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fpnc
Oct 12, 2010, 02:08 AM
Since this issue or question seems to be coming up fairly often ("slow" streaming from the iTunes Store) I thought I'd create a table showing the buffering delays you might expect for HD content given the speed of your internet connection. These calculations assume that the iTunes HD content averages about 4.5Mbps and thus a one hour show will require about 2.0GB of data from start to finish (4.5Mbps x 60 seconds x 60 minutes / 8 bits per byte). As a check point, I have a 24.5 minute HDTV show from iTunes that occupies 818.4MB of disk space, thus scaling that example up to one hour results in a estimated size of just over 2GBs.

The first set a values assumes an essentially PERFECT performance with no overhead in the software/hardware or streaming protocol (an unlikely case, but we'll provide it just as a reference). The first column shows your internet transfer speed, following that the amount of time to transfer the full 60 minutes of HD content. If the time is over 60 minutes then you obviously can't watch the show in realtime and some buffering will have to occur (in this case, we'll assume that the difference is the time you must wait before the show can begin playback).

5.0Mbps -- 54 minutes
4.5Mbps -- 60 minutes
4.0Mbps -- 68 minutes (8 minute wait)
3.5Mbps -- 77 minutes (17 minute wait)
3.0Mbps -- 90 minutes (30 minute wait)
2.5Mbps -- 108 minutes (48 minute wait)
2.0Mbps -- 135 minutes (75 minute wait)
1.5Mbps -- 180 minutes (120 minute wait)
1.0Mbps -- 270 minutes (210 minute wait)

Unfortunately, in the real world you'll very likely never see performance this good. So, to avoid stutters or complete interruptions in the streaming playback Apple has to allow a safety margin whose value is anyone's guess. For the sake of argument, I'll assume that this safety margin might be something like 15%. Thus, for any measured broadband transfer speed (average) Apple could assume that it will take about 15% longer to transfer the complete video stream. Easy case, just add 15% to the above transfer times and you get the new waits. So...

5.2Mbps -- 60 minutes (threshold for essentially "instant" streaming)
5.0Mbps -- 62 minutes (2 minute wait)
4.7Mbps -- 66 minutes (6 minute wait, U.S. average broadband speed)
4.5Mbps -- 69 minutes (9 minute wait)
4.0Mbps -- 78 minutes (18 minute wait)
3.5Mbps -- 89 minutes (29 minute wait)
3.0Mbps -- 104 minutes (44 minute wait)
2.5Mbps -- 124 minutes (64 minute wait)
2.0Mbps -- 155 minutes (95 minute wait)
1.5Mbps -- 207 minutes (147 minute wait)
1.0Mbps -- 311 minutes (251 minute wait)

As a case in reference, I have a rated 3Mbps DSL connection which routinely tests out to a real-world transfer speed of about 2.5Mbps. Further, I know that the last HDTV show I streamed from iTunes reported an estimated wait before being ready to view of just over 40 minutes. That particular show was about 45 minutes long, so from the 2.5Mbps entry in the above table you can see very good agreement between what I experienced and what I estimated using the 15% safety factor (since for a 2.5Mbps connection you have to wait to begin viewing just about as long as the total length of the content itself).

Note that standard definition (SD) streaming is an obvious different case. For SD, if you have a connection at 2Mbps or better then the playback should begin almost immediately.



SonicStomp
Oct 12, 2010, 02:26 PM
Good work!

I guess the 'so what' is that for HD streaming in the real-world you need to be getting > 5mbps for instant streaming.

This would explain why I am having trouble during periods of peak internet usage where my speedtests are returning results of < 3mbps!

Time to boost my package!

Slower Pace
Nov 13, 2010, 05:24 PM
Thank you for such a fine job of making download times clear to the rest of us who may be byte/bit challenged! What you have seems to verify what I experienced on my one and only HD download from iTunes. And as for SD, you are right, it begins almost immediately. Your charts really help, thank you!

liquidsuns
Nov 13, 2010, 07:54 PM
Man do I wish I could get faster internet! I've been thinking that about 5mbps was the magical HD streaming number.

b11051973
Nov 13, 2010, 08:36 PM
I have Comcast with pretty fast speeds. I think I have 16 mbps down or something. I've rented a couple HD movies on my Apple TV over the years. I think I've rented 2 on the new Apple TV. I usually have to wait like 30 seconds before the option to play comes up. Fast internet is pretty awesome. :)

Pressure
Nov 14, 2010, 07:59 AM
I feel for you, really.

That is sad, pathetic, low speed internet there.

Even my 3G internet speed is faster :p

itsmemuffins
Nov 28, 2010, 03:53 PM
I have a 7 meg connection, it takes about 30 sec for an hd movie to star and there is no more buffeting during the movie.