[x264-devel] X264 to iPad

[Dennis Munsie]

I'm not familiar with the specifics of the Microsoft Expression encoder, but even if you were to get a gain in encoding speed from x264, HTTP live streaming is going to introduce some latency of it's own.  There are some buffering requirements in HLS that are not negotiable -- you need to make sure there are at least N number of buffers available before starting the stream, the buffers have to be at least N seconds long.  I don't have the specific numbers in front of me, but the Internet draft has the details (http://tools.ietf.org/html/draft-pantos-http-live-streaming-05).  I recall that you might be able to get it down to around 10 seconds or so, but that might off by a bit.

Also while the server can control how many segments it has available on the server and how long those segments are, there is no control over when the client chooses to start streaming the content.  So those 20 iPads will all have slightly different places they will be at in regards to presentation time.  So if you are trying to keep them synchronized, that will have it's own problems.

HLS is really better suited for situations where latency isn't a priority.  Since it's based on TCP, it has to be able to smooth out any network jitter that might arise by packets being dropped.  And as soon as you start adding buffers to smooth out jitter, you start increasing the latency.

If latency is a huge issue, you might want to start looking for other solutions tailored more for this kind of problem rather than trying to force HLS into a role that it wasn't meant for.

dennis

On Jan 24, 2011, at 9:28 AM, Avi Shvartz wrote:

> Hello list,
> I am new in this list so please forgive me if my question is not inline with the 
> list. Purpose:
> - Transmit a Power Point presentation, both slides and movies, or other video 
> resources such as TV, DVD etc..
> via WiFi to ~20iPads within a meeting room .
> Note: the presenter is in the same room , so the delay between the presentation 
> show on the PC 
> 
> and the display on the iPads should be minimal.
> Problem:
> - A ~30sec delay between the video changes on the PC and the appearance on the 
> iPads.
> 
> Topology:
> - PC XP with power point/movies.
> - 100MHZ TCP/IP connection to a video streamer machine.
> - Video streamer machine:
>      - i7950 cpu.
>      - 4GB memory.
>      - 2xSSD (one for the operating system and one for the video capture card).
>      - Video capture card : SKYDIGITAL CAPTUREX-X
>      - Operating system: most updated WIN7 professional.
>      - Microsoft Expression Encoder 4 pro: version 4.0.1639.0
>      - IIS 7.
> - WiFi : Two Access points 802.11n (for high availability and avoiding bandwidth 
> problems)
>    configured with WPA2-Entreprise.
> - iPad: 4.2.1 
> 
> Taxonomy:
> - From brief analysis it seems that most of the delay is in the Expression 
> Encoder side. 
> 
> 
> Some questions:
> - Can the Expression Encoder be replaced with X264 software?
> Note: iPad use - Any other ideas ?
> Thank you very much for your kind help
> Avi"Apple Live HTTP Streaming protocol"as explained in:
> http://developer.apple.com/library/ios/#documentation/networkinginternet/conceptual/streamingmediaguide/HTTPStreamingArchitecture/HTTPStreamingArchitecture.html#//apple_ref/doc/uid/TP40008332-CH101-SW2
> 
> 
> 
> 
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