits) - 2 byte pre-gap (16 bits) - <optional channel mapping?>
- 8 byte magic signature 'OpusTags' (64 bits) - rest follows the vorbis-comment header design used in OggVorbis, OggTheora, and Speex. ** Vendor string (always present) ** tag=value metadata strings (zero or more)