Doing 25Mb/s virtual 720p on the HC1/A1. at
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Sony HVR-A1 and HDR-HC Series
Sony's latest single-CMOS additions to their HDV camcorder line.

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Old August 24th, 2005, 02:02 AM   #1
Inner Circle
Join Date: May 2003
Location: Australia
Posts: 2,762
Doing 25Mb/s virtual 720p on the HC1/A1.

I have thought out a procedure capable of reproducing a 1280*720 pixel screen at 25Mb/s, that anybody is welcome to try.

I bemoaned the fact that the Sony didn't give us better compression, and the JVC didn't use the full 25Mb/s (apart from only running at 30fps in the HD1/10 models). Then I thought, hey you can get the best of both worlds. You could letter box the screen (top and bottom, and left and right) to get an approximate 720p screen that could be extracted from the 1080i frame (extra pixels for letter box border left behind). I do not know any software to perform such a cut and paste action though.

In the thread below I described the possibility of lower compression by using letter boxing on a DV camera, but this can also be used for a HD camera. Please see the first thread first for an explanation:

The difference between 1080/60i HDV (Ntsc market camera) and HDV 720/25p (pal market camera) is approximately half the compression, making it good for frame rate shift to 24fp for movies. This should also clean up compression/movement artifacts a lot more for the big screen. The difference between 1440 and 1280 pixels across is 160 pixels, just over 1/tenth field of view width reduction.

Using approximate figures:

1080i 60i 373.248 Mb/s / 25Mb/s = 14.93:1
1080i 50i 311.040 Mb/s / 25Mb/s = 12.44:1

720p 30p 221.184 Mb/s / 19Mb/s = 11.64:1
720p 25p 184.320 Mb/s / 19Mb/s = 9.70:1

720pV 30p 221.184 Mb/s / 25Mb/s = 8.85:1
720pV 25p 184.320 Mb/s / 25Mb/s = 7.37:1
2.35:1 gets over another 24% reduction in data rate, but too few vertical lines might be left for your liking on 720p (around 545 lines) but around 5.58:1 compression. 2.35:1 is equivalent to approximately 16:6.81

To letter box you could setup a frame, or you could put the frame on your GG adaptor, but because the HDV 1080i pixel is stretched compared to the real HDTV picture pixel, 1280*720p pixels won't be the right 16:9 ratio shape or fill field of view, unless you use an optical adaptor to squeeze the horizontal and vertical to compensate. So it would be complex.

Going one step further, you could letter box top and bottom the 1080i screen to 2.35:1 cinema screen frame, or the 720p virtual screen and get even better compression quality.

Progressive comes from cinema mode de-interlacing. We are conjecturing that de-interlacing in cinema mode happens before compression, therefore compression quality will be much better in this mode. It is not true progressive (according to some we don't know, hooking up the camera to a HD tv through the component and reading the status might help to determine) but a good compromise.

I don't think I forgot anything, any comments?
Wayne Morellini is offline   Reply With Quote
Old August 24th, 2005, 08:00 AM   #2
Join Date: Mar 2004
Location: Milwaukee, WI
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Very interesting.

I have actually been thinking about also using a 1080i camera to shoot 720p video. What I was planning however was to convert 1440x1080i 60i to 1280x720 60p for broadcast work. It may be a little softer than true 720p since each field only has 400 to 540 lines of resolution but at least it would be a way to shoot HDV at 60p without having to shoot 720x480 at 60p.
Thomas Smet is offline   Reply With Quote
Old August 28th, 2005, 10:48 PM   #3
Inner Circle
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Interesting Thomas.

I got back to it, and researched, cineframe mode, little information on cineframe effect mode for the HC1, but some people reckon it is the same as the cineframe 24 mode.

Doesn't look very suitable as it throws out one interlace field, unless you want to shoot 540 lines vertical, which means you instantly halve your compression ratio (assuming they don't reduce the data-rate recorded to tape at the same time). With an optical adaptor a unique way to do 2:35:1. But, the down side, it actually chucks out every second line, so there will be problems when something moves vertically through the image, and lines may look more jagged. On the negative side this could be considered throwing the baby out with the bath water than de-interlacing. A good dream while it lasted :)
Wayne Morellini is offline   Reply With Quote
Old August 29th, 2005, 03:26 AM   #4
Join Date: Jun 2004
Location: Chiyoda-ku, Tokyo
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Ah ha,

Wayne, you were thinking of the same thing as I was wondering but you had it further. My subject is not getting HDV compression on interlace than achieving progressive, but I will try to make cranky letter box adaptor and just to make the active shoot area smaller. It should be good enough for the examination.

I can resize it in realtime in Final Cut Pro 5.
Kaku Ito is offline   Reply

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