HD olympic coverage...can you stream it without any problems

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HD olympic coverage...can you stream it without any problems

Postby dwalton22 » Tue Feb 21, 2006 6:15 pm

I'm using MythRoku to watch olympic coverage that I recorded with mythtv but I've been having lots of problems where the sound cuts out or the video stutters. I've been working on MythRoku making some tweaks here and there and from what I can see in my debugs olympic speed skating in HD is very very difficult to stream.

- My speed skating file is 5.46GB or 5862630359.0 bytes for 55 minutes
Average rate of 14212437 bits per second

- Within MythRoku I have 8 393216 byte buffers that I am constantly
filling that I use to satisfy the OnFillBuffer calls.

- OnFillBuffer drained 13 of these buffers in one second
13 * 393216 * 8 = 40894464 bits
I know speed skating is fast moving but 40M for one second of video seems a bit much. I don't know a ton about mpeg2 though...is this possible or do I have some bug in my debug output that is giving me unrealistic data?

- MythRoku was able to replenish 5 in that second
5 * 393216 * 8 = 15728640 bits

The options that I've come up with are:

- use a lot more than 8 393k buffers so I can withstand bursts like the one above. The only downside with that is everytime I skip forward or back in the video I have to wait for all of these buffers to fill before I can resume playback.

- find a way to increase transfer speed with my backend. Does the HD1000 support full duplex on its FastEth port? I've already followed the recomendations in this post:

http://www.rokulabs.com/forums/viewtopi ... fillbuffer

Does anyone know of other tricks to increase TCP/UDP throughput?

Thanks
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Postby KevMcK » Wed Feb 22, 2006 6:11 pm

What was the source of your video? Cable/satellite or off-air ATSC? How was it recorded?

The maximum bitrate of a transport stream off-air is 19.39Mbps, and either 26.97 Mbps (64QAM) or 38.8 Mbps (256QAM) depending on the cable operator.

It seems very unlikely that anything from a broadcast/cable source would hit 40Mbps as you describe. The transport medium can't handle that much data on a single program, let alone as part of an Multi-Program TS. The highest I've seen coming off the satellite into a cable headend for a single HD program stream is around 20Mbps (most are around 15), and terrestrial broadcasters rarely use their whole 19.39 for one video stream. Their HD is often between 12 and 14, leaving room for an SD or two. The 14-15 range matches your calculated bitrate of 14.2Mbps.

If your file is a transport stream, you could check it with a tool like TSReader (free version at http://www.coolstf.com/tsreader/index.html) to see if the recorded stream has been stuffed with null packets. Nulls could be padding the transport stream to reach an arbitrary target bitrate. TSReader may also indicate whether there's continuity problems in the stream (dropped packets) that could affect video or sound. Note that TSReader only works on transport streams though, not program or elementary streams.

The key here will be to determine why OnFillBuffer drained 13 buffers in a second. Does it happen consistently at the same point in the video? Perhaps there's some bad/corrupt data in your source?
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Postby dwalton22 » Thu Feb 23, 2006 11:00 am

It was recorded via 256QAM. Most of the time the local cable company does show a couple of SD streams too but during the olympics they cut out one of those altogether and the other is just radar. They are intentionally dedicating a lot of bandwidth to the HD stream because it is the olympics.

I'll have to take a look at TSReader but I think you might be on the right track about the mpeg stream having some glitches. I've been able to speed up the download rate from the myth backend to the roku but when I play the speed skating file there is one place where the video skips everytime and the sound cuts out.
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Postby timh » Thu Feb 23, 2006 5:56 pm

I don't know if this helps, but I'm watching the Olympics (well some) over (Adelphia) cable with an S-A Explorer 8300HD (High Def) Set Top Box. I periodically see the same video and audio glitches, but the box recovers nicely (resyncs audio and video). I suppose the problem occurs during production or transmission and affects all of us.
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Postby dgburns » Fri Feb 24, 2006 1:22 pm

timh wrote:I don't know if this helps, but I'm watching the Olympics (well some) over (Adelphia) cable with an S-A Explorer 8300HD (High Def) Set Top Box. I periodically see the same video and audio glitches, but the box recovers nicely (resyncs audio and video). I suppose the problem occurs during production or transmission and affects all of us.

Same cable box here, Comcast cable company. I see glitches too but haven't specifically noticed any more or less that I recall on speed skating.
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Postby leenuxg33k » Fri Feb 24, 2006 1:35 pm

I watched last nights Olympics with OTA through mythRoku and saw a glitch once but it picked right back up with no audio sync issues.
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Postby jor » Fri Feb 24, 2006 2:38 pm

I too have seen a healthy number of blackouts - on my hdtivo using directv's satellite feed for NBC-LA. Maybe they're sending out a marginal or vaguely noncompliant feed.

That said, I think NBC delivered on HDTV this time, for the first time. Still the crap with them tape delaying, and saving the critical segments for 11:30 at night, and the human interest BS, but in terms of the picture on what seems like every single camera, it has been great. The dropouts don't really hurt me too much.
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