Streaming bitrates

Originally Posted by michaeltscott 


Roku runs dozens of different video services and all of them will have different bandwidth requirements. Presuming that you're talking about Netflix (and this goes for any device streaming Netflix, not just Roku), a title with HD video will have encodings at 235-, 375-, 560-, 750-, 1050-, 1400-, 1750-, 2350-, 3600- and 4800 Kpbs. 2350 and 3600 are 720p24, 4800 is 1080p24 and all the rest are 480p24 (p30 for older TV or p25 for British content). Add 192 for stereo sound or 384 for 5.1 channel DD+ (for that subset of titles with 5.1 sound). Multiply by 1.3 and you'll get roughly the amount of bandwidth necessary to sustain that encoding. This will vary a little with the title, since the encodings are typically highly variable bit rate; you might manage to stream the 1080p encoding of a non-action title for significantly less bandwidth than the 6.7 Mbps that my forumula indicates.

There are so many encodings because Netflix on newer devices (and PS3, Xbox and the PC web player) uses some tech called Adaptive Bit Rate Streaming. To put it in simple terms, the player will read content as fast as it can until it fills it's buffer; if it cannot keep the buffer full with its currently selected encoding it will request the next lower quality, lower bit rate encoding; if it's keeping the buffer full easily it will try a higher bit rate encoding with higher picture quality. If it changes encodings it will do it smoothly without pausing; on the best players this looks like the focus of a lens becoming gradually sharper or softer. The bit rate that a player can keep up with depends on multiple factors, principally available bandwidth on your connection to Netflix's servers and those servers' current responsiveness; if you can accept content faster than your server can find time to shovel it to you, it won't do you any good. Note that available bandwidth on your connection to Netflix's servers is somewhat independent of your Internet service's rated speed--that's only a limit, and you're not guaranteed to get that on a connection to any arbitrary node on the net.