Resolution Issues in Downconverting HDV

J. Beale Feb. 15 2006

The Sony FX1 camera can record a lot of detail. Sometimes, when downconverting to produce a standard-definition DVD, you have too much.  Here is one example from an interview that I did, examining a detail area by a shirt collar. The images below are a 1:1 crop from an original HDV frame (using square pixels at 1920x1080). The camera "sharpness" was set to 14; a high setting which for some cases yields good results. In the original frame shown below at top left, the fine pattern of a shirt is reproduced with some moire patterns, but the image looks fine at the original resolution on a laptop screen.  When I simply exported the project from Vegas 6 as a 720x480 widescreen AVI for conversion to DVD, I saw severe moire artifacts on the shirt (see below). This was further compounded by the MPEG2 compression on the DVD, causing the moire bands to pop in and out of view depending on the bit-budget in each separate half-second GOP. As a result, the DVD playback looked truly awful on the same laptop that showed the HDV looking just fine.

You can reduce the fine detail by checking "reduce interlace flicker" properties but even that may not be adequate. I used a linear blur filter set at 90 degrees (vertical) and a strength of 0.001 (the minimum Vegas allows), as shown at the lower left.  This resulted in a reasonable looking downconverted video for DVD.

Original (crop from 1920x1280)
Crop from original HDV frame (1920x1080)
detail with "reduce interlace flicker" checked
Original with Properties / Reduce Interlace Flicker" checked
crop w/linear filter at 90 deg, 0.001
Original plus linear filter at 90 degrees, strength 0.001
crop with both filters
Both linear filter (90/.001) and Reduce Interlace Flicker

Below are three photos of the same Dell Inspiron 9300 laptop (1440x900 pixel, 17" LCD display) playing the two DVDs and the original HDV for comparison. At left is the first DVD created from the original HDV project without pre-filtering. On the right, another photo with everything the same but playing a DVD using the linear filter as described above, prior to downconversion to SD resolution.  The original HDV file is shown below left. Note that these images are only a small portion of the full HDV frame, as indicated below right. In each case I am using Nero Showtime 2 for display, which turns out to be particularly bad for moire artifacts.  WinDVD does not show as severe a problem playing  this particular DVD.

photo of laptop screen, with moire
photo of laptop screen showing moire pattern
photo of screen with corrected DVD
photo of same laptop playing corrected DVD
HDV displayed on laptop
photo of  laptop showing original HDV image
HDV frame detail area

detail area shown relative to full HDV frame

Ideally,  interview subjects shouldn't wear clothing with fine patterns. In this particular situation I couldn't ask my subject to go home and change.  
I didn't realize the problem until I got home, since the shirt looks fine on the FX1 LCD screen and in HDV playback on the laptop.
Incidentally, I only blanked out the interviewee's face since I didn't ask permission to put his photo online.  He's not suspected of anything.

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