wav64_opus.c File Reference

Support for opus-compressed WAV64 files. More...

Data Structures
struct	wav64_opus_header_ext
	Wav64 Opus header extension. More...
struct	wav64_opus_state
	Wav64 Opus state. More...

Functions
void	wav64_opus_init (wav64_t *wav)
	Initialize opus decompression on a wav64 file.
void	wav64_opus_close (wav64_t *wav)
	Shut down opus decompression on a wav64 file.
int	wav64_opus_get_bitrate (wav64_t *wav)
	Return the bitrate for a wav64 file.

Detailed Description

Support for opus-compressed WAV64 files.

Author

Giovanni Bajo (giova.nosp@m.nnib.nosp@m.ajo@g.nosp@m.mail.nosp@m..com)

Opus notes

This section details how the Opus format is used in wav64. Opus is made by a mix of two different coders: CELT and SILK. CELT is used for larger frames and is more apt for music, while SILK is used for smaller frames and is more apt for speech. Our N64 implementation only uses CELT. In fact, the whole Opus layer (which is a framing layer) is not used at all.

A WAV64 file compressed with Opus contains a sequence of raw CELT frames. Since CELT requires framing (that is, the length of the compressed frame must be known in advance), a very simple framing is used: each frame is preceded by a 16-bit integer that contains the compressed length of the frame itself. Moreover, frames are forced to be 2-byte aligned, so that they're easier to read them via DMA.

At the API level, we use the opus_custom API which is a CELT-only API that allows to implement custom "modes". A "mode" is the configuration of the codec, in terms of sample rate and frame size. Standard CELT only supports 48kHz with frames of some specific length (from 2.5ms to 60ms in various steps). For N64, we want to flexibility of experimenting with different sample rates and frame sizes. For instance, currently the implementation defaults to 32 Khz and 20ms frames (640 samples per frame), which seems a good compromise between quality and performance.

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Data Structure Documentation

wav64_opus_header_ext

struct wav64_opus_header_ext

Wav64 Opus header extension.

Data Fields
uint32_t	frame_size	Size of an audioframe in samples.
uint32_t	max_cmp_frame_size	Maximum compressed frame size in bytes.
uint32_t	bitrate_bps	Bitrate in bits per second.

wav64_opus_state

struct wav64_opus_state

Wav64 Opus state.

Data Fields
wav64_opus_header_ext	xhead	Opus header extension.
OpusCustomMode *	mode	Opus custom mode for this file.
OpusCustomDecoder *	dec	Opus decoder for this file.