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MillimeterHDV@Work
Oct 7, 1997 5:52 PM
24fps HDV Production: Part 1
By Steve Mullen
Over the last few months, two new HDV camcorders have come to market that offer 24 frames-per-second HDV video. The first to ship was JVC's GY-HD100, which offers progressive-scan CCDs and true 24p. The second, Canon's XL H1 that provides "24fps," which Canon is calling "24F." The Canon XL H1 uses 1080-line interlace CCDs, while the JVC HD100 has 720-line progressive CCDs.
Canon does not claim it captures at 24p because its three CCDs do not capture a "progressively scanned" image. In this way, the Canon is like the Sony Z1 and FX1, which also use interlace CCDs.
Canon claims its 24F implementation has three characteristics: the CCDs are clocked at 48Hz; the data from the CCDs is sent at 48Hz to the DIGIC DV II chip where 2:3:2:3 pulldown is incorporated; and the resulting 1080i60 video has neither interlace artifacts nor a loss in effective vertical resolution. (The latter claim must be qualified by the necessary “as compared to?”)
Interlace-scanning CCDs normally employ row-pair summation to keep light sensitivity high and reduce interlace line flicker and twitter. Row-pair summation acts as a filter that reduces effective vertical resolution by about 25-percent. Therefore, effective 1080i vertical frame resolution is about 810-lines.
By clocking the CCDs at 48Hz, a constant stream of video frames is captured. This eliminates the non-equal time between frames provided by Sony’s CineFrame 24 mode. It also supports a 1/48th second shutter. Next the stream of frames is “de-interlaced.” There are several ways deinterlacing can be accomplished.
One option is to drop every other field and interpolate a frame from the remaining field. This naturally causes a 50-percent loss in effective vertical resolution -- leading to “frames” of only about 405-lines.
Another option in to use Smart Deinterlacing to create a frame from both fields. While static resolution will be that of a full frame, dynamic resolution will be that of a field. Resolution will, therefore, range from about 405-lines to 810-lines depending on motion within the frame. Canon could also use its DV Frame Mode technique to read-out the CCDs. This mode employs every other field and creates the luma signal by adding Red+Blue pixels from rows 1 and 3 together with the sum of Green pixels in rows 2 and 4. This solution would yield about 608-lines of effective vertical resolution -- a close match to 720p24 resolution. Were row-pair summation disabled in 24F mode, these resolution values would increase to 540-lines, 540- to 810-lines, and 810-lines.
Until Canon clarifies the DIGIC DV II chip’s processing, it is not clear if the 24F mode’s effective vertical resolution is: 405-, 540-, 608-, or 810-lines. Or some value that changes with motion.
Now, 2:3:2:3 pulldown is applied to the synthetic frames and the result is then presented to the MPEG-2 codec. The codec output 1080i60 that carries 24F.
Both DV and HDV can be recorded at 24fps. The Region 60 model also supports "30F" (plus 60i), while the Region 50 model provides only "25F" and 50i—with no 24F capability. In the next issue, I will examine JVC's implementation of 24fps recording in its GY-HD100.
