[opus] Gapless concatenation of Opus frames

[Jean-Marc Valin]

Hi Andreas,

So if I understand your question correctly, what you want is really
short "files" that are independent, but yet create a glitchless stream
when concatenated, right. For Ogg, this can be implemented with
libopusenc and chaining. It works pretty well (even for really tiny
files). For WebM, I'm not sure how to handle the details at the
container level, but for how to handle the transition details (reset and
all), I suggest you have a look at the libopusenc code. In general, the
idea is to disable the prediction at the point of the transition between
two files and to include the transition frames in both files.

Cheers,

	Jean-Marc

On 11/08/2017 03:43 AM, Andreas Stöckel wrote:
> Hi!
> 
> Short version of my question: How to produce Opus frames which can be
> safely concatenated and how to embed them into a WebM file?
> 
> Long version:
> 
> I'm currently implementing a web-based audio player which streams
> audio as opus/WebM using the HTML5 media source extensions. Currently,
> the server decodes a set of input files to a fixed RAW audio format
> (stereo, 48000 kHz) and encodes the resulting continuous RAW stream as
> Opus/WebM. Having a single, uninterrupted RAW stream allows for
> perfect gapless playback on the client (which only sees a single live
> WebM stream), e.g. there are no interruptions whatsoever when
> transitioning between continuous tracks from the same music album.
> 
> An early tech-demo of the technique can be found here [1], the source
> file http_audio_server/encoder.cpp implements the relevant
> opus-encoding and webm-encapsulation (but see also [2] for a condensed
> version).
> 
> 
> Now, for performance reasons I'd like to split my RAW audio into
> independent blocks (say, as an example, 50 frames or 1s each), encode
> these as raw Opus frames and cache them on disc ahead of time. For
> each block I'd like to reset the encoder to ensure independence
> between the first frame of each block and the last frames in the
> previous block, e.g., using
> 
> opus_encoder_ctl(enc_ctx, OPUS_RESET_STATE)
> 
> When the client requests a certain sequence of blocks (which may
> originate from various input files in (let's pretend) any order) my
> goal is to (on-demand) encapsulate the pre-encoded frames as WebM and
> send them to the client.
> 
> However, in early experiments [2], resetting the encoder state at the
> beginning of each block and then concatenating the frames in the WebM
> container leads to clearly audible gaps in the decoded WebM stream
> whenever the opus encoder has been reset.
> 
> Interestingly, such artifacts are far less pronounced (if they exist
> at all), if I don't explicitly reset the encoder. However, in my real
> application the encoder will at least be reset implicitly (e.g. by
> starting the encoding process in multiple threads for two files which
> may be played consecutively).
> 
> See [2] for a MWE which expresses what I've tried to describe above.
> 
> So to rephrase my question: if it is possible at all, how can I
> independently pre-encode blocks of Opus audio frames, such that I can
> concatenate them during WebM muxing without audible glitches?
> 
> 
> In advance, thank you for your help. Please let me know I anything I
> wrote is unclear, or you need more information to answer my question.
> 
> 
> Andreas
> 
> 
> [1] https://github.com/astoeckel/http_audio_server/
> [2] https://github.com/astoeckel/opus_gapless_webm/
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