Really bugging questions about interlacing and Canon's 25f

Discussion in 'Digital Video' started by rainerfilm, Feb 18, 2009.

  1. rainerfilm macrumors newbie

    Feb 18, 2009
    Hi there. I have a few really bugging questions about interlaced and progressive footage. I am fresh in the industry and will soon own my own "Canon xh a1" that I will use to make a living. I am 21 years old and just finished my studies as a cinematographer.

    Please correct me if I am wrong. my email address is

    Lets recap first:

    25p (progressive) is the digital format that resembles real film. This is where the camera takes 25 images per second and saves it just like that on your dv tape or firestore in a digital format.

    Canon (xh a1) has its own version of progressive, thy call it "F" for "Full frame". Rumor has it that thy dint want to pay for a progressive license so thy made their own version of 25p. As far as I know it works the same but somehow there is a 20 % vertical detail loss compared to progressive.

    [B]50i (interlaced)[/B] is where the camera takes 50 photos per second but only uses 1/2 of the information on each photo to make 25 interlaced frames per second. You get 2 fields, (the "odd" field and the "even" field) but that is not so important.

    This is how interlacing works according to
    1) Record field 1
    2) Record field 2
    3) Mix (=interlace) field 1 and field 2 into one frame and save the frame as frame 1.
    4) Record field 3
    5) Record field 4
    6) Mix (=interlace) field 3 and field 4 into one frame and save the frame as frame 2.


    So we are shooting 50i but we end up with only 25 frames. ( I don't know if this is true or not )

    This is how the average TV set works:
    The average TV set displays 50 interlaced frames per second. each field changes 25 times to give a total of 50 interlaced frames per second.

    My questions:

    When a camera records interlaced, does it take the odd field (the first field) from the first image, and the even field (the second field) from the second image and combines it to make 25 complete images per second?
    (Like listed above)
    OR does it work like this?:

    field no 1 = the odd field
    field no 2 = the even field

    STEP 1) record field one (photo no1)

    STEP 2) record field two (photo no 2)

    STEP 3) Mix (=interlace) photo 1 and photo 2 into one frame and save the frame as frame 1.

    STEP 4) record field one (photo no 3)

    STEP 5) Mix (=interlace) photo 2 and photo 3 into one frame and save the frame as frame 2.

    STEP 6) record field two (photo no 4)

    STEP 7) Mix (=interlace) photo 2 and photo 3 into one frame and save the frame as frame 3.

    STEP 8) record field one (photo no 5)

    STEP 9) Mix (=interlace) photo 2 and photo 3 into one frame and save the frame as frame 4.


    step 1, 2 and 3 is just to get the process going.

    This way there is 50 fps (frames per second) and not 25, similar to how a tv set works but uses the same processing power as the first method.

    If this is not true, why so???

    IF THIS IS TRUE:), How does a computer deinterlace 50i footage?

    Will the computer remove one of the fields completely and digitally re generate the missing field to give 50 progressive frames per second?:confused:

    How does a computer or tv set interlace 25p footage?

    Now CANON!!!!!!!!! and 25F.:confused::confused::confused::confused:

    Does anyone know of any differences between 25p and 25f?

    Will 25f cause any problems when I use it on my computer or any other device like a HD TV?

    Will I be able to play edited footage (25f) on a normal DVD player?

    Will 25f cause ANY problems that I don't know about?

    My email address is
  2. LethalWolfe macrumors G3


    Jan 11, 2002
    Los Angeles
    24fps, not 25fps, is the standard for film. PAL countries operate on a 50Hz cycle (25/50) while NTSC countries operate on a 60Hz cycle (30/60).

    The camera takes 50 'half pictures' a second. For example, a frame of SD PAL DV is 720x576 so an SD PAL DV camera will record two 720x288 images (fields) each second to create one 720x576 frame of video.

    Yes. 50i in this sense refers to *fields* per second.

    A PAL TV will display 50 *fields* a second which equals 25 *frames* per second.

    No. Each frame is composed of its own two fields.
    Odd field + Even field = Frame #1
    Odd field + EVen field = Frame #2

  3. ChrisA macrumors G4

    Jan 5, 2006
    Redondo Beach, California
    Two corrections: (1) Film is typicaly shot at 24 fps not 25.

    To figureout and remember how interlaced video works,think about an analog camera based on tube technology works. The camera and TV set do NOT have any built-in storage.

    In both the camera and picture tube the beam scans the odds lines, blanks and retraces to the top then scans out the even lines. The electron beams in both the camera and pcture tubes are synced and move together. (yes if there are 100,000 TV sets watching the same channel then everyone of then moves the electron beam to exactly the same spot on the screen. So, the feilds are not combined -- or maybe they are. The phosphor on the picture tube has a "half life" and will continue the glow after the beam has left for abot 1/69th second. This is why interlaced was developed so that TVs would flicker less even with "fast" phophor and to reduce the bandwidth of the transmitted signal and because it is easier (in the 1940's) to build a stable 60hz scanning system then a 30Hz system.
  4. rainerfilm thread starter macrumors newbie

    Feb 18, 2009
    Film in South Africa is shot on 25 fps. it corresponds with the 50 Hz of the power phases that we use. Oversea (for me) thy use 24 fps, why it is 24 with a 60 Hz power source, I don't know. I shot 2x 16mm films on 25 fps.
  5. rainerfilm thread starter macrumors newbie

    Feb 18, 2009
    In SA we shoot real film on 25 fps.

    So the first method that I method is true and the second method is false?

    This is the same as I explained in my first example.

    So do you know how 'f' works? The way I see it, the camera uses the same progressive image in both interlaced fields to save it in a interlaced format, but when you play back the 2 fields will give a progressive image because both fields comes out of the same image.
  6. LethalWolfe macrumors G3


    Jan 11, 2002
    Los Angeles
    Do you shoot film at 25fps when the intended output is just television and 24fps for theatrical or do you shoot 25fps for everything? The standard of 24fps for film shooting was chosen basically because it was the slowest frame rate (i.e. least expensive) that could be used to adequately playback sound.

    The examples you used were a little confusing because interlacing is a continuous recording/displaying of a new field every 1/50th or 1/60th of a second. There is not really a "mix" step. Viewing an interlaced signal on a progressive TV could be said to "mix" the fields because it will draw both fields at the same time, but on an interlaced TV only one field will be drawn on screen at a time.

    IIRC the reason Canon has 24f instead of 24p is two fold. First, the imagers on the Canon cameras record interlaced, not progressive, and second Canon doesn't license the patented technology typically used to generate progressive images from an interlaced sensor. They use their own technology to do this.

    What you are describing, one progressive image split into two interlaced fields, is called Progressive Segmented Frame (or PsF for short) and was developed to help make progressive video more backwards compatible w/interlaced-based video gear.

  7. rainerfilm thread starter macrumors newbie

    Feb 18, 2009
    We shoot 25 fps for film, and for digital we use 25p (progressive) or 50i (interlaced). we don't use 30p at all. All of our frame rates are usable with our 50hz power sours. It just works better that way. Don't you get flickering problems when you shoot at 24 fps?
  8. LethalWolfe macrumors G3


    Jan 11, 2002
    Los Angeles

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