Dolby maps GOP tradeoffs in LL-HLS streaming
An analysis from Dolby OptiView details how Group of Pictures (GOP) size impacts video bitrate, quality, and latency in LL-HLS streaming. The study demonstrates that increasing GOP size can reduce bandwidth consumption and improve video quality, but at the cost of increased zapping time and potential latency. Different video types show varying degrees of impact on bitrate and VMAF scores.
Key Takeaways
- Apple’s recommended GOP size is 2 seconds; Dolby says 1-second GOPs can support roughly 3-second end-to-end LL-HLS latency.
- Across four test videos, bitrate fell as GOP size increased, with static screen streaming dropping from 1,934 Kbps at 0.5 seconds to 453 Kbps at 10 seconds.
- Static video saw up to 70% less bandwidth at 10-second GOPs versus 0.5-second GOPs; other video types saw up to about 20% reduction.
- VMAF improved with larger GOPs in every test case, with Big Buck Bunny rising from 49.5 at 0.5 seconds to 64.8 at 10 seconds.
- Dolby says larger GOPs force a tradeoff: either wait for the next keyframe or start sooner with latency that can reach the GOP size.
Why It Matters
For LL-HLS, GOP size is not a tuning detail — it determines the tradeoff between bandwidth, quality, and startup latency. Dolby’s tests show that longer GOPs can lower bitrate and raise VMAF, especially for static content, but they also push zapping time and can add up to the GOP length in latency. That makes encoder settings a live product decision, not just an infrastructure one. The practical question for streaming teams is how far they can push GOP length before startup delay becomes visible. Watch which GOP settings are used for low-latency live services, especially around the 1-second, 2-second, and 6-second examples Dolby cites.
Read full article at optiview.dolby.com
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