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Overview
For optimal pucture picture quality with available channel bandwith bandwidth video bitrate should be managed while capturing WebRTC stream in browser. WCS server allows to limit minimum and maximum video bitrate for stream published. Audio bitrate is not managed.
WCS builds before 5.2.1825 use REMB to manage a publishing bitrate. Since build 5.2.1825 TWCC is used by default.
Platforms and browsers supported
Chrome | Firefox | Safari 11 | Edge | |
---|---|---|---|---|
Windows | + | + | + | |
Mac OS | + | + | + | |
Android | + | + | ||
iOS | - | - | + |
Settings
REMB
Since build 5.2.1825 REMB support should be enabled by the following parameter
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webrtc_cc2_twcc=false |
The following WCS settings are intended to limit publishing bitrate:
...
These settings work in most modern browsers and define the REMB bitrate management limits.
How it works
If maxBitrate is set, WCS server will send REMB command to decrease bitrate when this limit is reached.
If minBitrate is set, WCS server will stop sending REMB command to decrease bitrate when this limit is reached.
Therefore, the settings define 3 ranges with its own bitrate management algorithm:
№ | Range | Algorithm |
---|---|---|
1 | [0, minBitrate] | WCS stops bitrate management and not send any REMB commands |
2 | [minBitrate, maxBitrate] | WCS server makes active bitrate management: depending on jitter and incoming traffic uniformity WCS decides to send REMB commands to decrease bitrate. If channel is good, WCS does nothing and bitrate is not decreasing |
3 | [maxBitrate, ...] | WCS constantly sends bitrate decrease commands until it reduce to maxBitrate |
TWCC
Since build 5.2.1825, TWCC is enabled by default
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webrtc_cc2_twcc=true |
When TWCC is used, bitrate may be limited via publishing constraints at browser side only, in kbps
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constraints.video.minBitrate=500
constraints.video.maxBitrate=1000 |
TWCC bitrate settings work in Android SDK 1.1.0.62, iOS SDK 2.6.122 and WebSDK 2.0.239.
How to enforce bitrate increasing
Now, bitrate increasing can be enforced in Chrome browser Chromium browsers only by setting x-google-max-bitrate
and x-google-min-bitrate
parameters via SDP hook.
It is impossible to enforce bitrate rising with client and server settings, only bitrate decreasing can be managed.
In this case, Chrome specific settings take precedence if they are set, i.e. constraints
and server settings will be ignored. Note that Chrome default settings found by experience is
Code Block | ||
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x-google-max-bitrate=2500 |
In the latest Chrome builds, the option videoContentHint: "motion"
should be set when enforcing bitrate on browser side, because Chrome drops publishing bitrate in prefer to resolution when one of the other values is set
Code Block | ||||
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session.createStream({
name: streamName,
display: localVideo,
...
videoContentHint: "motion"
}).publish(); |
Enforcing bitrate increasing in Chrome browser with server parameters
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These settings are intended for Chromium-based browsers and work with, e.g., Opera, Vivaldi and Yandex.Browser. Also, they are applied when iOS Safari 12 or newer is used. They do
Bitrate enforcing does not work with in Firefox and Edge.
Usage
Bitrate limiting in certain borders can be useful for example when publishing video to Safari browser subscribers. This browser is sensitive to bitrate jumps, in this case picture quality loses until freeze and browser hangs. It is recommended to stabilize bitrate when publishing streams for Safari viewers by setting narrow limits of bitrate change, for example
...
In this case picture quality in Safari browser will be acceptable depending on channel bandwith bandwidth and state.
Bitrate rising needs to be enforced when publishsing publishing HD and 4K streams. In this case, Chrome browser is recommended for publishing.
Known issues
1. In lates latest Chrome versions (Chrome 75 for example) browser holds publishing bitrate on low limit x-google-min-bitrate when publishing WebRTC H264 stream
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