Gapless stream from pre-encoded chunks of Ogg/Opus data

This repository contains a C++ program that generates independently encoded Ogg/Opus audio segments. These audio segments can be seamlessly played back in a webbrowser using WebAudio. Indepdentently encoded means that the individual segments could have been encoded in parallel or in random order. Since all segments are deterministic, they can be served using RESTful APIs, or stored in content-addressed filesystems such as IPFS.

An online demo can be found at

https://somweyr.de/opus/demo.html

Gapless playback is achieved by two independent measures:

Lead-in and lead-out frames: The encoder adds an additional frame to the beginning/end of each chunk. These frames contain artificial audio generated with Linear Predictive Coding and naturally extend the audio signal at the beginning/end of each segment. The extra data is thrown away after decoding as indicated by the pre_skip and granule metadata in the Ogg/Opus container. As a whole, this suppresses ringing-artifacts caused by encoding the discontinuities at the beginning/end of a chunk.
Overlap: All chunks slightly overlap each other and are cross-faded by the browser. The number of audio samples to be cross-faded is stored in each segment as proprietary CF_IN, CF_OUT metadata fields, which are then parsed by the JavaScript client. The demo uses extreme overlap values (250ms) and very short segments (1s) for demo purposes, but 1ms overlap will work just as well.

This code is part of a larger project I'm currently working on and will not receive any further updates in this repository (unless I find severe bugs).

Building

The supplied Makefile searches for an installed libopus using pkgconfig.

To build:

git clone https://github.com/astoeckel/opus_gapless
cd opus_gapless
make

Running

Feed raw stereo floating point audio data at 48000 samples/s into the opus_gapless program. This will create a number of files in the blocks subdirectory.

mkdir -p blocks && rm -f blocks/* && ffmpeg -loglevel error -i <AUDIO FILE> -ac 2 -ar 48000 -f f32le - | ./opus_gapless

Then serve this directory via HTTP, e.g. by running

python3 -m http.server

and go to http://localhost:8000/demo.html

License

Opus Gapless
Copyright (C) 2017  Andreas Stöckel

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>.