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Clarification regarding H.264 AVC

Discussion in 'Other video questions' started by morphosix, Jan 9, 2007.

  1. morphosix

    morphosix Member

    Dec 25, 2006
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    I recently started converting videos to MP4 to put in my Sony PSP. I was reading about my format options and apparently, I have two---standard MP4 or AVC. And according to everyone, AVC is the way to go due to its high compression.

    Here is where my confusion starts.

    I converted a video in standard MP4 format and AVC format with same settings for both (bitrate, resolution, fps, etc.) to see the difference. The first thing I noticed were the filesizes for both are roughly the same, give or take a few kilobytes. I had the impression that AVC format produces smaller filesizes, but in this case it didn't.

    Okay...so then I thought, maybe it produces twice the quality in half the bitrate. I converted the video again, but this time AVC in 384kbps. Results...great quality with the AVC still and since it's half the bitrate, I expected the smaller filesize. At this point, I'm understanding that AVC at lower bitrates still produces good quality picture and minimizes the size because of the obvious lower bitrate.

    But then I came across this website which shows side-to-side image quality comparison of different settings. It shows (looking at the two bottom ones) that at 768kbps, standard MPEG4 produced a 304.2MB file while AVC produced a 114.5MB file.

    So now...I don't get it. How did they get less than half the filesize on AVC at the same bitrate? Am I missing something here or am I not understanding something? Sorry for the long post, by the way. Hopefully someone would still clarify this for me.
  2. celtic_d

    celtic_d Regular member

    Jan 23, 2005
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    File size = bitrate * length

    If the filesize is smaller, then the bitrate is lower or the length is shorter. Also the set bitrate and actual bitrate may not always be the same. For instance with AVC you might reach a saturation point where you can't throw any more bits at it (especially if you are limiting quants).

    So to sum up, you aren't missing anything. They are since if the files have the same bitrate, are encoded from the same source (means the same length) with the same audio in the same container then the filesizes will basically be identical. Only difference would be possible a possible increase in overhead for one of the video streams over the other.

    One thing also to consider is that I think the PSP only supports SP MPEG-4, which is really limiting the quality since bframes can really help at lower bitrates as can custom quant matrices.

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