Why Don't Digicams Use Hard Disks?

Discussion in 'Video Cameras' started by keef, Sep 20, 2003.

  1. keef

    keef Guest

    As above.

    Only recently started looking at buying a digicam & had assumed they use
    Hard Disks to write to. Now I see they use tape ... that's not very
    digital.

    Why? Why not stick a mini 100GB Hard Disk in instead with some RAM to
    buffer? It can't be a capacity or write-speed issue as neither would be a
    problem. My background/work is UNIX systems support so perhaps I'm biased
    :eek:)

    Just wondering.

    Cheers,

    keef
     
    keef, Sep 20, 2003
    #1
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  2. keef

    Trev Guest

    And how much is it going to cost to keep your vids on a pile of HD
     
    Trev, Sep 21, 2003
    #2
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  3. keef

    keef Guest

    Doesn't everyone have a PC these days? :eek:)
     
    keef, Sep 21, 2003
    #3
  4. keef

    Tony Morgan Guest

    Of course it is. In fact from 1950 through to 1990 tapes were almost
    exclusively the (only) digital storage medium. And today, its rare to
    find larger computer systems that don't employ tape as a storage medium.
    Ummm.... Have you peeked inside your PC had a look at the size of a
    100GB hard drive. And not only are 2.5" drives larger than a miniDV
    tape, anything larger than 30GB is inordinately expensive.
    Then I'm a little surprised that you haven't worked it out. Especially
    since most UNIX based systems invariably use DAT tape (for the very same
    obvious reasons as outlined below).

    Your 100GB hard drive will only hold the equivalent of 7 miniDV tapes
    worth of video. Then what are you going to do with your precious video
    clips (as well as your finished edited movies)? I haven't looked, but I
    imagine that a 100GB 2.5" hard drive will set you back something in the
    order of £150. Equate that with the cost 7 miniDV tapes - £26 or
    thereabouts. The way that a lot of people skimp by buying the cheapest
    tapes they can find, I hardly thing that your idea is a very practical
    or economic once - nor is it likely to catch on :)
     
    Tony Morgan, Sep 21, 2003
    #4
  5. keef

    Jerry. Guest

    150 ukp on the price of a half decent camera is hardly going to break the
    bank, the reason why HD's are not used in domestic camcorders is due to the
    fact that the average 'Joe Public' don't want a large camera - rather
    something that is so small that it fits into a breast pocket or ladies
    handbag, not to mention image stabilisation to make any hand held footage
    viewable....

    But it is dear Tony, admittedly only on ENG camera's ATM, but then DV was in
    that domain before it became cheap enough for domestic use.

    As for what to do with any footage on the cameras HD, you do exactly what
    most do now, download it to your computer, edit it, and then either burn it
    to DVD etc. or make a VHS tape.
     
    Jerry., Sep 21, 2003
    #5
  6. keef

    Guest Guest

    Tape isn't digital? Don't Unix systems use tape to back up?
    Gyroscopic effects during pans? ;-)

    PeterS
    Remove my pants to reply.
     
    Guest, Sep 21, 2003
    #6
  7. keef

    lpp Guest

    Why? Why not stick a mini 100GB Hard Disk in instead with some RAM to
    An affordable StediCam :=-)
     
    lpp, Sep 21, 2003
    #7
  8. keef

    SjT Guest

    They will be on media card in the future (i.e. simm cards) But i dont
    know what capacity or format these will be.

    I have heard that cams will record in DVD Quality footage, using 4GB
    cards, hope not though the loss will be terrible when editing, not to
    mention slow working with high compressed video.
     
    SjT, Sep 22, 2003
    #8
  9. keef

    Tony Morgan Guest

    "DVD Quality" is MPEG-2, so I suppose on a 4GB card you'd be able to get
    storage of about 90 minutes of video. Sounds reasonable until you look
    at the relative cost of tape/card. But there again, the cost of memory
    cards continues to come down.

    The problem in my view is that MPEG-2 is lossy. MicroDV hasn't really
    caught on because of the lossy nature of the medium - so I'm not holding
    my breath :)
     
    Tony Morgan, Sep 22, 2003
    #9
  10. keef

    Darcy O'Bree Guest

    They do if you've got the big bucks!

    http://www.ikegami.com/br/products/sdtv/dng_frame1.html

    http://www.sonybiz.net (search for XDCAM or DSR-DU1)

    http://www.jvcpro.co.uk/prod/item/index_html?item=DR-DV5000E

    And there's one in development from Panasonic
    http://www.panasonic-broadcast.co.uk/_web/index.cfm?uuid=96D7125DD7445163A95
    9B494CF78B6C8

    Although not all strictly hard disks they do eliminate tape.

    Darcy O¹Bree
    Digital Media Studios Manager
    Faculty of Arts, Media and Design
    Staffordshire University

    http://www.staffs.ac.uk/directory/viewperson.php?person_id=1320
    http://www.staffs.ac.uk/vrtour/stoke/media/
     
    Darcy O'Bree, Sep 22, 2003
    #10
  11. Some do. Others write direct to DVD media.
    miniDV provides adequate quality, and the tapes survive being dropped (up to
    a point!)
     
