For optimal picture quality with available channel 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.
Chrome | Firefox | Safari 11 | Edge | |
|---|---|---|---|---|
Windows | + | + | + | |
Mac OS | + | + | + | |
Android | + | + | ||
iOS | - | - | + |
Since build 5.2.1825 REMB support should be enabled by the following parameter
webrtc_cc2_twcc=false |
The following WCS settings are intended to limit publishing bitrate:
| On browser side (JavaScript) | On server side (flashphoner.properties) | |
|---|---|---|
| Minimum bitrate limit | constraints.video.minBitrate | webrtc_cc_min_bitrate |
| Maximum bitrate limit | constraints.video.maxBitrate | webrtc_cc_max_bitrate |
On browser side, bitrate limits are set in kilobits per second, for example
constraints.video.maxBitrate=600 |
and on server side limits are set in bytes per second
webrtc_cc_max_bitrate=600000 |
If the limits are set on both sides, browser settings take precedence over server settings.
If browser settings are not defined, server settings are applied,
If none of limits are set, default settings are applied
webrtc_cc_min_bitrate=30000 webrtc_cc_max_bitrate=10000000 |
These settings work in most modern browsers and define the REMB bitrate management limits.
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 |
Since build 5.2.1825, TWCC is enabled by default
webrtc_cc2_twcc=true |
When TWCC is used, bitrate may be limited via publishing constraints at browser side only, in kbps
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.
Now, bitrate increasing can be enforced in 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
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
session.createStream({
name: streamName,
display: localVideo,
...
videoContentHint: "motion"
}).publish(); |
Chrome browser SDP bitrate settings can be adjusted on server side using the following parameters
webrtc_sdp_min_bitrate_bps=3000000 webrtc_sdp_max_bitrate_bps=7000000 |
These parameters are set in bps. In the example above, the settings have a same effect as
x-google-max-bitrate=7000;x-google-min-bitrate=3000 |
These settings are intended for Chromium-based browsers. Also, they are applied when Safari 12 or newer is used.
Bitrate enforcing does not work in Firefox.
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
constraints.video.minBitrate=600 constraints.video.maxBitrate=600 |
In this case picture quality in Safari browser will be acceptable depending on channel bandwidth and state.
Bitrate rising needs to be enforced when publishing HD and 4K streams. In this case, Chrome browser is recommended for publishing.
1. In latest Chrome versions (Chrome 75 for example) browser holds publishing bitrate on low limit x-google-min-bitrate when publishing WebRTC H264 stream
Symptoms: with SDP settings
x-google-min-bitrate=3000;x-google-max-bitrate=7000 |
the publishing bitrate hols on 3000 kbps while channel bandwidth is on 20 Mbps.
Solution: double bitrate limits, for example
webrtc_sdp_min_bitrate_bps=6000000 webrtc_sdp_max_bitrate_bps=14000000 |