VHS ABR (as of September 2022)

vhs-abr-09-2022.md

Goals (priority order)

1. Least disturbance

current: rebuffering
future: rebuffering + dropped frames

2. Highest quality

current: bitrate + resolution
future: bitrate + resolution + framerate + quality score + codec

3. Least wasted bandwidth

avoid exceeding width and height of player

ABR algorithms

Algorithm	systemBandwidth Value	Low Water Line	High Water Line	When is it run?
default	most recent	0-30s (dynamic)	N/A	bandwidthupdate events2
experimentalBufferBasedABR	moving average1	0-16s (dynamic)	30s	timer (250ms)3

¹ decay of 0.55: average = 0.55 * systemBandwidth + (1 - 0.55) * average

² bandwidthupdate events occur on the following

• XHR progress events (~250ms)
• XHR timeout
• source buffer updateend events

³ decay does not run on the timer

Further comparing the algorithms

Similarities

• Dynamic GOAL_BUFFER_LENGTH of 30-60s: buffer occupancy at which to stop buffering
• Use of simpleSelector
• Use of Config.BUFFER_LOW_WATER_LINE: buffer occupancy before allowing upswitches
• Use of shouldSwitchToMedia
  • if live: don't consider low water line
  • if LL-HLS: don't switch before playback starts
• MIN_SEGMENT_DURATION_TO_SAVE_STATS = 1 / 60

Differences

• Config.BUFFER_LOW_WATER_LINE values
• experimentalBufferBasedABR uses Config.BUFFER_HIGH_WATER_LINE: buffer occupancy before disallowing downswitches
• experimentalBufferBasedABR uses a moving average instead of most recent value
• default uses earlyAbortWhenNeeded on XHR progress events

How is systemBandwidth calculated?

bandwidth

• uses NetworkInformation.downlink (max 10 Mbps) if available
• otherwise (or if over 10 Mbps) uses XHR bits / time taken
• timeout means bandwidth goes to 1

throughput

• decryption + transmuxing + appending
• cumulative moving average: new = old + (sample - old) / (sample count + 1)

systemBandwidth = 1 / (1 / bandwidth + 1 / throughput)

Playlist selectors

• simpleSelector: basic function for choosing based on bandwidth, resolution, and player dimensions
• lastBandwidthSelector: default
• movingAverageBandwidthSelector: used by experimentalBufferBasedABR with 0.55 decay
• minRebufferMaxBandwidthSelector: earlyAbortWhenNeeded comparison
• lowestBitrateCompatibleVariantSelector: for enableLowInitialPlaylist option

In the future

• dropped frames
• segment size
• quality score (e.g., VMAF)
• codec comparison
• framerates