CalloraVoipSdk 4.13.0

[BechsteinDigital]

CalloraVoipSdk 4.13.0

Inbound WebRTC audio can now be buffered before it is raised, so a consumer that mixes gets a steady
cadence instead of whatever the network delivered. One new opt-in setting,
WebRtcConfiguration.AudioReceivePlayoutDelayMs, default 0 — every existing consumer is byte-identical to
4.12.0. Purely additive: one new public member, nothing removed or changed (verified against
PublicApi.approved.txt).

Highlights

A jitter buffer on the WebRTC receive path (#381)

Until now, packets arriving on a WebRTC audio track were raised the moment they came off the wire. For a
peer that forwards that is correct and should stay that way: the browser at the far end runs its own
jitter buffer (NetEQ), and a second one here would only add latency to the same job.

A peer whose consumer mixes is in the opposite position. It must produce one frame every frame interval
from whatever each source has delivered by then, and it cannot wait. Handed raw arrivals it reads a burst as
a single usable frame and the rest as silence.

Opus DTX makes that the normal case rather than the exception. A browser sends nothing at all while
nobody speaks, so the packets that follow a pause arrive together. The audible result is audio that cuts out
after every pause and returns a few seconds later — which is how this was found, in a two-party conference
between a browser and a telephone: phone to browser was clean, browser to phone stuttered.

Two things confirm the shape of the fix rather than merely the fix itself:

• The SIP path in this SDK has always worked this way. RtpCallMediaSession buffers arrivals and drains
  them from a playout loop. Only the WebRTC receive path did not.
• Janus reached the same design for its mixing plugin: put the packet in the jitter buffer on arrival and
  decode from the buffer on the participant's own cadence. A pure SFU needs no buffer; a mixer does.

Using it

var config = new WebRtcConfiguration
{
    // 0 (default) raises packets on arrival — correct for forwarding.
    // A mixing consumer sets a starting delay; the buffer adapts from there.
    AudioReceivePlayoutDelayMs = 60,
};

Buffering costs latency, which is why it is off by default: a forwarding peer would pay it for nothing.

Behaviour, for anyone enabling it

Behaviour	Why it is that way
One buffer per audio m-line, at that track's clock rate	48 kHz Opus and 8 kHz G.711 in one bundle would otherwise be scheduled six times off
The buffer resets when the SSRC changes	A new source brings its own sequence space; without the reset it reads as wild reordering — a leg that goes silent after a renegotiation and returns seconds later
Packets due together are released together	A burst that arrived together is due together; releasing one per interval would rebuild the backlog the buffer exists to undo
A gap is passed on as a gap, never concealed	This layer carries encoded RTP and does not know the codec — repeating an Opus frame is an artefact. The SIP path conceals because it delivers into G.711, where repetition is benign and a silent gap is not
A throwing subscriber is logged, not fatal	One bad subscriber must not end the playout loop and take every other track on the session with it

Arrival and playout both read MonotonicClock: a wall-clock step mid-call would otherwise corrupt the
schedule.

Compatibility

Purely additive. One new public member (WebRtcConfiguration.AudioReceivePlayoutDelayMs); nothing removed or
changed, verified against PublicApi.approved.txt. With the setting left at its default of 0, sessions are
byte-identical to 4.12.0. SemVer: MINOR.

See CHANGELOG.md for the itemised list.

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This discussion was created from the release CalloraVoipSdk 4.13.0.