*Frame* Rate vs. *Refresh* Rate

Discussion in 'Digital SLR' started by Radium, Jul 6, 2007.

  1. Radium

    Radium Guest


    What is the difference between "frame rate" and "refresh rate"?

    Is it best for the frame rate to have a frequency the same as the
    power supply [60 Hz in USA/Canada, 50 Hz in Europe/Asia]? Is this true
    even if the video is digital and played through a digital plasma

    In digital video, how is the sample rate mathematically related to the
    frame rate?

    In digital video, how is the sample rate mathematically related to the
    refresh rate?


    Radium, Jul 6, 2007
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  2. I am not aware that they are related in any way.

    The highest frame rate of any DSLRs that I am aware of is 10 frames per
    second on high-end Canon SRLs. According to Ct (which in turn cited sources
    in Canon corporation) this frame rate is artificially lowered because at
    many venues a higher fps rate is considered filming and the fees are
    significantly higher than for photography.

    On the other hand the only refresh rate that comes to my mind would be the
    time until a flash can fire again.
    Huuu? Where do you find a DSLR with a frame rate of 60 fps? That's nuts.
    Huuuu? What does digital video have to do with SLR cameras?

    Jürgen Exner, Jul 6, 2007
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  3. Radium

    mrdarrett Guest

    That's really interesting... because I have a Canon Powershot A530,
    and the manual (which is for both the A530 and A540) mentions that the
    A540 can shoot 640x480, 30fps, and 320x240, 30fps.

    My A530 can only shoot at 20 fps at 320x240 (or 10 fps at 640x480).
    Using Nero's VCD Burning Tools, I was able to create a Video CD that
    would play on my DVD player... video was a bit jerky, though.

    Makes me wonder whether I should just get an A540 (smaller), or
    actually buy a digital camcorder next time...

    mrdarrett, Jul 6, 2007
  4. Radium

    Alan Guest

    One is how fast information frames come, one is how fast the screen is

    Not really. It helps in reducing the visibility of hum bars if all three
    are the same, but power supplies are way better now, so that is of little

    Yes, it is unimportant even if the video is digital and shown on plasma.

    By an integer multiple.
    What makes you think it is?

    Alan, Jul 6, 2007
  5. Well, but how many professional photographers are out there using a
    Powershot for their work?

    Jürgen Exner, Jul 6, 2007
  6. That confused me too but look at where the question has been
    crossposted and its obvious the OP is erm... clutching at straws for
    an answer.

    Dr Hfuhruhurr, Jul 6, 2007
  7. Radium

    contrex Guest

    What baffles me is how does he type wearing a straitjacket?
    contrex, Jul 6, 2007
  8. Radium

    Flasherly Guest

    Twice. 24 to 48 for high motion film or field rates using two stops at
    each frame. As applied interlacing, and apart from detail,
    deinterlaced line rates are accountable for a multiple again twice
    over to equal perceived smoothness.
    Depends on the digital video card capabilities and whether or not it's
    limited to the vertical sync of the monitor. A monitor maximum sync
    at 60Hz isn't going to help a card capable of rendering 100 frames a
    second, so it's a parity function while within specification limits of
    the hardware.
    Flasherly, Jul 6, 2007
  9. Crayon in mouth. It's common practice. Apparently.

    Tarquin Fin-tim-lin-bin-whin-bim-lim bus stop F'ta, Jul 6, 2007
  10. I see that you guys already know Radium :)
    Gene E. Bloch, Jul 7, 2007
  11. Radium

    Stephen Guest

    In digital video, how is the sample rate mathematically related to the
    For PAL television the field rate is 50 Hz and the frame rate is 25 Hz. For
    NTSC television the field rate is 59.94 Hz and the frame rate is 29.97 Hz
    (or more accurately 60 times 1000/1001 and 30 times 1000/1001 respectively).

    Both PAL and NTSC use 2 to 1 interlace which means that each field contains
    only alternate horizontal scan lines. So in "625 line" PAL with 576
    horizontal active lines per frame, each field refresh contains only 288
    lines. In "525 line" NTSC with 480 horizontal active lines per frame, each
    field refresh contains only 240 lines.

    When viewed on a large scale the sample rate for the picture as a whole is
    the same as the field rate which is the same as the refresh rate, i.e. 50 Hz
    (PAL) or just under 60 Hz (NTSC). Movement is generally perceived on a large
    scale so it is seen as though it were sampled at 50 or 60 Hz.

    Fine vertical detail (i.e. detail which goes vertically across the
    horizontal scan lines) is only sampled once per frame, on alternate field
    refreshes, so this has a sample rate of only 25 or 30 Hz, and can flicker
    violently if there is too much difference in intensity between adjacent
    horizontal lines.

