4 / 8 Media, Streaming & Storage

Rungs Too Close Together

Netflix draws per-title rungs from a finite set of bitrates whose adjacent values differ by roughly 5 percent, and it wants the quality step between neighbouring rungs to land just below one just-noticeable-difference.

An analysis proposes keeping both of these 1080p points for one title: 4640 kbps and 4820 kbps. They are under 4 percent apart in bitrate and well inside one JND in measured quality.

Why does the ladder ship one of them and not both?

A

A CDN cache can only hold one variant per resolution, so the second rung would never be served.

B

A rung costs an encode of the whole title, a copy on every appliance in the delivery network, and one more variant for the client to switch between, and a sub-JND step buys the viewer nothing.

C

Adaptive streaming clients cannot hold more than six variants in a manifest.

D

Quality metrics cannot resolve a difference smaller than one JND, so the second rung would be chosen from noise.