    Clickety-Click, Sep 22, 2003
    #11
  12. keef

    Guest Guest

    I have to admit to not having seen any MicroDV content, but DV is lossy, and
    that caught on.

    PeterS
    Remove my PANTS to reply.
     
    Guest, Sep 22, 2003
    #12
  13. keef

    SjT Guest

    I think this would work great for those people only wanting to record
    straight from Camcorder to DVD disc, i.e no conversions etc.
    Some people claim that their mpeg-2 codecs are good for 3 generations
    of rendering, i think these people need glasses personally.

    But who knows?!

    Do you know if the AVI DV format operates similar to mpeg, in that it
    stores the changes from one picture to the next? or is the frame
    re-drawn each time from scratch? ive always wondered that
     
    SjT, Sep 23, 2003
    #13
  14. keef

    Tony Morgan Guest

    Personally I can't see any real advantage, since AFAIK you can't
    directly stream from the camcorder to the DVD burner, so it's got to be
    at least buffered to the hard disk in one way or another when burning.
    Me too. I wonder if the folk who come out with this sort of observation
    only talk about it and have never actually done it (or maybe read it
    "somewhere"). Such is the way that urban myths get started :)
    Not AFAIK. AVI DV is an exact facsimile of what comes off the miniDV
    tape except that it is contained in an "envelope". And DV of this sort
    is frame-by-frame.
    AFAIK MPEG-2 is also frame-by-frame. The main *practical* difference is
    that DV is compressed but non-lossy while MPEG-2 is compressed much more
    and is lossy. Naturally the compression algorithms of the two are
    completely different.
     
    Tony Morgan, Sep 23, 2003
    #14
  15. keef

    Radiohead Guest

    Doh! And you work in technical support? You confirm all my worse prejudices.
     
    Radiohead, Sep 23, 2003
    #15
  16. keef

    SjT Guest

    Is that not a video signal that has to be re-rendered when streaming
    into a dvd burner though, rather than a direct digital duplication?!
    Well, it must be frame-by-frame looking at the filesizes, 30FPS would
    be 30 seperate interlaced images, so if each image was a high quality
    JPEG (At TV Resolution) that flicked through to build the image on the
    screen, for example, then each image would be around the 100k mark
    (100k * 30 = 3mb/sec).

    Damn im rambling on, thinking out loud.. :)
    I thought all the video MPEG codecs only stored the differences
    'between' frames, hence when alot changes on screen with a low bitrate
    it takes a while for the picture to regain clarity again?

    Also MPEG file sizes vary even when the running-length is exact, which
    suggests to me that their must be a variation happening somewhere, and
    that could either be down to less detail on each image being stored,
    or less movement (If MPEG is only recording the 'changes' to each
    frame) i may try and film a scene that is rock solid-still for half
    hour and one that is moving to see if there is a big difference when
    compressed to MPEG-2 for example.

    I dont know why it bothers me really, i guess that i like to
    understand what is happening, and why DV AVI creates such massive
    files.

    Then again, it could be the audio in DV AVI files that take alot of
    the 3.5mb/second up (!?). im just gonna confuse myself now :)
     
    SjT, Sep 23, 2003
    #16
  17. keef

    Dave R Guest

    Sort of... in MPEG there are 3 different "frame types". I, B and P. The
    I-frames are the initial frames and consist of a complete snapshot of the
    original frame. The B-frames then contain info about the current frame
    plus differences with the previous and next frames. As the picture
    generally doesn't change that much, this is quite a good saving on space.
    The P-frame I think is based only on the previous frame, or previous I-
    frame.

    - Dave.
     
    Dave R, Sep 23, 2003
    #17
  18. keef

    SjT Guest

    And whatabout DV AVI is it made entirely of I-Frames?
     
    SjT, Sep 23, 2003
    #18
  19. Sort-of-ish! In fact the compression on DV is on each individual
    frame, without reference to adjacent frames. It uses DCT in a similar
    fashion to MPEG, (and this *is* lossy).

    The reason that MPEG is not liked for editiing is that to do good
    transitions the in-between frames must be reconstituted and
    recompressed, and this causes much more degradation. A cut also has
    to rework several frames either side, whereas with DV a cut is just a
    simple join.

    Regards,
    Harry.
     
    Harry Broomhall, Sep 23, 2003
    #19
  20. keef

    Guest Guest

    Nope - check the spec.
    while MPEG-2 is compressed much more

    Of course DV is lossy! Do the sums!
    720 x 576 pixels by 25 frames per second at 4:2:0 is 15.5x10^6 bytes per
    second (assuming eight bit samples), or 124 Mbit/sec.
    DV is 25Mbit/sec - that is 5:1 compression.
    Basic information theory says the best compression you can get lossless on
    uncorrelated data is around 2:1 - anything more MUST be lossy (yes, the
    video data will not be completely random).
    No, they're both based on the discrete cosine transform - they are not
    "completely" different.
    The difference is in the motion prediction (inter-frame coding) in MPEG2.

    PeterS
    Remove my PANTS to reply.
     
    Guest, Sep 23, 2003
    #20
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