    Large LCD and Plasma screens use "de-interlacing" to avoid this, but this
    has drawbacks because it has to either display one field for twice it's
    original sample period (i.e. display a 1/50th second exposure for 1/25th
    second) or start doing clever things like only doing this for fine details,
    or guessing where the patterns of the image should be in the missing lines
    of each field by examining the picture details from adjacent fields and
    lines. Most large screens use the "clever" approach, but none of this
    processing can be done perfectly and it tends to introduce lag, smearing or
    exaggerate mpeg artefacts by freezing them on the screen for twice the
    length of time they would be displayed by a CRT display which is not

    Modern large screens also have to resample the whole picture from 576 lines
    to the 768 lines of the display which inevitably reduces the resolution to
    about half of 768. This is why standard definition looks poor on an "HD
    ready" screen but looks virtually the same as 768 line HD when displayed on
    a CRT at 576 lines from an RGB Scart.

    Modern HD ready screens also use heavy edge sharpening to try and offset
    their lower-than-SD resolution when displaying standard definition, but this
    also exaggerates mpeg artefacts (erroneous detail information), which is
    another reason why these artefacts are objectionable and intrusive on HD
    ready screens, when they are barely noticeable on a CRT.

    It is an unfortunate fact that, since most TV we actually watch is still SD
    but most TV's on sale now are "HD ready" with poor SD performance, it is no
    longer possible to buy a new TV which is capable of displaying the majority
    of TV broadcasts properly. For most broadcasts we can only get proper
    pictures on "legacy" CRT TV's which have a limited lifespan and cannot be
    Stephen, Jul 8, 2007
  12. Nice to see such a clear explanation. If only the programme makers understood
    it so well.
    It's unfortunate that nobody is ever likely to see such clear HD pictures with
    such a smooth rendition of movement as the ones that were being shown at trade
    shows like IBC more than twenty years ago. The interlaced signals were derived
    from Plumbicon tube cameras, subject to control by vision engineers, and
    displayed on cathode ray tubes with no digital processing in between. For a
    while I thought it a normal symptom of the approach of old age that caused me
    to be cynical about the notion that "progress" always represents an
    improvement, but since then its even closer approach has disabused me of this.
    It seems a waste of money to keep buying ever newer, ever more elaborate and
    allegedly innovative domestic television equipment while the quality of the
    material available to show on it proceeds on its present downward spiral.

    Roderick Stewart, Jul 9, 2007
  13. Radium

    Ray Fischer Guest

    In a year and a half it will be moot as there will be no more of the
    same old TV broadcasts. They will all be replaced with digital
    signals, and probably of better quality.
    Ray Fischer, Jul 9, 2007
  14. Here's an interim solution if you have one of them new fangled HD TVs-
    may not be possible on all, but on my 4 year old Sony 60" what I do for
    NTSC broadcasts is to split screen, and size the lo rez broadcast down
    to where it's sharpest. It's about equal to the rez. I'd get on a non HD
    tv. The other screen can be tuned into something slow, such as baseball
    or golf, or Planet Earth.
    John McWilliams, Jul 9, 2007
  15. So what do you think old CRT's did? Nothing of course, but they had
    phosphors with a persistence of 40mS or more, so the lines drawn last time
    had not faded when the alternate ones were drawn - which amount to the same

    Many modern CRT sets draw the whole screen 100 times a second.
    I think it is more to do with jpeg images, and sitting too close to the set,
    so you can resolve (see individual) the pixels.
    R. Mark Clayton, Jul 10, 2007
  16. Radium

    Bob Myers Guest

    Very few CRTs ever had persistence that long. Typical persistance
    figures for TV/monitor tubes (and especially color CRTs) are on the
    order of a few milliseconds (to 10% of initial brightness), tops.
    Longer persistence tubes were available, of course (up to several
    seconds), but were generally restricted to very specialized uses
    (such as radar displays).

    Bob M.
    Bob Myers, Jul 10, 2007
  17. Radium

    Sjouke Burry Guest

    All those old IBM green phosphor monitors
    were very SLOOOOW....
    That was in the early XT pc era.
    Sjouke Burry, Jul 10, 2007
  18. Radium

    Ashley Booth Guest

    So was the first TV I ever saw. It was in the early 50s and my father
    used a radar crt (green phosphor) and a few other radar bits to make it.
    Ashley Booth, Jul 10, 2007
  19. Radium

    Bob Myers Guest

    Right - that was one of the exceptions. The original IBM 8514
    monitor (which is the one I believe you're thinking of) was a
    fixed-frequency 1024 x 768 monitor - one of the earliest to
    provide that format - but at the atrociously slow rate of
    43.5 Hz, interlaced. Had it not used a long-persistence
    phosphor, the slow refresh would have driven all its users
    crazy in short order. Unfortunately, long-persistence phosphors
    also tend to produce fairly large spot sizes, so this was a tradeoff
    between trying to avoid flicker and still produce an acceptable
    image at this "high resolution."

    Bob M.
    Bob Myers, Jul 10, 2007
  20. Same here. One of our neighbours had built a huge box to receive this newfangled
    kind of wireless programme with pictures. I remember the picture was very small,
    and round, and green. With hindsight it was probably a VCR97 or something of
    that ilk, which would have made it 6" diameter. It seemed to require a lot of
    fiddling about with some brass stair-rods which had been woven into a lot of
    wire netting in the attic, which meant nothing to me at the time of course, but
    I think would probably have been a slot aerial, for either Sandale or Kirk
    o'Shotts on Band 1, so quite big.

    Roderick Stewart, Jul 11, 2007
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