HD Bit Rates?

Discussion in 'TiVo Home Media Features & TiVoToGo' started by larrs, May 23, 2011.

  1. larrs

    larrs Movie Fan-Addict

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    While attempting to archive my video collection, including hundreds of HD movies captured from my Tivo in 1080i, I am looking for input on acceptable bit rates.

    I am converting to mp4 and mp2 using VideoRedo and MeGUI.

    With a 1080i video in mpeg 2 with 5.1 DD which has a bit rate of +/- 18Mbps, what can I generally get by with when reducing the bit rate in mpeg2?

    mp4?


    If I downscale to 720P@ mpeg2?

    mp4?


    I appologize if this sounds like a first grader to those of you experienced in this sort of thing, but I am probably in Kindergarten when it comes to video conversion.
     
  2. orangeboy

    orangeboy yes, I AM orangeboy!

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    I use ffmpeg + x264, and use a constant ratefactor, so I don't worry much about figuring bitrates.

    Here's a sample command line:

    Code:
    ffmpeg -i "<input file>" -acodec copy -vcodec libx264 -fpre "D:\ffmpeg\.ffmpeg\libx264-veryfast.ffpreset" -crf 20 -level 41 -y "<output file>"
    
     
  3. jcthorne

    jcthorne Well-Known Member

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    I too use x264 CRF of 20 and have been very pleased with the results. I use meGUI as an interface for almost all encoding/remuxing etc. Its a nice set of tools and keeps them all current.

    Oh, to answer your original question, 1080p24 video with a CRF of 20 usually comes in at about 4GB per hour with 640kbps AC3 audio. It varies depending on content but thats about average.
     
  4. dlfl

    dlfl Cranky old novice

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    That calculates to 8.9 Mbps for the video track bitrate. What is your typical input MPEG2 video bitrate? Doesn't sound like much compression. :confused:
     
  5. txporter

    txporter One sec, almost done

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    If you want to take 1080i video and compress it as MPEG2 @ 1280x720, I have been able to do this and see good results at 6kbps (this is using avisynth to IVTC back to 23.976fps and encoding with HCenc).

    If you want to still use MPEG2 but stay at 1920x1080, then you can still probably get away with reduced bitrate as long as you IVTC (simply because you have fewer frames per second to encode). Something in the neighborhood of 13kbps is likely transparent.

    Like others, I generally compress with x264. I tend to reduce to 720p as well with x264 since I don't notice the difference. I use CRF 21.
     
  6. jcthorne

    jcthorne Well-Known Member

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    The only HD Mpeg2 streams I have are from the tivo and those run 10 to 12 GB per hour, sometimes more.
     
  7. dlfl

    dlfl Cranky old novice

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    :confused: Something seems off here. 12 GB/hr computes to 26.7 Mbps. :eek: The highest bit rate I've seen in HD recordings from my TiVo HD is 15 Mbps. What channel(s) are you getting these rates on?
     
  8. larrs

    larrs Movie Fan-Addict

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    I get 18 on my system for 1080i and it is an all fiber network. I would think 27 would have to be OTA from a channel with no sub channels whatsoever.
     
  9. orangeboy

    orangeboy yes, I AM orangeboy!

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    Using kmttg as my gauge, the highest OTA recording I have is just over 16½Mbps. Transferred 1080p, ~25½Mbps.
     
  10. larrs

    larrs Movie Fan-Addict

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    THis is exactly what I needed to hear. I'll look into it.
     
  11. wmcbrine

    wmcbrine Well-Known Mumbler

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    I would not even consider reencoding to MPEG-2 at a lower bitrate.* Go h.264, or leave it alone. Preferably the latter.

    Back in the day, I used to use a "requantizing" program to reduce the bitrate of some videos to get them to fit onto single-sided DVDs, without completely reencoding them -- it was much faster, and higher-quality. If you feel you have to reduce the bitrate while staying in MPEG-2, I'd try that approach first.

    * Except to make SD DVDs from HD video. But I don't really do that anymore, either.
     
  12. lrhorer

    lrhorer Active Member

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    I'm not at home now to double-check, but I think the highest average bit rate I have ever seenfrom an MPEG2 CATV source is from Discovery HD Theater, averaging at just over 17 Mbps and peaking at just a bit over 20. They were episodes of Planet Earth
     
  13. lrhorer

    lrhorer Active Member

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    Actually, in absolute terms, it's a huge amount of compression. A raw 1080i video stream tops out at over a Gigabit per second.
     
  14. lrhorer

    lrhorer Active Member

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    I think you mean Mbps, not Kbps. A bit rate of 6 Kbps isn't even enough to encode an uncompressed phone conversation.
     
  15. orangeboy

    orangeboy yes, I AM orangeboy!

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    That made me literally made me laugh out loud! :D

    I ventured into the Kbps range when I was using VLC to stream on-the fly video to my sister's house. I think I settled on ~1450 Kbps due to paltry upload capacity with my RoadRunner ISP.
     
  16. dlfl

    dlfl Cranky old novice

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    True but I meant compression relative to the MPEG2 he was re-compressing. (I thought that was obvious from the context but ..... ;))
     
  17. jcthorne

    jcthorne Well-Known Member

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    I was obviously mistaken. I need to learn to rely on my memory less as I get older. Looking through what is stored on my tivo right now, most broadcast video is stored as 720p mpeg2 and runs 5 to 7 GB/hr.

    Movies stored as 1080p24 h264 run 2.5 to 3.5 GB/hr although some encodes are much smaller.

    So roughly half the data size for twice the resolution using h264
     
  18. txporter

    txporter One sec, almost done

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    Re-quantizing (ala Rejig, DVDshrink, etc) is definitely faster, but definitely not higher-quality. You are simply changing the DCT coefficients without redoing motion analysis. A full re-encode will provide more efficient re-distribution of the lower bitrate. This is post for more details on requant vs re-encode/transcode.

    Oops. Yep. 6000kbps or 6mbps.
     

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