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GH1 I frame codec with gop1
  • I was thinking about the possibility of creating an iframe avc recording by forcing all frames to be iframes.

    This seems really interesting.
  • 29 Replies sorted by
  • I'm doing some tests on this right now. I think 90mb is too high to push GOP 1.
  • GOP 6 results in an error which doesn't allow the camera to go above 18mb.
  • Ok. I've done a little testing and I can do 35mb GOP1 and it is stable with an anamorphic lens. All ifames in stream parser. Unfortunately it is buggy in playback
  • 35mbps as iframes seems like it has potential for better quality than a higher bitrate without?

    What do you think?
  • Yeah. If it worked correctly 35mb with just Iframes should be better quality.
  • What are the other settings? Can you post your .ini here? I played with GOP1 around 35mb and only had fails.
  • Well it looks correct in StreamParser, but in the NLE and in VLC and Splash Lite its obviously not correct. 458MB for 1:49 of footage as well

  • The 85mb gop1 setting was crashing too often for me with hacked GH17 /SanDisk Extreme 30MB/s/kit lens..so i went down a bit on the numbers, about 15% from the original values posted on the other forum..still errors,but not as often, at lest not at f 2.8 , and most clips you see in this fast test gave a bit rate between 37-47
    , i think to be able to use f4 and above with lots of detail with gop1 i will try to go down in value a bit more..60p still lots of errors at gop1
  • Here is a question, is it better to have lower bit rate with all iframes or higher bit rate iframes w pframes? Im sorry if its a stupid question ..
  • Lowering gop always decrease quality with the same bitrate, because I-frame compression is much worse than P-Frame.
  • Not a stupid question to me, I would like to know the answer as well, because shooting with the native 24p blackout powell patch averages in streamparser show a low average anyways, so if I used all I frames OR mostly all I frames I do not know the technical differences enough to know which of the variations would have the best visual output in comparason to the blackout powell native 24p patch. Perhaps someone with technical knowledge about I, P, etc frames and 24p/60p can give details about this idea from telkitachki and how it possibly could (at least in theory) produce better results, or if not now at least in the future when sdhc cards can run faster.

  • thanks Vitaliy, so you think is good idea to stay away from gop1 even if we get stable patch with lower bit rate?
    I also want to thank you for the GH17 hack.
  • what s the advantage of gop1 avchd over mjpeg?
  • 24p versus 30p for one I'd think.
  • isnt mjpeg at 24p possible with gh13/gh17?
  • It's news to me if it is. Anyone?
  • its possible, but not true 24p, but it rather drop frames
  • sammy asked: "...is it better to have lower bit rate with all iframes or higher bit rate iframes w pframes? "

    If you examine the data size of AVCHD frames in Stream Parser, you'll find that I-frames are typically about 3-4 times larger than P-frames. As Vitaliy pointed out, I-frame compression is much less effective than P-frame compression. Thus, if you construct a video entirely out of I-frames, to maintain the same quality level you would need to increase the bitrate by 3-4 times. This is why professional AVC-Intra encoders produce bitrates of over 100Mbps, while AVCCAM encoders (which use I, P, and B frames) record at 50Mbps.
  • > you'll find that I-frames are typically about 3-4 times larger than P-frames...
    > I-frame compression is much less effective than P-frame compression...
    > Thus, a video entirely out of I-frames would [be] 3-4 times [the bitrate]

    Uncompressed P-frames contain less data than uncompressed I-frames, if I get it right, so I do not know if the compression itself plays a factor as well or if it is just the same for both.

    The size increase for only I-frames would limit to 3-4 times, I guess, depending on what GOP you use: each GOP already has one "large sized" I-frame, of course...
  • Thanks for the response LPowell, so do you think its best to stay at a gop where we get the best of both worlds ,say about gop of 6?
  • Mere speculation here - but does anyone think 100mbps is possible on gh2?
  • 100 Mbps is already possible in GH13. But not with GOP 1.
  • Telkitatchki's GOP 1 opens up interesting possibilities although I can't get smooth video with my cards. The bit rate was high but the video stuttered. There were regularly spaced skipped frames (the upper limit was set at 48mbps, my class 10 cards cant reliably handle anything higher). With GOP15 I only got an average of 20mbps for the same scene.
    Copy of gh17_gop1.jpg
    800 x 425 - 58K
  • redsky, you just made something wrong. It is not possible to loose frames with my settings. It looks like you did not increase bitrate in ptool, and just set gop1. Max 100 to 130 mbps is a tipical bitrate for this settings, and average bitrate is always very high even at dark low detailed scenes. Try this some changed more stable settings. I set GOP2 for 720p 60fps and it works great, of course not for all scense, but for most scenes which i need. Just try this settings for real shooting, not for shooting big bushes in daylight and you will see that it is stable. Of course if you work on animal planet and everytime shoot deers under the trees, this settings not for you.

    Native 24p/25p=Checked
    Video Bitrate FHD/SH=119900000
    Video Bitrate H=74000000
    Video Bitrate L=74000000
    Overall Bitrate=119900000
    720p60 GOP Size=2
    1080p24 GOP Size=1
    Preset bitrate=1199
    Preset bitrate 2=1199
    Overall Bitrate 2=119900000
    Constant for 1080p=3888000
    Constant for 720p=466560
    Video buffer=84.000.000
  • sammy asked: "...do you think its best to stay at a gop where we get the best of both worlds ,say about gop of 6? "

    Selection of GOP-size is a trade-off between static image quality versus dynamic image quality. With a limited average bitrate, shortening the GOP-size will reduce the bitrate available for encoding image details in each key frame, which will degrade static image quality in highly detailed scenes. On the other hand, a shorter GOP-length will improve the encoder's ability to track rapidly moving objects, which will increase dynamic image quality in highly agitated scenes.

    At the 50-100Mbps bitrates that can be reliably used with fast Class 10 SD cards, I've found that extremely short GOP-sizes as low as 3-5 frames can be used on 1280x720 videos recorded at 25p or 30p. 1280x720 videos recorded at 50p or 60p work more reliably with a GOP-size of 15 frames or more. 1920x1080 videos also need moderately long GOP-sizes of 12-15 frames.

    Reducing GOP-size to 1 causes the AVCHD encoder to encode each frame as an individual I-frame. As long the amount of data required to encode all the details in each frame does not exceed the available bitrate, an I-frame-only encoder will track all moving objects as quickly as the frame rate allows. However, this mode of operation forces the AVCHD encoder to work more like an MJPEG encoder, and sacrifices AVCHD's most efficient compression techniques. As a result, image quality will rapidly degrade in highly detailed scenes, since the encoder is forced to use excessive amounts of bitrate to redundantly encode the same details repeatedly in each individual frame.

    For aesthetic reasons, full-HD 24-25p films are shot with only a limited amount of object motion or fast panning action. For this type of filming, GOP-sizes up to 15-frames can be used without noticeable motion-tracking artifacts - so long as the encoder's maximum bitrate is high enough to handle the amount of image detail required. With the excellent compression efficiency provided by P-frames (and when available, B-frames), average bitrates do not need to reach very high levels. With the most highly detailed scenes, maximizing the peak bitrate is what really counts.