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How TiVo cheated death

Discussion in 'TiVo Coffee House - TiVo Discussion' started by Malcontent, Feb 2, 2009.

  1. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    These are NOT "exactly the same issue"! The behavior may be similar but the solution is almost certainly not.

    I'm not sure if you know how MPEG-2 encoding works but basically there are these groups of frames called GOPs. At the start of every GOP there is an I frame, which is a full frame of video compressed with basically the same technology used for JPEG images. After that there are P and B frames. These frame are not full frames. They only store the differences between them and the previously decoded frame. What this means is that in order for the decoder to display one of the frames in the middle, or at the end, of a GOP it has to first decode all the frames that came before it. (over simplification, but you get the idea)

    This system works fine for realtime playback, and even for slightly faster then realtime decoding (depending on the speed of the hardware doing the decoding), but it's impossible to do high speed decoding of every single frame using an inexpensive hardware decoder like TiVo uses. Which means that TiVo is forced to use tricks that allow it to display enough frames to make FF seem smooth while also eliminating enough frames so that it moves at the proper speed.

    One trick commonly used for lower speed FF is to simply skip B frames. B frames take the longest to decode, and the P frames can still be decoded without them, so skipping them allows the stream to be decoded significantly faster creating a decently smooth and moderately fast FF speed. However for super high speed FF they usually just display the I frames. Since they are full frames and require minimal power to decompress they can be skipped to and displayed very quickly.

    For DVDs there is a strict standard which says that the GOP can be no longer then 18 frames. Which means that they have an I frame every 2/3 second or so and TiVo can use simple I frame decoding to get a decent FF. I think the only reason TiVo had trouble with DVDs was due to media latency and not the streams themselves. DVDs are slow and somewhat erratic to read from. TiVo probably had to employ a little extra buffering and maybe a little timecode monitoring to make FF smooth from DVDs.

    Now for digital cable there is no standard. Most cable providers stick to pretty normal GOP lengths and use a repetitive pattern of IBBPBBPBBP... frames so that the tricks above work OK. However I've seen some digital cable recordings that had GOPs 50+ frames long and used an excessive number of B frames to maximize compression. It's those types of streams that TiVo most likely has trouble with, and unfortunately there isn't really anything else they can do to handle them better. It's just a fact of life when you're dealing with a system where you have no control over how the MPEG-2 stream is encoded.

    All that being said I've never had any real trouble with FF/RW on my S3. On occasion it will seem like FF is a little faster or slower on certain shows, but it's never enough to totally screw up the whole process. Perhaps the reason it's an issue for you is because your cable company is over compressing the signal to save bandwidth. You should try transferring one of the problem recordings to your PC some time and then opening it in VideoReDo and checking the B/P frame pattern and the GOP length. I bet they either have really long GOPs or erratic and/or excessive use of B frames and that's why FF does work correctly with them.

    Dan

    P.S. If I had to guess I'd say that TiVo probably uses IP only decoding for 1x FF and I only decoding for 2-3X FF. So even 1x FF could get messed up if a stream has an excessive number of B frames.
     
  2. berkshires

    berkshires New Member

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    Feb 22, 2007
    That would be interesting. We should have a whole thread devoted just to examining those and observations about FF under different circumstances in the TTG Forum.
     
  3. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    I agree. If anyone does start such a thread shoot me a PM so I can check it out. (I don't always catch every thread on my own)

    Dan
     
  4. HerronScott

    HerronScott Well-Known Member

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    Jan 1, 2002
    Staunton, VA
    Are you still watching in SD? HD would certainly qualify as a post-Series1 feature (and the primary reason we moved from our Series1 TiVos to Series 3 TiVos when they offered the lifetime transfer).

    Scott
     
  5. jrm01

    jrm01 New Member

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    Oct 17, 2003
    Pittsburgh
    Wow. Thanks for all the great information. I do get the impression that you know what you're talking about.

    My S3 has had problems with the FF ever since 9.3. Starge thing is that FFx1 will start out at realtime, and then if held long enough will finally speed up. Also, on some channels the FFx2 and FFx3 look very choppy, but that is usually on the channels where Comcast has decided to place 3 channels per QAM instead of 2, probably resulting in longer gaps between I-Frames.

    Of course, once this problem started I switched to 30-sec skip for the vast majority of my use and don't really miss the FF reliability.
     

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