Wihich way to use Half D1- there are conflicting opinions out there

Discussion in 'Amateur Video Production' started by Sanman, Apr 20, 2004.

  1. Sanman

    Sanman Guest

    Strait to the point... Do you use half D1 to fit more on a DVD, or to use
    same bitrate and get better quality?

    In my experience, encoding to half D1 doesn't reduce the file size to half,
    so if you use half the bitrate to encode, you will lose quality in the mpeg
    file. I haven't got numbers, but encoding to half D1 only reduces the file
    size by about 25%. (which makes sense because you're only cutting one of
    the two dimensions in half - the vertical) So, you're only getting a bit
    more time on a DVD.. For VHS, isn't it better to keep the same bitrate and
    take advantage of less info per frame, thus better quality encoding at that
    bitrate? For example; VHS to a DVD at 4000kbps gives 2 hours at
    not-so-good quality, vs., VHS to DVD at 4000kpbs at half D1, which needs
    less compression to fit on a 2 hour DVD because it's smaller. Less
    compression means better quality (for VHS).

    I have read that using half D1 at a high bitrate is useless. I have done
    tests and found that a video encoded at half D1 - 6000kbps has less mpeg
    artifacts than a full D1 file at the same bitrate, and it looks better,
    (Again, with VHS source)
    So, how do we use half D1? ... To save space, or to get better quality
    encoding for VHS archiving?

    Sanman, Apr 20, 2004
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  2. Sanman

    Toshi1873 Guest

    Only if you haven't tweaked your encoding parameters.
    (Motion Search Precision is the key one that needs to be
    considered, and probably a few others.)
    Uhhhhh... the *only* factors that affect the size of a
    DVD clip are the bitrate and the run length. Resolution
    (full-D1 or half-D1) has *zero* impact on file size.

    .... and I'd suggest you go back and re-visit Geometry
    101 (704x480 is 337920 pixels, 352x480 is 168960 pixels
    or *half* as many pixels)
    Half-D1 probably stops showing improvement after 4000-
    6000. However, where you draw the line depends on the
    quality of the encoder, VBR vs CBR, Motion Search
    Precision, and any other tweaks that have been made in
    the encoder settings.

    The fact that you noticed that half-D1 encodes better at
    a given bitrate then full-D1 is a bit silly. It's half
    the pixels, so naturally it will require a lower bitrate
    then the full-D1 to achieve the same quality.
    Toshi1873, Apr 20, 2004
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  3. Sanman

    Ken Maltby Guest

    I agree, yes to both parts of your question.
    Very important points, and they can be, and most often
    are, impacted if not controlled by your capture HW&/ SW.
    (While this seems self evident, these factors are seldom
    included in the selection/consideration of hardware or
    software for this purpose.)
    Basically true, but using the lower resolution can allow a
    lower bitrate, and still handle image changes better than
    would be the case at a higher resolution. As long as the
    source image (analog) can be digitized without significant
    loss, at a any particular resolution (digital); a "true to
    source" result is possible. Increasing the resolution above
    what is needed for reproduction of the source, shows little
    if any improvement, in terms of the resulting image detail
    or other factors associated with the benefits of higher (digital)
    resolution. There is only so much image data in an analog
    TV or VCR signal.
    Now what is here, is what was made of the original analog image
    by the A/D chip (as a baseband, probably close to 100Mbps)
    and then, for hardware sent to the MPEG Encoder chip or for
    software provided in some "uncompressed" DV format.

    If NO compression was applied then :

    337,920 x 8bits = 2,703,360bits + 6,400bits (audio @192kbs)
    = 2,709,760 bits per frame x 30fps = 81,292,800bps

    168,960 x 8 bits = 1,351,680bits + 6,400bits
    = 1,358,080 bits per frame x 30fps = 40,742,400bps

    Every pixel rendered at 8 bits. ( of course there are 10 bit and
    greater A/D encoder chips.)

    Of course you never see the above, as MPEGs whole point
    is to provide a compression to the images.
    All true, but I don't see why it's silly? It's kinda the point
    isn't it?

    Both, it's part of a balancing act that includes the points
    that "Toshi1873" mentioned.

    Ken Maltby, Apr 21, 2004
  4. Sanman

    Sanman Guest

    Yes, I'm aware of that. My mistake, I was thinking of capturing
    uncompressed video at lower resolutions, not compressing.
    Good point. Lose the sarcasm.
    I disagree. Encoding half the data with the same bitrate will render less
    artifacts in the encoding than with full D1. It makes perfect sense that
    the encoder will have less information to compress at the same bitrate, and
    will have to compress *less* to stay within that bitrate.
    Exactly, which is why it would look better at the same bitrate, other than
    it being lower resolution. My tests show that there is far less motion
    artifacting with Half D1 at the same bitrate as Full D1. The difference was
    clear, especially during <pause>.

    Thank you. You've answered my question.

    Sanman, Apr 21, 2004
  5. Sanman

    Bariloche Guest

    Both. For instance, I'm encoding material from VHS as half-D1 in 100%
    CQ mode. This usually amounts to a bitrate between 6500 and 7000,
    although at times it can even go beyond 7000. I get the best in
    compression terms, and some benefit in storage ones.
    Bariloche, Apr 21, 2004
  6. Sanman

    Bariloche Guest

    Obviously. But the half-D1 / full D1 question is a complex one. If one
    needs encode at very low bitrates, say 2500, because of storage
    considerations, one would use half-D1 in all cases, because that
    bitrate is very low for a full D1. But at higher bitrates, and for the
    same given bitrate, half-D1 gives you better encoding quality while D1
    gives you a sharper image. And there are factors that can very much
    affect the result. For instance, low motion would favour the use of D1
    (for a better detail), while fast motion favours half-D1 (reducing
    artifacts). The use of filters (denoise, sharpening...) would also
    favour one or the other. Commercial movies use D1, thus evaluating
    better the resolution factor than the compression one. But that is
    because they use of 8000 and above. At lower bitrates things may be
    not so clear. I do not have enough experience to aseverate on this,
    and would gladly hear from those that have it, but wonder if a 3000
    kbps half-D1 may not look better than a 4000 kbps D1, in spite of the
    lower bitrate -at least, in some cases.
    Bariloche, Apr 21, 2004
  7. Sanman

    Will Dormann Guest

    I still think that in most cases, the Half D1 at 3000 will look better
    than Full D1 at 4000. But it may depend on your source material.
    High motion scenes are better represented with Half D1, and low motion
    with Full D1. And if your source material isn't high resolution and
    very sharp to start with, I see no reason to go Full D1 for your output.

    See some examples for yourself:

    Will Dormann, Apr 21, 2004
  8. Sanman

    Bariloche Guest

    Nice and instructive photos. Pity the DVD standard does not allow a
    per-scene resolution!
    Bariloche, Apr 22, 2004
  9. Sanman

    Bariloche Guest

    One more consideration: if you multiply 1150 (the Vcd bitrate) by 4,
    you get 4600, which would be the bits/pixel equivalent of a 704x80
    mpeg-2 for Dvd. For 720x480, it becomes 4705 kbps. Thus, 4000 kbps is
    a really low bitrate for D1.

    Now, I'm still wondering if Rempeg2 can do a better job than
    DvdShrink, as it would use a resolution more appropriate to the
    lowered bitrate; the con is that it re-encodes, which is a factor of
    degradation. As Rempeg2 doesn't work for me, I may have to do the test
    myself by a different and longer route.
    Bariloche, Apr 22, 2004
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